b-decay properties of 72Ni and 72Cu

Abstract

The

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PHYSICAL REVIEW C 74, 054309 (2006)
β-decay properties of72Ni and72Cu
J.-C. Thomas,*H. De Witte, M. Gorska,†M. Huyse, K. Kruglov, Y. Kudryavtsev, D. Pauwels, N. V. S. V. Prasad,
K. Van de Vel,‡P. Van Duppen, and J. Van Roosbroeck
Instituut voor Kern- en Stralingsfysica, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
S. Franchoo,§J. Cederkall, H. O. U. Fynbo,||U. Georg, O. Jonsson, and U. K¨ oster¶
ISOLDE, CERN, CH-1211 Gen` eve 23, Switzerland
L. Weissman and W. F. Mueller
National Superconducting Cyclotron Laboratory, Michigan State University, 164 S. Shaw Lane, 48824-1312 Michigan, USA
V. N. Fedosseev∗∗and V. I. Mishin
Institute of Spectroscopy, Russian Academy of Sciences, RU-142092 Troitsk, Russia
D. Fedorov
St. Petersburg Nuclear Physics Institute, RU-188350 Gatchina, Russia
A. De Maesschalck and N. A. Smirnova
Vakgroep Subatomaire en Stralingsfysica, Universiteit Gent, Proeftuinstraat 86, B-9000 Gent, Belgium
(Received 1 September 2005; revised manuscript received 6 August 2006; published 21 November 2006)
The β-decay properties of72
at the CERN-ISOLDE facility, respectively. These neutron-rich nuclei have been produced in the proton-induced
fission of238U. Their decay schemes are presented and the lifetime T1/2= 6.63(3) s of72Cu was measured. No
β-decaying isomeric state was found in72Cu, in line with a suggested spin (2) for its ground state. Spin and parity
assignments of the observed excited states in the odd-odd nucleus72Cu are proposed and discussed in terms of
coupling between the valence proton and neutrons. Comparison is made with a schematic shell-model picture of
72Cu and with large-scale shell-model calculations performed in the (2p3/21f5/22p1/21g9/2) shell space outside
the doubly magic56
28Ni44and72
29Cu43have been studied at the LISOL facility of Louvain-La-Neuve and
28Ni28core.
DOI: 10.1103/PhysRevC.74.054309 PACS number(s): 21.10.−k, 23.40.−s, 25.85.Ge, 27.50.+e
I. INTRODUCTION
A. Motivation
The nuclear structure and the decay properties of neutron-
rich nuclei in the vicinity of the magic nucleus68
been extensively investigated in the past few years (see
[1] and references therein). The aim of these studies is to
evaluatethestrengthoftheneutronsubshellclosureatN = 40
and, generally speaking, to test the reliability of nuclear
shell models in this far-off stability region. Depending on
the robustness of the neutron subshell closure at N = 40,
28Ni40have
*Electronic address: thomasjc@ganil.fr; present address: GANIL,
B.P. 55027, F-14076 Caen Cedex 5, France.
†Present address: GSI, Planckstrasse 1, D-64291 Darmstadt,
Germany.
‡Present address: VITO, IMS, Mol, Belgium.
§Presentaddress:IPNOrsay,15rueG.Cl´ emenceau,F-91406Orsay
Cedex, France.
||Also at Department of Physics and Astronomy, University of
˚ Arhus, Dk-8000˚ Arhus C, Denmark.
¶Present address: Institut Laue-Langevin, 6 rue J. Horowitz,
BP 156, F-38042 Grenoble, France.
∗∗Also at CERN, 1211 Gen` eve 23, Switzerland.
collective driving nucleon-nucleon forces are indeed expected
to balance shell effects in nuclei having Z ≈ 28 protons and
N = 40 to 50 neutrons.
In addition, the decay properties and the nuclear structure
of the neutron-rich nuclei lying between68Ni and the doubly
magic78
physical processes [2,3].
Copper isotopes with A > 69 are good candidates to
study the strength and the evolution of nuclear shell effects
since their excited states may be partly described in terms
of particle-particle and particle-hole couplings between the
valence proton and neutrons [4]. The systematic study of the
nuclear structure of the odd69,71,73Cu isotopes has already
stressed the prominent role of the monopole part of the
residual proton-neutron interaction [5]. In a recent work, the
low-energy spectrum of70Cu has as well been successfully
described by means of large-scale shell-model calculations in
terms of coupling between the valence proton and the valence
neutron [6,7]. Extended nuclear structure studies in this mass
region should therefore lead to a better understanding of the
residual proton-neutron interaction.
In the following, we will discuss new results obtained in
β-decay studies of72Ni and72Cu at the LISOL facility of the
Centre de Recherche du Cyclotron at Louvain-La-Neuve and
28Ni50can be relevant for the modelization of astro-
0556-2813/2006/74(5)/054309(16)054309-1 ©2006 The American Physical Society

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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
at the ISOLDE facility of CERN, respectively. Data related to
the β decay of72Cu comprise part of a systematic study of
the decay properties of very neutron rich copper isotopes. The
decay properties of74,76,78Cu are discussed in [8] and the one
of71,73,75,77Cu will be presented in a forthcoming article [9].
B. Previous work
The β decay of72Cu was first investigated 20 years ago by
Runte et al. [10]. In this work,72Cu was produced by means
of multinucleon transfer reactions. A half-life of 6.6(1) s and
a partial decay scheme toward excited states in the daughter
nucleus72Zn were reported. Additional information on the
nuclearstructureof72Znresultsfromanearlier70Zn(t,p)72Zn
transfer reaction work by Hudson and Glover [11] and from
multinucleon transfer reactions performed recently by Wilson
et al. [12].
72Ni was first identified by Armbruster et al. in the thermal-
neutron-induced fission of235U [13]. A recent β-decay study
performed at the LISOL facility of Louvain-La-Neuve by
Franchoo et al. [14] led to a lifetime value of 1.57(5) s, in
agreement with the first measurement of 2.20(41) s reported
in [15]. A fragmentation study by Grzywacz et al. [16] led to
the identification of a microsecond isomeric state at 270 keV
in72Cu.Inthatwork,threeγ transitionsat51,82,and138keV
were associated with the decay of the isomeric state toward
the ground state of72Cu. These three γ transitions were also
identified ina fragmentation reaction experiment performed at
the GANIL Facility [17,18]. The spin and parity of the ground
stateandofthethreeinvolvedexcitedstateswerederivedfrom
half-life measurements and multipolarity considerations.
II. EXPERIMENTAL CONDITIONS
A. Production methods and detection setups
In the present work,72Ni and72Cu have been studied by
means of the ISOL technique in a four-step process consisting
of (i) the fission of238U induced by a proton beam of several
µA, (ii) the selective laser ionization of Ni or Cu atoms
inside a laser ionization source, (iii) the mass separation of the
selected ions, and (iv) the implantation of the separated ions
on a movable tape surrounded by β and γ detection devices.
72Cu was produced at the ISOLDE facility whereas72Ni was
studied at the LISOL facility. In the first case, a 1-GeV proton
beamof2-µAaveragebeamintensitywasimpingedonathick
UCx/graphite target of 50 g/cm2; in the latter case, a 30-MeV
proton beam of 6.4 µA was used with two thin238U targets of
10 mg/cm2tilted at 20◦with respect to the beam and placed in
a 500-mbar Ar gas cell.
For the decay study of72Cu at ISOLDE, two separate
detection setups were used. In the first one, three thin plastic
?E detectorsandtwoHPGedetectorsof64%and90%relative
efficiency were used in close geometry. In the second setup
devoted to lifetime measurements, the activity was deposited
on a movable tape passing through a 4π β detector. The β-γ
decay of72Ni was observed at LISOL by means of four plastic
TABLEI. Experimentalconditionsforthedecaystudyof72Niand
72Cu. For each nucleus, the time structure (implantation and decay
periods) of the cycles is given as well as the number of cycles after
which the tape was moved. The total measurement time with and
without laser ionization is shown in the next column. The last column
gives the average production yields obtained for the two selectively
ionized isotopes.
NucleusCycle
(impl./dec.)
TapeLaser onLaser
off
Yield
(ions/µC)
72Ni
72Cu
0.6 s/0 s
0.5 s/7 s
15013 h 14 min
20 min
50 min
—
∼10
11.3 × 107
?E detectors, three of them being associated with HPGe
detectors of 70%, 75%, and 90% relative efficiency.
Table I gives a summary of the experimental conditions
for the decay study of72Ni and72Cu: Each run consisted
in the repetition of cycles of implantation and decay periods.
The proton beam was pulsed during the implantation part of
the cycles. Laser-off runs were taken for the setting on72Ni
only.Moredetailsontheproductionmethodsandthedetection
setups used for the β-decay study of neutron-rich isotopes
at the LISOL and ISOLDE facilities can be found in [5,19]
and [7,20], respectively.
B. Analysis procedure
For each setting, both β-conditioned and nonconditioned
γ spectra were taken. The comparison between the two
sets of data was made to identify γ rays originating
from the environment (background lines) or associated with
the decay of long-lived contaminants (contamination lines).
The latter contribution had different origins, depending on the
installationwheretheexperimentwasperformed:Forthestudy
of72Cu at ISOLDE, the contaminating activity was related
to the implantation of long-lived isotopes next to the tape
during previous runs and from surface ionization of fission
fragmentsinsidetheionsource.Forthestudyof72NiatLISOL,
long-livedfissionfragmentsextractedfromthelaserionization
source in a 2+charge state with the same A/q ratio as the
selected ion were also transmitted to the detection setup.
To reduce the activity from implanted contaminants, the
tape was moved after a given number of cycles. As shown in
Table I, the tape was moved every cycle of 7.5 s for72Cu and
after 150 cycles (90 s) for the setting on72Ni.
As reported in Table I, the production yield of 10 ions/µC
measuredatLISOLfor72Niwassixordersofmagnitudelower
than the one obtained at ISOLDE for the production of72Cu.
As a consequence, the β-conditioned γ data for the setting
on72Ni are dominated by contamination lines. Laser-off runs
were therefore performed to disentangle the γ rays following
the β decay of72Ni (and of its short-lived daughter72Cu) and
the contamination lines.
Once the main γ transitions have been identified in72Zn
(72Cu β decay) and in72Cu (72Ni β decay), γ-γ coincidences
were performed to detect low-intensity γ rays and to place
them in the β-decay scheme of the dedicated nucleus.
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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
The relative intensity of the observed γ rays was derived
from the β-gated γ spectra. By considering the high energy of
the β particles emitted in the decay of72Cu (Qβ= 8.3 MeV
[21]) and72Ni (Qβ= 5.8 MeV [21]) and the low detection
threshold(aboutonehundredkeV)oftheplastic?E detectors,
the β-detection efficiency was taken as a constant in each
experiment whatever the energy of the level fed. As a result,
no energy-dependent correction was applied to the relative
intensity of the γ rays.
Standardγ calibrationsourcesdonotallowustodetermine
with a good precision the γ-detection efficiency at energies
above2MeV.However,thebehaviorofHPGedetectorscanbe
reliablyreproducedbymeansofMonteCarlosimulations[22].
The γ-detection efficiency was therefore derived in the 0- to
4-MeV range for the two experiments from simulations using
the GEANTcode[23].Theywerevalidatedupto1.4MeVusing
60Co and152Eu calibration sources. In case cascading γ rays
or cross-over transitions were observed in the same detector,
the relative intensities were corrected to account for possible
summing effects.1
III. EXPERIMENTAL RESULTS
A. The β-decay study of72Cu at ISOLDE
1. γ-ray identification
Representative single and β-gated coincidence spectra
obtained in the decay of72Cu with the HPGe detector having
the best resolution are shown in Fig. 1. The strongest γ rays
are listed in Table II. The peak areas, also shown in the table,
were extracted from the β-gated γ spectrum obtained for the
full data set.
The time behavior of the identified γ lines was checked
in the following way: The counting rate obtained for each
photo peak during the decay part of the cycles was fitted by
means of a single-component exponential decay function after
a background subtraction was applied. The γ-ray identifica-
tion was refined by means of γ-γ coincidences. For some
γ rays, there is only weak evidence for a real coincidence.
Their peak areas are indicated with a question mark in
Table II.
Three γ rays at 145.1, 352.4, and 1461.1 keV disappear
when the β-coincident condition is applied. The latter two are
well-known background lines originating from214Pb and40K,
respectively. The 145.1-keV γ line was attributed to the β-γ
decay of72Zn (T1/2= 46.5 h, Qβ= 458.4 keV [21]). The
associated β particles have a maximum energy of 458.4 −
145.1 = 313.3 keV, and most of them may not have enough
energy to trigger the β detectors.
The time behavior of the γ rays was fitted with a single
exponential, revealing a half-life value reported in Table II.
Mostoftheγ raysagreewiththe72Culifetimeof6.6(1)sgiven
in [10]. However, the fitting procedure gives negative half-life
1The probability that two cascading γ rays contribute to the photo
peakoftheassociatedcross-overtransitionisequaltoεγ1×εγ2,where
εγ1,2are the photo peak efficiencies of the cascading γ rays.
values for the γ rays at 264.6, 487.6, and 834.4 keV. This
indicates that the half-life of their parent nucleus is probably
much longer than the 7-s decay period of the cycles. The
264.6-keV γ line does not show any clear coincidence with
anyotherlineanditwasattributedtothedecayof75Ge(T1/2=
82.78min[21]).Theγ lineat487.6keVisincoincidencewith
thetwolinesat386.5and619.4keV,whichareassociatedwith
long decay times. The three γ rays were therefore attributed
to the decay of71Znm(T1/2= 3.96 h [21]). Both75Ge and
71Zn may have been implanted next to the tape in previous
settings. The γ ray at 834.4 keV is coincident with other
γ rays from the decay of72Ga (T1/2= 14.10 h [21]). The
latter is the daughter nucleus of72Zn, originating from the
β decay of72Cu and directly produced from surface ionization
in the ISOLDE ion source.
The remaining 55 γ lines were attributed to the decay of
72Cu because they exhibit a short half-life or because they are
coincident with known transitions in72Zn. Furthermore, the
γ ray at 3355.0 keV was considered as an escape peak from
the intense γ transition at 3865 keV, because of the energy and
coincidence relations between the 511.00- and 3355.0-keV
γ rays.
2. Decay scheme
The decay scheme of72Cu was derived from the γ-γ
coincidence relations quoted in Table II. The result is shown
in Fig. 2. Most of the γ rays could be placed in the decay
scheme unambiguously. Some weak lines, such as the 753.1,
928.6, and 1469.0-keV transitions, were placed on the basis of
energy matching.
The relative γ-ray intensities were deduced from the area
of the β-gated peaks listedinTable II,following the procedure
described in Sec. IIB. They are summarized in Table III and
absoluteintensitiesper100β decays aregiveninFig.2.These
values were used to derive the β branching ratios toward the
deduced excited states in72Zn.
In contrast to the previous β-decay study of72Cu by Runte
et al. [10], no β-singles spectra where taken during the present
experiment. As a consequence, the intensity of the direct
β feeding to the ground state of72Zn could not be inferred
from a comparison between β-singles and β-gated γ spectra.
However, the ground-state spin of72Cu was assumed to be
Iπ= (2±) from a comparison between the decay schemes of
70,72,74,76Cu (see next section) and from the decay study of
72Ni (see Sec. IIIB). A direct feeding to the72Zn ground state
wouldthereforeinvolveaforbidden2±→0+β transitionwith
?I = 2and?π = ±1.AccordingtothecompilationofSingh
et al. [24], the corresponding log(ft) values are expected to be
respectively larger than 10.6 (?π = +1) and 7.5 (?π = −1),
depending on the parity of the72Cu ground state. Thus, the
directfeedingtothegroundstateof72Znwouldhaveaintensity
weaker than 0.02% and 2.5%, respectively. We assumed in the
present work that it can be neglected.
The β and γ-ray branching ratios shown in Fig. 2 are
normalized to the total γ-decay strength obtained by summing
the relative intensities of all the γ rays decaying directly to
the ground state of72Zn. The reported log(ft) values were
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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
100200 300400500600700800900 10001100
10
2
10
3
10
4
10
2
10
3
10
4
10
5
10
Counts/keV
145.1
264.6
352.4
386.5
487.6
511.00
535.3
599.6612.3619.4
652.68
716.18
753.1
798.7
834.4
846.75
858.3
928.6
942.4959.9
988.24
1004.7
1016.0
1100 1200 1300 1400 1500 160017001800190020002100
2
10
3
10
2
10
3
10
1146.4
1251.5
1404.31408.3
1461.1 1469.015161540.0
1610.5
1657.6
1789.3
1896.0
1917.3
1993.12004.4
2040.5 2050.3
2094.9
210022002300 24002500 2600 2700280029003000
2
10
3
10
2
10
2169.0 2206.7 2236.5
2255.6
2409.2
2517.52540
2594.0
2769.8
2859.2
2921.9
300031003200330034003500 3600 37003800 39004000
10
2
10
3
10
10
2
10
Energy (keV)
3008.9
3054.7
3100
3212.93246.3
3348.43355.0
3477.4
3706.9
3865
3941.6
FIG. 1. Single (upper histogram, left scale) compared to β-gated coincident (lower histogram, right scale) γ spectra obtained for the setting
on72Cu with the HPGe detector having the best resolution. Histograms are represented with a binning of 1 keV/channel. The energy of the
γ rays identified in the β decay of72Cu is reported.
calculated by assuming no β feeding to the ground state and
have to be considered as lower limits. Levels identified in
previous works [25] are marked with an x symbol in Fig. 2.
The72Cu lifetime of 6.63(3) s was measured in the present
work as described in Sec. IIIA4.
In general, the decay scheme proposed in Fig. 2 agrees
well with the one of Runte et al. [10]. The decay scheme
from the present work is more complete and it contains all the
γ transitionsreferencedin[10].Onlythe612.3-and858.3-keV
γ transitions have a different placement: In [10], they link the
excitedstatesat3662and2193keVviaanintermediatestateat
3050 keV. In the present work, a coincidence between the two
γ rays was excluded (see Table II). Hence, the intermediate
excited state proposed in the work [10] at 3050 keV is
questionable.
3. Spin and parity assignment
In the previous β-decay study of72Cu by Runte et al. [10],
the spin and parity of the excited states at 653, 1499, and
1658keVwereadoptedfromthe70Zn(t,p)72Znreactionwork
of Hudson and Glover [11]. In the latter, excited states at
652(10), 1505(10), and 1652(10) keV were observed and Iπ
assignments of 2+, 0+, and 2+were made by comparing
the experimental (t,p) cross sections with optical model
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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
TABLE II. Energy values, peak areas, and half-life values of the β-gated γ lines identified in the decay of72Cu. γ lines with a negative
value for T1/2were associated with the decay of parent nuclei with a half-life much longer than the fitting time interval. Coincident γ rays are
given in the last column of the table. Their peak areas are indicated within brackets and with a question mark when only weak evidence for a
coincidence was found.
E (keV)Area
T1/2(s)Coincident γ rays
145.1(3)
264.6(1)
352.4(4)
386.5(1)
487.6(1)
511.00(6)
535.3(2)
599.6(3)
612.3(1)
619.4(2)
652.68(5)
———
—
—
900(100)
—
1000(100)
430(90)
3500(100)
670(50)
310(40)
1400(100)
300(70)
89700(300)
−15(1)
—
170(90)
−6(2)
11(2)
10(8)
—
8(2)
400(3500)
8.2(1)
488[70], 511[30], 596[40], 619[70], 1107[20]
387[60], 619[60]
511[520], 653[70], 3355[40]
653[30], 1005[10]
653[20], 1540[10]
535[10], 653[130], 1540[30]
387[60], 488[60]
600[20], 612[120], 847[770], 858[190], 988[50], 1005[1180], 1016[240], 1252[170],
1408[40], 1469[40], 1516[50],1540[220], 1993[240], 2004[50], 2041[100],
2207[60], 2256[200], 2409[200], 2540[50], 2594[30], 2922[50], 3009[130],
3055[60], 3100[100], 3213[50], 3348[60], 3477[80], 3942[20]
653[50], 1540[30]
653[30]
653[60], 1005[40], 1252[40], 1658[20], 2256[10]
630[20], 894[10], 2202[20]
653[710], 942[20], 1146[30], 1896[20], 2207[20], 2859[20]
653[150]
653[10], 1005[?], 1658[?]
653[30], 847[20]
653[20]
653[30], 1005[20], 1016[20], 1658[30]
535[30], 653[1080], 799[30], 988[50], 1252[100], 1917[10], 2004[50], 2095[20]
653[170], 847[20], 988[30], 1005[?], 1146[10], 1658[10], 1993[90]
653[40], 847[30]
653[120], 799[50], 1005[110], 1658[80]
653[10]
653[20], 847[20]
—
653[20], 1540[?]
653[30], 1540[10]
612[40], 653[260], 716[30], 1469[20], 2237[20]
653[30], 847[20]
799[20], 988[20], 1252[100], 2004[50]
653[30]
653[20], 847[20]
653[30], 1005[20], 1658[10]
653[240], 1016[80]
653[40], 1005[40], 1658[40]
653[40], 1005[10]
653[10], 1005[10], 1658[?]
653[20], 1005[20], 1658[?]
—
653[30], 847[20], 1005[?]
653[80], 1540[20]
653[130], 799[10]
653[200]
653[20], 1005[10], 1658[?]
653[50]
653[30]
653[10], 1005[10], 1658[10]
847[40]
716.18(7)
753.1(2)
798.7(1)
834.4(2)
846.75(7)
858.3(1)
928.6(7)
942.4(3)
959.9(3)
988.24(9)
1004.7(2)
1016.0(1)
1146.4(2)
1251.5(1)
1404.3(5)
1408.3(3)
1461.1(4)
1469.0(2)
1516(2)
1540.0(2)
1610.5(5)
1657.6(2)
1789.3(3)
1896.0(6)
1917.3(3)
1993.1(9)
2004.4(3)
2040.5(3)
2050.3(5)
2094.9(3)
2169.0(3)
2206.7(3)
2236.5(3)
2255.6(3)
2409.2(3)
2517.5(4)
2540(2)
2594.0(4)
2769.8(4)
2859.2(4)
610(60)
350(50)
1360(60)
380(60)
8400(100)
1490(50)
160(40)
240(40)
230(40)
1270(40)
11200(100)
3150(60)
730(60)
4170(80)
190(30)
370(40)
9(6)
6(3)
12(4)
−40(90)
9.1(7)
8(2)
—
10(9)
4(2)
14(5)
8.4(5)
9(1)
—
8.3(9)
3(2)
4(1)
—
—
—
9(1)
3(2)
8.9(8)
7(5)
8(6)
6(3)
9(1)
15(6)
12(8)
5(3)
5(2)
9(4)
7(2)
9(3)
7.1(9)
9(2)
9(6)
—
8(3)
7(4)
8(3)
350(30)
540(50)
3200(70)
140(30)
5950(70)
260(30)
310(30)
540(40)
2700(60)
1530(50)
470(40)
230(30)
430(40)
530(50)
560(40)
830(40)
2050(60)
1780(50)
360(30)
330(30)
500(40)
290(30)
770(40)
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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
TABLE II. (Continued.)
E (keV)Area
T1/2(s) Coincident γ rays
2921.9(5)
3008.9(5)
3054.7(5)
3100(2)
3212.9(5)
3246.3(6)
3348.4(6)
3355.0(7)
3477.4(6)
3706.9(7)
3865(1)
3941.6(7)
450(30)
1050(40)
650(40)
820(40)
360(40)
200(40)
690(30)
700(30)
620(40)
490(30)
1900(30)
250(30)
10(6)
8(2)
4.4(8)
8(3)
10(8)
6(3)
6(1)
7(2)
7(2)
12(8)
8(1)
3.3(7)
653[50]
653[140]
653[50]
653[90]
653[40]
—
653[100]
511[40], 653[20]
653[60]
—
653[?]
653[20]
calculations. Similarly, we will adopt the Iπ= 2+assignment
for both the 653- and 1658-keV states. However, it is unclear
whetherthe0+stateat1505(10)keVfromHudsonandGlover
corresponds to the 1499-keV state, as Runte et al. assumed, or
to the state at 1511 keV additionally observed in the present
work. The answer is suggested by recent results obtained in
multinucleon transfer reactions by Wilson et al. [12]. In this
work, a (4+) state at 1500 keV is reported, which decays
further down to the ground state of72Zn via two cascading
γ rays of 847.1 and 653.0 keV.2They suggest this state to be
2In multinucleon transfer reactions, yrast excited states are more
likely produced (i.e., the higher the excitation energy of a state is, the
higher its spin is). Since the 1500-keV state in [12] decays to the 2+
state at 653 keV, an Iπvalue of 4+is more likely than Iπ= 0+.
TABLE III. Relative intensities (Irel) of the γ rays identified in the β decay of72Cu. Values are
normalized with respect to the intensity of the transition at 652.68 keV. For absolute intensities per 100
β decays, Irelhas to be multiplied by 0.79.
E (keV)
Irel
E (keV)
Irel
535.3(2)
599.6(3)
612.3(1)
652.68(5)
716.18(7)
753.1(2)
798.7(1)
846.75(7)
858.3(1)
928.6(7)
942.4(3)
959.9(3)
988.24(9)
1004.7(2)
1016.0(1)
1146.4(2)
1251.5(1)
1404.3(5)
1408.3(3)
1469.0(2)
1516(2)
1540.0(2)
1610.5(5)
1657.6(2)
1789.3(3)
1896.0(6)
1917.3(3)
0.64(6)
0.32(5)
1.5(1)
100(5)
0.73(8)
0.43(6)
1.8(1)
11.4(6)
2.0(1)
0.23(5)
0.35(6)
0.34(6)
1.9(1)
17.3(9)
4.9(3)
1.2(1)
7.6(4)
0.38(6)
0.74(9)
0.71(7)
1.1(1)
6.8(4)
0.31(6)
11.7(6)
0.59(9)
0.80(9)
1.4(1)
1993.1(9)
2004.4(3)
2040.5(3)
2050.3(5)
2094.9(3)
2169.0(3)
2206.7(3)
2236.5(3)
2255.6(3)
2409.2(3)
2517.5(4)
2540(2)
2594.0(4)
2769.8(4)
2859.2(4)
2921.9(5)
3008.9(5)
3054.7(5)
3100(2)
3212.9(5)
3246.3(6)
3348.4(6)
3477.4(6)
3706.9(7)
3865(1)
3941.6(7)
7.0(4)
4.0(2)
1.3(1)
0.55(9)
1.2(1)
1.5(1)
1.6(1)
2.4(2)
5.6(3)
5.6(3)
1.2(1)
1.1(1)
1.7(2)
1.1(1)
2.9(2)
1.6(2)
3.6(2)
2.3(2)
3.3(2)
1.5(2)
0.7(2)
3.1(2)
2.9(2)
2.2(2)
9.9(5)
1.4(2)
054309-6

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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
FIG. 2. Proposed level scheme for72Zn. Levels labeled with “x” were observed in previous experiments [25].
different from the 1499-keV state from Runte et al. because of
the difference in Iπand in energy (?E ∼ 1 keV). Both states
are finally assumed to be possible members of a two-phonon
multiplet.However,withthepresenceofthenewlydetermined
state at 1511 keV from this work, it is more likely that the
Iπ= 4+state at 1500 keV from [12] is the same than the
1499-keV state reported in [10] and that the 1511-keV state is
the 0+state observed by Hudson and Glover. Hence, we
propose a Iπ= (4+) assignment for the 1499-keV excited
state and Iπ= (0+) for the state at 1511 keV.
054309-7

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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
TABLE IV. Intensity ratio of the (4+
γ transitions in70,72,74,76Zn observed in the β decay of70,72,74,76Cu.
1) → (2+
1) to (2+
1) → 0+
g.s.
72Cu, g.s.
74Cu, (2,3)
70Cu, (3−)
76Cu, (3,4)
70Cu, (6−)
11.4(9)%13.0(9)%52(6)%60(4)% 94.7(6)%
The spin and parity assumption of (4+) for the 1499-keV
state also agrees well with the observed decay patterns in
Fig. 2: The states at 2442, 2646, and 2909 keV decay to the
2+state(s) at 653 keV (and 1658 keV) and to the 1499-keV
state, but not to the 0+ground state of72Zn. If the 1499-keV
state would be a 0+state, we would expect these states to
decay to the ground state as well. The absence of the decay
to the ground state is explained3if the 1499-keV state has a
spinvalueIπ= 4+andifthe2442,2646,and2909-keVstates
have spin values of 3 or 4.
Using the same argument indicates that the states at 3247
and 3865 keV have possible spin values of 1 or 2 since they
populate the 0+ground state of72Zn and the 2+state at
653 keV. The state at 3707 keV is restricted to a spin value of
(2) since it decays to the 0+ground state, to the two 2+states
at 653 and 1658 keV, and to the (4+) state at 1499 keV.
The largest amount of β-decay strength (57.2%) feeds the
states at 653, 1658, 2909, 3662, 3707, and 3865 keV: Most of
these states have spin ranging from (2) to (4). This suggests
that the ground state of72Cu has a spin value ranging from
(1) to (3). The spin value can be tentatively inferred from
a comparison of the ground-state β decays of neighboring
even-even copper isotopes. No direct β feeding nor γ decay
to the first 4+excited state of70Zn was observed in the
β decay of the 1+isomeric state of70Cu [7]. Hence, the
feeding to the (4+) excited state at 1499 keV in72Zn observed
in the present work suggests a spin I > 1 for the ground
state of72Cu. Furthermore, Table IV shows that the intensity
ratio of the (4+
agrees well with the one obtained in74Zn in the β decay of
the I = (2,3) ground state of74Cu [8], whereas it disagrees
with the yields measured in the decays of the Iπ= 3−
isomeric state and Iπ= 6−ground state of70Cu [7] and in
the decay of the I = (3,4) ground state of76Cu [8]. A spin
assignment I = (2) is therefore favored for the ground state of
72Cu.
During an in-source laser spectroscopy experiment dedi-
cated to neutron-rich Cu isotopes, three β-decaying isomers
wereobservedin70Cu[7].Usingthistechniquefor72Cu,how-
ever, revealed no evidence for different β-decaying isomeric
states.
1) → (2+
1) to (2+
1) → 0+
g.s.γ transitions in72Zn
4. Lifetime evaluation
Thelifetimeevaluationof72Cuwasperformedbyusingthe
time spectrum delivered by the 4π β detector being part of the
second detection setup. The daughter nucleus72Zn has a half-
3Here and in the following, the argument is based on the selection
rule ?I ?2 (no 0 → 0) for fast γ decays.
020 4060
t (s)
80100 120
1
10
2
10
3
10
4
10
5
10
counts
β
FIG. 3. Number of counts in the β detector (dots) as a function
of time. The solid line is the result of a fit of a single-component
exponential decay function on a constant background.
life of 46.5 h [21] and its contribution to the β-decay activity
was considered as a constant background in view of the time
scale used. The same holds for possible surface ionized72Ga
(T1/2= 14.10 h [21]). Thus, the72Cu β-decay time spectrum
wasfittedwithasingle-componentexponentialdecayfunction
added to a constant background. The fit of the spectrum is
shown by the solid line in Fig. 3. A half-life of 6.63(3) s
was obtained for72Cu, in perfect agreement with the value of
6.6(1) s reported in [10].
The influence of the dead time of the data-acquisition
systemwasinvestigatedbystartingthefitofthetimespectrum
at different positions. No significant change in the derived
lifetime value was observed.
B. The β-decay study of72Ni at LISOL
1. γ-ray identification
Figure 4 shows a superposition of the β-gated γ spectra
obtained in the decay of72Ni with (upper histogram) and
without (lower histogram) selective laser ionization of Ni
atoms. Because the beam time devoted to laser-off runs was
quite short (see Table I), the corresponding spectrum in Fig. 4
was normalized by a factor of 16 for comparison with the
laser-on data. For visibility, the laser-on spectrum is plotted
with an additional offset of 3000 counts per channel in the top
part of the picture, of 500 counts per channel in the middle
part, and of 200 counts per channel in the bottom part.
Nonresonant γ lines were attributed to the decay of
144Ba (T1/2= 11.5 s [21]) and of its daughter nucleus144La
(T1/2= 40.8 s [21]). As shown in Fig. 4, the most intense
γ rays following their β decay are visible in the laser-on
as well as in the laser-off spectra. Owing to their relatively
short lifetimes, most of their activity was observed before
the implantation tape was moved after each cycle of 90 s
(Table I).144Ba is strongly produced in the fission of238U
and is partly extracted from the laser ion source in a 2+charge
state.ThemassresolutionM/?M ≈ 1000oftheLISOLmass
separator does not allow filtering of this contamination since
the A/Q values of72Ni and144Ba are too close to each
other: [M(144Ba)/2 − M(72Ni)]/M(72Ni) ≈ 3. × 10−4.4The
4Atomic masses were taken from [26].
054309-8

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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
100200300400500600700800900
3
10
4
10
Counts/keV
74.8
94.0
137.4
202.6
297.0
314.3
376.4
451.7
470476.0
Cu decay
72
Ba decay
144
La decay
144
9001000 110012001300140015001600 1700
500
1000
1500
2000
2500
987.3
1141.3
1332.5
1387.5
1421.81443.4
1517.4 + 1518.0
1590.0
1684.0
17001800 1900 200021002200 2300
200
400
600
800
Energy (keV)
1726.2
1745.2
1762.2
1820.2
1895.0
2060.3
2120
2221
FIG. 4. β-gated laser-on (upper histogram) and normalized laser-off (lower histogram) spectra obtained in the setting on72Ni. Offsets of
3000,500,and200countsperchannelwereaddedtothelaser-ondatadisplayedonthetop,middle,andbottompartsofthepicture,respectively.
Histograms are represented with a binning of 1 keV/channel. The energy of the γ rays identified in the β decay of72Ni is reported.
activity of144La is supposed to originate from the β decay
of144Ba and not from a direct contamination. Its second
ionization potential is indeed higher than the one of144Ba
and it is presumably it does not significantly survive in a 2+
charge state inside the laser ion source [27].
The resonant γ lines were attributed to the decay of72Ni,
unless their energy and intensity matched the ones of the
γ lines previously identified in the decay of72Cu. They are
listed in Table V with their associated areas. The peaks at
1332.5, 1421.8, and 1518.0 keV could not be identified by
comparing the β-gated laser-on and laser-off γ spectra. They
were attributed to the β-γ decay of72Ni on the basis of γ-γ
coincidence relations: As illustrated in Fig. 5, the γ ray at
1332.5 keV is clearly coincident with the one at 376.4 keV
whereasitishardlynoticeableintheβ-gatedlaseronspectrum
of Fig. 4.
Many of the γ transitions associated with the decay of
72Ni are contaminated by low-intensity γ rays originating
from the β decay of72Cu,144Ba, and144La. As a result, the
area of the peaks of interest was most often corrected for the
expected contribution of the contamination lines. The peak
areas corrected at a level of more than 10% are quoted with a
asterisk in Table V. The main consequence of this correction
procedureisthatlargeuncertaintieswereobtainedfortheareas
of some low-intensity γ rays.
In Fig. 4, the peak at 470 keV was found to originate from
true summing of the intense coincident γ transitions at 94 and
376 keV. It is therefore not reported in Table V. True summing
corrections of less than 20% were also applied to a few other
γ rays.
2. Decay scheme
The decay scheme of72Ni is proposed in Fig. 6. It was
derived from the coincidence relations between the identified
γ lines. As shown in Table V and in Fig. 5, half of the
γ transitions are coincident with the one at 376.4 keV. They
were placed in the decay scheme with respect to their energy,
relativeintensity,andcoincidencerelationbetweeneachother.
AsshowninFig.6,ninetransitionswerefoundtofeeddirectly
054309-9

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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
TABLE V. Energy values, peak areas, and coincident γ rays
(whichpeakareasareshowninbrackets)oftheγ linesidentifiedinthe
β decay of72Ni. Peak areas that were corrected more than 10%
for the contribution of the144Ba and144La contaminants or for the
β decay of the daughter nucleus72Cu are marked with an asterisk (∗).
E (keV)AreaCoincident γ rays
74.8(1)
94.0(1)
800(100)
25900(200)
376[260]
203[400], 376[5870],
1388[50],
1590[50], 1726[110]
314[950], 1443[30], 1745[70]
94[370], 376[280], 470[70],
1388[40]
376[450], 1388[90]
137[910], 1443[?], 1745[40]
75[190], 94[5600], 203[300],
297[470], 1141[100],
1333[40],
1388[140], 1518[140],
1590[50],
1684[390], 1726[160],
1820[100],
2221[40]
1443[30], 1745[60]
1422[30]
653[20], 1004[30], 1016[10],
1654[10]
376[100]
376[30]
94[40], 203[50], 297[100],
376[100]
476[30]
75[10], 137[30], 314[20],
452[30]
137.4(1)
202.6(1)
5900(500)∗
2000(300)∗
297.0(1)
314.3(1)
376.4(1)
1600(400)∗
2300(100)
62200(900)
451.7(1)
476.0(1)
987.3(1)
2700(800)∗
800(100)∗
4000(200)
1141.3(5)
1332.5(2)
1387.5(2)
<590
180(40)
450(80)∗
1421.8(9)
1443.4(9)
<150∗
>130
1517.4(1)
1518.0(2)
1590.0(1)
1684.0(4)
1726.2(1)
1745.2(3)
<320∗
550(60)
<300
1300(100)∗
<920
630(50)
376[130]
94[40], 376[30]
376[380]
94[140], 376[160], 470[20]
75[20], 137[90], 314[40],
376[30], 452[60]
1762.2(2)
1820.2(7)
1895.0(4)
2060.3(1)
2120(1)
2221(1)
400(200)∗
400(200)∗
190(70)∗
1200(100)
60(20)
160(60)
376[100]
476[20]
376[30]
the ground state of72Cu, with the one at 376.4 keV accounting
more than 70% of the total direct γ-decay strength.
The relative intensities of the γ transitions are presented
in Table VI. The large uncertainties of about 30% are due to
the poor precision reached in determining experimentally the
relative γ-detection efficiency and to the corrections applied
to take into account the contribution to the photo peaks of
γ lines originating from the β decay of144Ba,144La, and72Cu.
Theβ branchingratiosIβtoward72Cuexcitedstatesaswell
as their associated log(ft) values are given in Fig. 6. They were
200400600 8001000
0
500
1000
1500
2000
2500
3000
3500
Counts/3 keV
74.8
94.0
202.6
297.0
100012001400
Energy (keV)
1600 18002000
0
50
100
150
200
250
300
1141.3
1332.5
1387.5
1518.0
1684.0
1726.2
1820.2
1590.0
2000210022002300
0
10
20
30
2221
FIG. 5. γ spectrumobtainedforthesettingon72Niincoincidence
with the β-conditioned γ ray at 376.4 keV. The histogram is
represented with a binning of 3 keV/channel.
calculatedbyusingthelifetimevalueof72Nideterminedinthe
work [14] and assuming no direct feeding to the presumably
I = (2)groundstateof72Cu:Suchadecayfromthe0+ground
stateof72Niwouldindeedinvolveaforbiddenβ transitionwith
?I = 2and?π = ±1.AccordingtothecompilationofSingh
et al. [24], the corresponding log(ft) values are expected to be
largerthan10.6(?π = +1)and7.5(?π = −1),respectively,
which means that the direct feeding to the72Cu ground state is
expectedtohaveanabsoluteintensitylowerthan(8. × 10−5)%
or 0.1%, depending on its parity.
The log(ft) values were corrected to take into account
the contribution of the electronic conversion process to the
electromagneticdecayofthelowestexcitedstates.Onthebasis
ofspinandparityassignmentsdiscussedinthenextsection,the
most probable multipolarities were adopted for the transitions
at74.8,94.0,137.4,314.3,376.4,and451.7keV.Accordingto
previous studies [16–18], none of the related decaying states
is likely to be a long-lived isomer and we checked that the
expected lifetimes associated with the adopted multipolarities
were indeed shorter than 1 ps. The largest correction (13%)
concerned the very low intensity γ line at 74.8 keV and has
therefore little influence on the evaluated β branching ratio
toward the excited state at 452 keV. However, a correction of
6% had to be applied to the transition at 94.0 keV having a
relative intensity of 27%. This leads to an increase of the β
feeding to the excited state at 470 keV of less than 10%.
For comparison, the low-energy part of the level structure
of72Cu obtained in the work of Mach et al. [17] is shown in
the right part of Fig. 6. In addition to the (3−) excited state
at 138 keV observed as well in the present work, two other
negative-parity states were reported at 219 (4−) and 270 keV
054309-10

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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
FIG. 6. Proposed level scheme for72Cu. Spin and parity assignments were partly derived from a previous study [17,18]. Results from the
latter work are presented in the right part of the figure.
(6−). The fact that they were not seen in the present work can
be explained by the low spin and the positive parity of the
0+ground state of72Ni. The same argument can be used to
question the direct feeding to the (3−) excited state at 137 keV.
One may assume that there is actually no direct feeding to this
state in the β decay of72Ni.
3. Spin and parity assignment
In the β-decay study of70Ni [7], the 0+ground state was
found to decay preferably toward two 1+excited states at
242 keV and 1278 keV in70Cu, with log(ft) values lower
than 5. In the same way, a spin of (1+) can be attributed
to the excited states of72Cu at 376 [log(ft) = 4.8], 2060
[log(ft) = 4.6], and 2197 keV [log(ft) = 4.7]. The excited
state at 470 keV has a slightly higher log(ft) value of 5.1, but
it only decays toward the (1+) excited state at 376 keV. This
behavior is closely related to that of the (1+) excited state at
1278 keV in70Cu, decaying mainly toward the 1+excited
state at 242 keV. The excited state at 470 keV is therefore
presumably a (1+) state as well.
Inthework[17],theground-statespinof72Cuwasassumed
to be Iπ= (2+), based on multipole and lifetime arguments.
Considering the lifetime of the 138-keV transition from the
upper lying (3−) state (see Fig. 6) gives its most probable
multipolarity of E1,M1, or E2, with an associated transition
rate being, respectively, 105times slower, 2. × 103times
slower, or 30 times faster than the corresponding Weisskopf
054309-11

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J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
TABLE VI. Relative intensities (Irel) of
the γ rays identified in the β decay of72Ni.
Values are normalized with respect to the
intensity of the transition at 376.4 keV. For
absolute intensities per 100 β decays, Irel
has to be multiplied by 0.74.
E (keV)
Irel
74.8(1)
94.0(1)
137.4(1)
202.6(1)
297.0(1)
314.3(1)
376.4(1)
451.7(1)
476.0(1)
987.3(1)
1141.3(5)
1332.5(2)
1387.5(2)
1421.8(9)
1443.4(9)
1517.4(1)
1518.0(2)
1590.0(1)
1684.0(4)
1726.2(1)
1745.2(3)
1762.2(2)
1820.2(7)
1895.0(4)
2060.3(1)
2120(1)
2221(1)
0.9(3)
27(7)
5(2)
2.1(6)
2.2(6)
3.3(6)
100(16)
5(2)
1.4(3)
12(2)
<2.2
0.7(2)
1.8(4)
<0.6
>0.5
<1.3
2.3(4)
<1.3
6(1)
<4.6
2.9(5)
2(1)
1.9(8)
0.9(4)
6(1)
0.3(1)
0.9(3)
estimate. A systematic study of the pure electromagnetic
transitions observed [28] in the 10 to 200-keV range in the
62?A?82 region reveals five pure E1, two pure M1, and
eight pure E2 transitions with transition rates, respectively,
5. × 103to 105times slower, 102to 103times slower, and 1
to 100 times faster than the associated Weisskopf estimates.
An E2 transition would result in a (1−) or a (5−) ground state
for72Cu. As shown in the next section, a 1−state cannot
be obtained by considering the valence proton and neutron
orbitals involved in72Cu. A 5−ground state can be rejected
as well since the intense transition at 376.4 keV from the
upper lying (1+) state would be of M4 character and therefore
strongly hindered. However, an E1 or an M1 character of
the transition at 137 keV is consistent with observations in
the same mass region. Thus, the spin of the72Cu ground state
is presumably I = (2), in agreement with the expected value
derived earlier from its β-decay study.
TwoI = (2−)and(2+)stateswereidentifiedinthework[7]
inthelow-energyspectrumof70Cu.Theyarefedintheγ decay
of two high-energy (1+) states, which are mainly connected
to lower lying (1+) states. In the present work, the two (1+)
states identified at 2060 and 2197 keV are in the same way
decaying preferably toward low-lying (1+) states, but they are
also decaying toward the I = (2) ground state of72Cu and
toward an excited state at 452 keV (see Fig. 6). The latter
decays toward the I = (2) ground state of72Cu, and toward
the I = (3−) and I = (1+) excited states at 137 and 376 keV,
respectively. We therefore assigned a spin I = (2) to the state
at 452 keV.
IV. DISCUSSION
A. Shell-model interpretation of the low-energy spectrum
of72Cu
Shell-model calculations have been performed following
two different approaches.
(i) First, we used a schematic model. We have considered
thespectrumofthelow-lyingstatesin72Cuasoriginatingfrom
thecouplingofthevalenceprotonwithaneutronquasi-particle
beyond the70Ni core. Thus, the proton could occupy any
of the four valence orbitals (2p3/21f5/22p1/21g9/2), whereas
neutrons were restricted to the 1g9/2orbital (where we will
refer to ν1˜ g9/2as a neutron quasi-particle). As a result, only
states of negative parity could be calculated. The interaction
between the valence proton and neutrons was taken to be of
δ type or a quadrupole-quadrupole one. The change of the
single-proton energies reulting from the interaction with the
ν1˜ g9/2neutrons(with68Nibeingconsideredasacore)hasbeen
estimated via a monopole shift formula as described in [4]. All
the details of this approach as well as the parameters of the
interactions used can be found in [8].
(ii) In the second approach, the energy levels have been
obtained via a large-scale shell-model diagonalization using
the ANTOINE code [29] in the (2p3/21f5/22p1/21g9/2) space
outside the doubly magic56
effective interaction as derived by Hjorth-Jensen, Kuo, and
Osnes [30] and modified further for the monopole part by
Nowacki [31]. The interaction used in the present work differs
from the one used in Refs. [4,6–8] by changes imposed onto
three two-body centroids:
28Ni28 core. We used a realistic
?VT=0
2p3/22p3/2= −0.2 MeV,
?VT=0
2p1/22p1/2= −0.1 MeV,
?VT=0
1g9/21g9/2= −0.05 MeV.
In addition, some multipole modifications have been per-
formed,whichwillbediscussedinmoredetailelsewhere[32].
The resulting interaction gives a better agreement with
the behavior of the lowest 5/2−state studied in neutron-rich
odd-copper isotopes [4,5]. It also reduces the energy of the
(2−
755 keV.
To test further the interaction, the level scheme of72Zn
was calculated with only four neutrons allowed to be excited
from the pf shell to the 1g9/2 orbital. As shown in Fig. 7,
the level scheme is in good agreement with the one obtained
experimentally below 2 MeV excitation energy, although the
state observed at 1613 keV is not reproduced. One can notice
that the excitation energy of the 2+
by more than 200 keV when going from70Zn [7] to72Zn. The
1) state observed in70Cu at 369 keV from 1275 keV [7] to
1state drops experimentally
054309-12

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β-DECAY PROPERTIES OF72Ni AND72Cu PHYSICAL REVIEW C 74, 054309 (2006)
0+
1612.6
1499.43
1510.9
652.68
00
755
1507
1652
1714
Realistic Interaction
0+
2+
2+
4+
0+
Experiment
E (keV)
2+
(4+)
(0+)
2+
1657.5
FIG. 7. Nuclear structure of72Zn below 2 MeV of excitation
energy. Nuclear states observed experimentally are compared to
the theoretical spectrum calculated using the realistic interaction
discussed in the text.
reason could be that the additional pair of neutrons occupying
the ν1g9/2 orbital in72Zn somewhat weakens the N = 40
subshell gap between the ν2p1/2and the ν1g9/2orbitals and
can induce deformation.
Figure 8 compares the predictions of the different shell-
model approaches to the low-energy spectrum (E∗< 1 MeV)
of72Cu observed experimentally. The two spectra obtained
with the schematic approach are shown in the right part of the
figure. They consist in two (3–6)−and (2–7)−multiplets of
π2p3/2ν1˜ g9/2and π1f5/2ν1˜ g9/2configurations, respectively,
almost without any mixing between them. The low-energy
positive-parity states, expected from the coupling of a proton
in π2p3/2and a neutron hole in ν2p−1
space.
The results of the large-scale diagonalization using the
realistic interaction are shown next to the experimental level
1/2, are outside the model
TABLE VII. Nuclear structure of72Cu at low excitation energy.
Nuclearstatesobservedexperimentallyarecomparedtothosederived
from a large-scale shell-model calculation using a realistic effective
interaction(seetextfordetails).The∗symbolsrefertostatesobserved
in [17]. For the 5−and 2+
2states only theoretical calculations exist.
Iπ
Eexp(keV)
Etheor(keV) Configuration(%)
(2+)0 or 451.6 431
π2p3/2ν(2p−1
π2p3/2ν(2p−1
π2p3/2ν1g3
π2p3/2ν(1f−2
π2p3/2ν1g3
π2p3/2ν(1f−2
π2p3/2ν1g3
π2p3/2ν(1f−2
π2p3/2ν1g3
π2p3/2ν(1f−2
π2p3/2ν(2p−1
π2p1/2ν(2p−1
π1f5/2ν1g3
π2p3/2ν1g3
π2p1/2ν(2p−1
π2p3/2ν(2p−1
π1f5/2ν(2p−1
π1f5/2ν(2p−1
π1f5/2ν(2p−1
1/21g4
3/21g4
9/2)
9/2)
(47)
(10)
(56)
(10)
(56)
(10)
(64)
(11)
(62)
(11)
(45)
(10)
(33)
(19)
(29)
(20)
(15)
(48)
(10)
(3−) 137.40
9/2
5/21g5
9/2)
(4−)∗
219140
9/2
5/21g5
9/2)
(6−)∗
27016
9/2
5/21g5
9/2)
5−
235
9/2
5/21g5
1/21g4
1/21g4
9/2)
9/2)
9/2)
(1+
1)376.4 418
(2−) 0 or 451.6 387
9/2
9/2
(1+
2)470.4 585
1/21g4
1/21g4
1/21g4
1/21g4
3/21g4
9/2)
9/2)
9/2)
9/2)
9/2)
2+
2
772
scheme. Table VII gives the π-ν configurations derived from
the large-scale shell-model calculation and contributing more
than 10% to the lowest excited states observed experimentally.
The low-energy spectrum of72Cu is found to be dominated
by the π2p3/2ν1g3
leading to the (3–6)−and (1,2)+multiplets of states, respec-
tively. The expected (2–7)−multiplet of the π1f5/2ν1g3
dominant configuration is also reproduced.
9/2and π2p3/2ν(2p−1
1/21g4
9/2) configurations,
9/2
1+
2−
δ
472
0
72
59
821
813
812
764
903
158
6−
96
5−
366
Realistic Interaction
5−
3−
3−
6−
4−
5−
7−
4−
3−
6−
2−
5−
4−
3−
7−
2−
4−
3−
QQ Interaction
971
904
833
700
252
0
x
x
Experiment
E (keV)
137.4
219
270
376.4
987.3
451.6
476.0
470.4
0
235
0
6−
16
418
387
431
673.3
(3−)
(4−)
(6−)
(1+)
(2)
(1+)
(2)
5851+
5−
609
2+
4−
3−
2−
6−
772
754
630
824
818
4−
140
2+
force
FIG. 8. Nuclear structure of72Cu below 1 MeV of excitation energy. Nuclear states observed experimentally are compared to theoretical
spectra derived from (i) large-scale shell-model calculations within the (2p3/21f5/22p1/21g9/2) shell space and using a realistic interaction and
from (ii) a schematic approach using two types of residual proton-neutron interactions (a δ force and a quadrupole-quadrupole interaction).
The states labeled with an “x” symbol are taken from [17].
054309-13

Page 14

J.-C. THOMAS et al.
PHYSICAL REVIEW C 74, 054309 (2006)
The (3−), (4−), and (6−) states observed experimentally at
low energy can be identified as members of the predicted
(3–6)−multiplet. According to Table VII, they have a
dominant (56% to 64%) π2p3/2ν1g3
energy difference between the states is quite well reproduced
in the realistic approach. The three calculations performed in
the present work predict a contraction of the (3–6)−multiplet
when two neutrons are added to70Cu [8]. It is difficult to
address this question since the 5−member of the multiplet
was not observed experimentally in72Cu. From the work of
Mach et al. [17], we can at least expect it to lie above the
(6−) state identified at 270 keV, which is in agreement with
the present calculation.
The (1+) state observed experimentally at 376 keV can be
identified as the first member of the (1,2)+doublet of states.
It is predicted by the realistic calculation to lie at 418 keV
with a dominant (more than 45%) π2p3/2ν(2p−1
configuration. The Iπ= 2+member of the doublet is presum-
ably one of the two I = (2) states observed in the low-energy
spectrum of72Cu, namely the excited state at 452 keV or the
groundstateof72Cu.Inthefirstcase,theenergypositionofthe
(1–2)+doublet would match pretty well with the large-scale
shell-model prediction (see Fig. 8). However, if the (2+) spin
partner of the (1+) state at 376 keV is the ground state of
72Cu, the large splitting of the doublet could be explained by
its strong mixing with another closely lying 2+
another dominant configuration such as π1f5/2ν(2p−1
Sucha2+
predicted by the present calculation to lie at 772 keV. The cal-
culation does not reproduce the desired effect, but the descrip-
tionoftheodd-oddnucleiisusuallynotasimpletask,sinceitis
rather sensitive to the details of the two-body matrix elements.
Alternatively, one of the two I = (2) states observed exper-
imentally can be identified as the low-energy 2−member of
the predicted (2–7)−multiplet, having according to Table VII
a dominant (more than 30%) π1f5/2ν1g3
In all theoretical approaches, the 2−state is expected to
lie significantly below the other members of the multiplet:
from about 130 keV in the schematic calculation using a
quadrupole-quadrupole interaction and up to almost 300 keV
in the realistic approach (see Fig. 8). In the latter case, the
2−state is expected at an excitation energy of 387 keV,
which would be in good agreement with the I = (2) state
observed at 452 keV. However, the realistic interaction used
in the present work predicts the 2−state in70Cu to lie at
755 keV, whereas it is observed at an excitation energy of
369 keV [7], that is, 386 keV below the expected value. If the
same situation occurs in72Cu, then the I = (2) ground state
of72Cu can be identified as the first member of the (2–7)−
multiplet. Such a lowering of the 2−state could be justified
by its leading configuration component, involving a π1f5/2
valence proton coupled to three neutrons located in the ν1g3
orbital. The lowering of the π1f5/2proton orbital associated
withthefillingoftheν1g9/2neutron orbitalisindeed expected
from earlier work on odd-copper isotopes [4,5]. In addition,
a drop in energy of the 2−state with respect to the other
members of a (2–7)−multiplet of similar character is also
observedin86Rb(Z = 37,N = 49),inwhichaπ1f5/2proton
9/2configuration. The
1/21g4
9/2)
2state, having
1/21g4
9/2).
2statewasnotobservedexperimentally,althoughitis
9/2configuration.
9/2
is coupled to a ν1g9/2 neutron hole [33]. It is therefore
unclear whether the predicted 2−state having a dominant
π1f5/2ν1g3
or the ground state of72Cu.
By now, the 3−to 7−members of the (2–7)−multiplet have
yet to be identified experimentally in72Cu. They appear in the
schematic approach below or slightly above 1 MeV, wheres
they are found to be between 609 keV (5−
in the realistic calculation. In the latter approach, they have a
dominant π1f5/2ν1g3
which the π2p3/2ν1g3
Except for the unexpected lowering of either the (2+
(2−
the theoretical calculations are in rather good agreement with
the experimental results. The same overall features have been
observed in70Cu [7]. In this lighter odd-odd Cu isotope, the
ground state was identified as the 6−state belonging to the
low-lying(3–6)−multipletofπ2p3/2ν1g+1
the (1,2)+doublet associated with the π2p3/2ν(2p−1
configuration was found at an excitation energy of about
300 keV. In72Cu, the corresponding (1+) state is located at
a similar energy of 376 keV and the (3–6)−multiplet is shifted
upward because of the lowering of either the (2+
state.
In [7], two high-energy (1+) excited states have been
identified at 1278 and 1980 keV in70Cu. The latter was
interpreted as being dominantly due to the coupling of
the valence proton and neutron in the π1g9/2 and ν1g9/2
orbitals, respectively. In72Cu, one of the two (1+) states
observed at about 2100 keV may have a similar π1g9/2ν1g3
configuration. As discussed in [7], the large-scale shell-model
calculation using a realistic interaction, even the modified
version discussed here, does not allow identification of any
high-lying 1+excited state having a dominant π1g9/2ν1g3
configuration. Nonetheless, such a state is well predicted
within the schematic approach to lie at 2112 and 1998 keV
for a δ force and QQ interaction, respectively.
9/2configuration is the one observed at 452 keV
2) and 941 keV (7−
1)
9/2configuration, except the 5−
9/2configuration prevails.
2state for
1) or the
1) state for which theoretical explanations could be given,
9/2configuration,and
1/21g2
9/2)
1) or the (2−
1)
9/2
9/2
B. Shell-model interpretation of the72Ni →72Cu →72Zn
β-decay chain
According to Fig. 6, the 0+ground state of72Ni decays
preferably toward the first (1+) excited state of
376 keV [B.R. = 42%, log(ft) = 4.8]. Within the extreme
single-particle shell-model picture, the decay mechanism can
be interpreted as the transformation of a ν2p1/2neutron into a
π2p3/2proton.
Two main β-decay branches feed the (1+) states of72Cu at
2060 and 2197 keV (B.R. = 11% and 7%, log(ft) = 4.6 and
4.7, respectively). As previously mentioned, one of these two
statesmaybeassociatedwithaπ1g9/2ν1g3
would therefore result from the conversion of a ν1g9/2neutron
into a π1g9/2proton. The further de-excitation of this state to
the (2−) ground state of72Cu or to the (2−) excited state at
452 keV can then be interpreted as the lowering of the π1g9/2
proton to the π1f5/2orbital across the Z = 40 subshell gap.
The log(ft) values associated with the allowed Gamow-
Teller transitions to the 1+states in72Cu were calculated
72Cu at
9/2configuration. It
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β-DECAY PROPERTIES OF72Ni AND72CuPHYSICAL REVIEW C 74, 054309 (2006)
TABLE VIII. Theoretical and experimental log(ft) values of
the Gamow-Teller β decay of72Ni to (1+) excited states in72Cu.
The shell-model calculations have been performed with the realistic
effective interaction described in the text.
Iπ
exp
Eexp(keV)
Iπ
th
Eth(keV)log(ft)exp
log(ft)th
(1+)
(1+)
(1+)
376.4(1)
470.4(1)
2060.4(1)
1+
1
1+
2
1+
3
1+
4
1+
5
418
585
1534
2273
2447
4.7
5.1
4.6
4.6
5.1
4.6
8.3
5.3
(1+) 2196.6(2)4.7
usingtherealisticinteractiondescribedhere.Theyarereported
in Table VIII and compared to the experimental results. The
overall agreement is good, although an additional high-lying
1+state is predicted to be fed with a high log(ft) value of
8.3. However, even if it exists, such a state could not have
been observed in the present experiment owing to its very low
feeding in the β decay of72Ni.
The ground state of72Cu was interpreted here either as a
(2+) of a π2p3/2ν(2p−1
with a dominant π1f5/2ν1g3
the β decay of the72Cu ground state toward the 2+
72Zn involves an allowed Gamow-Teller transition that can be
interpreted as the conversion of a valence neutron occupying
the ν2p1/2orbital into an extra π2p3/2proton in72Zn. The
decay rates of the hypothetical (2+) ground state of72Cu
toward the first two 2+states of72Zn calculated in Fig. 7
are compared in Table IX to the experimental values. They are
in rather good agreement with the measurements; the slight
discrepancy is possibly related to the imposed restrictions on
the model space.
However,ifthegroundstateof72CuhasanI = 2−spinand
parity, its decay toward the first two 2+and toward the first
(4+) excited states of72Zn would involve first-forbidden β
transitions of nonunique and unique character, respectively.
According to the compilation of Singh et al. [24], the
corresponding log(ft) values are expected to range between
5.1 and 11.0 for the two ?I = 0 transitions and between 7.5
and12.8forthe?I = 2transition.Thus,themeasuredlog(ft)
values associated with the β decay to the first two 2+states
are compatible with an I = 2−ground state of72Cu. As can
be seen in Fig. 2, the measured β-decay strength toward the
(4+
value of 7.0. However, the associated absolute β branching
1/21g4
9/2) configuration or as a (2−) state
9/2component. In the first case,
1state in
1) state is stronger that the compiled values, with a log(ft)
TABLE IX. Theoretical and experimental log(ft) values of the
β transitions from72Cu ground state to the first 2+excited states
in72Zn. The shell-model calculations have been performed with the
realistic effective interaction described in the text, assuming an I =
2+ground state for72Cu.
Iπ
exp
Eexp(keV)
Iπ
th
Eth(keV)log(ft)exp
log(ft)th
2+
2+
652.68(5)
1657.5(1)
2+
1
2+
2
755
1507
6.5
6.6
6.7
6.8
ratio is quite weak [Iβ= 2.8(5)%], and one cannot exclude an
underestimation of the feeding to the (4+
γ transitions from upper lying excited states. As a conclusion,
log(ft) arguments cannot be used to determine the parity of
72Cu ground state.
1) state by unobserved
V. CONCLUSION
The72Ni →72Cu →72Zn β-decay chain has been studied
in detail at the complementary facilities of LISOL (Centre
de Recherche du Cyclotron, Belgium) and ISOLDE (CERN,
Switzerland). Conventional β-γ detection techniques associ-
ated with a selective laser ionization of Ni and Cu atoms
led to detailed level schemes for72Cu, for which a half-life
T1/2= 6.63(3) s was measured, and72Zn. In contrast to70Cu,
an in-source laser spectroscopy study revealed no β-decaying
isomeric states in72Cu.
The nuclear structure of72Cu at low excitation energy was
discussed in terms of single-particle coupling between the
valence protons and neutrons. Two shell-model calculations
were performed using different modelization of the effective
residual proton-neutron interaction. They revealed that the
low-lying states in72Cu result predominantly from the cou-
pling of the valence proton occupying the π2p3/2or π1f5/2
orbitals to the valence neutrons located either in the ν1g9/2or
in the ν2p1/2orbital. Most of the low-energy states observed
in72Cu could be attributed to the (3–6)−, (1–2)+, and (2–7)−
multiplets of states already identified in70Cu.
In contrast to70Cu, for which the ground state spin is
Iπ= (6)−, the β-decay studies of72Ni and72Cu allowed
assignment of a spin value I = (2) to the ground state of
72Cu. Two theoretical explanations were proposed to justify
such a change. In both cases, an unexpected drop in energy
of either a (2+) state of π2p3/2ν(2p−1
or a (2−) state of π1f5/2ν1g3
invoked.Thefirstscenarioinvolvesastrongmixingofthe(2+
state with another (2+) state having a π1f5/2ν(2p−1
configuration. The second one suggests a dramatic lowering
of the π1f5/2 proton orbital as compared to
ing to the addition of two neutrons in the ν1g9/2 orbital.
These observations highlight the relevance of the nuclear
structure study of neutron-rich copper isotopes and the
need for a better understanding of the residual proton-
neutron interaction in the N = 40–50 region. Specifically,
the ground-state spin and parity of72Cu could be investi-
gated by means of laser spectroscopy or Coulomb excitation
studies.
1/21g4
9/2) configuration
9/2dominant component was
1)
1/21g4
9/2)
70Cu, ow-
ACKNOWLEDGMENTS
This work was performed within the frame of the IS365
Collaboration and the ISOLDE Collaboration. We gratefully
thank J. Gentens and P. Van den Bergh for running the LISOL
separator and the ISOLDE technical group for assistance
during the experiment performed at CERN. N.A.S. thanks
E. Caurier and F. Nowacki (IPHC, Strasbourg) for making
available the ANTOINE code and the interaction used here.
054309-15