Observation of isomeric decays and the high spin states in doubly-odd 208Fr

Abstract

Neutron deficient isotopes of Francium (Z=87, N=121-123) as excited nuclei were produced in the fusion-evaporation reaction: 197Au(16O,xn)[213-x]Fr at 100 MeV. The gamma-rays from the residues were observed through the high sensitivity Germanium Clover detector array INGA. The decay of the high spin states and the isomeric states of the doubly-odd 208Fr nuclei, identified from the known sequence of ground state transitions, were observed. The half lives of the 194(2) keV isomeric transition, known from earlier observations, was measured to be 233(18) ns. A second isomeric transition at 383(2) keV and half life of 33(7) ns was also found. The measured half lives were compared with the corresponding single particle estimates, based on a the level scheme obtained from the experiment. Comment: 11 pages, 7 figures

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arXiv:0911.2968v1 [nucl-ex] 16 Nov 2009
Observation of isomeric decays and the high spin states in doubly-odd208Fr
D. Kanjilala, S. Bhattacharyaa,b, A. Goswamia, R. Kshetria, R. Rauta, S. Saha∗,a,b, R. K. Bhowmikc, J. Gehlotc,
S. Muralitharc, R. P. Singhc, G. Jnaneswarid, G. Mukherjeee, B. Mukherjeef
aNuclear and Atomic Physics Division, Saha Institute of Nuclear Physics, Kolkata 700064, India
bCentre for Astro-Particle Physics, Saha Institute of Nuclear Physics, Kolkata 700064, India
cInter University Accelerator Centre, New Delhi 110067, India
dDepartment of Physics, Andhra University, Vishakhapatnam 530003, India
eVariable Energy Cyclotron Centre, Kolkata 700064, India
fDepartment of Physics, Visva Bharati, Santiniketan 731235, India
Abstract
Neutron deficient isotopes of Francium (Z=87, N ∼ 121 − 123) as excited nuclei were produced in the fusion-
evaporation reaction:197Au (16O, xn)213−xFr at 100 MeV. The γ rays from the residues were observed through
the high sensitivity Germanium Clover detector array INGA. The decay of the high spin states and the isomeric
states of the doubly-odd208Fr nuclei, identified from the known sequence of ground state transitions, were observed.
The half lives of the Eγ = 194(2) keV isomeric transition, known from earlier observations, was measured to be
T1/2= 233(18) ns. A second isomeric transition at Eγ = 383(2) keV and T1/2= 33(7) ns was also found. The
measured half lives were compared with the corresponding single particle estimates, based on a the level scheme
obtained from the experiment.
Key words:
PACS: 21.10.Tg, 23.20.Lv, 23.35.+g, 27.80.w
1. Introduction
Investigation on the nuclear structure of the trans-Lead neutron deficient nuclei (Z > 82, N < 126) have
attracted much attention in recent years[1, 2, 3]. For many of these nuclei, only the ground state spin and parity
are known from their g-factor/magnetic moment measurements, and perhaps a few low lying excited states are
observed so far. The major difficulties in populating the high spin states in these nuclei are: a) very low cross
sections for the formation of evaporation residues (ER), and b) very high probability of fission, which removes flux
from the ER channel and prevents the excited nucleus from sustaining large angular momenta needed to populate
the high spin states.
Experimental investigation of the high spin states of quite a few trans-Lead neutron deficient nuclei have been
of interest recently. A series of investigations on the211−214Fr isotopes[4, 5] showed that the structure can be
interpreted in terms of the shell model states, and the excited states reveal an interplay between the protons in the
(1h9/2,2f7/2,1i13/2) states and the neutron holes in the (2f5/2,3p3/2,1i13/2) orbitals, or the neutrons promoted to
the (2g9/2,1i11/2,1j15/2) high spin orbitals by core excitation, leading to the generation of high spin states. One of
the major interests in the spectroscopic investigation of these nuclei is the role played by the i13/2shell in creating
isomeric levels which decay through transitions of higher multipolarity or are hindered by the close proximity of
the levels below.
A few spectroscopic investigations on the proton rich lighter Francium isotopes have been made recently. While
a complete study of the205−207Fr nuclei[1], using Gammasphere and the HERCULES II array for filtering out
the evaporation residues from the fission background, revealed the existence of shears band in207Fr (N = 120),
investigations on the208−210Fr nuclei[2, 3] have resulted in contradictory conclusions. Spectroscopic studies made
using the YRAST BALL array, comprising six Compton suppressed Clover HPGe detectors[2], coupled to the
SASSYER recoil separator[6] for selecting the evaporation residues had concluded that the pair of intense gamma
∗Corresponding author
Email address: satyajit.saha@saha.ac.in (S. Saha)
Preprint submitted to arXivNovember 16, 2009

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Figure 1: A few gated spectra of the Fr isotopes.
rays of 632 keV (ground state transition) and 194 keV (isomeric transition) belong to209Fr. The half life of the
isomeric transition was measured to be 446(14) ns. At the same time, another independent study of isomeric decay
of proton rich nuclei produced by projectile fragmentation reaction of238U beam at 900 MeV/u on9Be target at the
Fragment Recoil Separator (FRS) facility of GSI, Darmstadt, Germany had assigned the same pair of gamma rays
to208Fr[3]. Clean isotopic and isobaric resolution of nuclei, which have low lying isomers with half lives ? 100 ns,
are routinely achieved using this facility[7]. Half life of the 194 keV isomeric transition was reported to be ∼ 200 ns,
though the prompt transitions above the isomer could not be observed because of experimental constraints. This
paper reports investigation of the isomeric transitions in208Fr, along with the high spin states above the isomers.
It is noteworthy to report here that at the time of final phase of data analysis and preparation of the manuscript, a
paper on assignments of levels in208Fr was published by G. D. Dracoulis et al.[8]. The differences in our methods,
the results of observations on the isomeric transitions, new γ transitions over and above those reported therein,
and the basis for our assignments of the isomeric transitions to208Fr are highlighted in this paper.
2. Experiment and data analysis
The experiment to produce208Fr was carried out at the Inter-University Accelerator Centre (IUAC), New Delhi.
The Fr isotopes were produced by bombarding a 3.5 mg.cm−2self-supporting Gold (99.95% purity) target with16O
beam at 88, 94 and 100 MeV. The nuclei of interest were produced as evaporation residues (ER) through (16O, xnγ)
reactions. The target thickness was chosen on the basis of energy loss calculations based on SRIM2003[9] to stop
more than 90% of the ER within the target, and allow most of the fission fragments to fly away from it. Estimation
of cross sections, angular distributions of the evaporation residues (ER) and the fission yield were done using the
code PACE[10]. Based on these calculations, ∼ 60 − 80 % of the fusion products at these bombarding energies
undergo fission, which causes a huge background. Therefore, an effective filter to clean up the spectra, and / or good
statistics are essential for extraction of meaningful results, as were done recently[1]. In the absence of such filters,
use of large gamma detector arrays with high resolving power, good efficiencies for γγ and γγγ coincidence events,
along with additional measurements of excitation functions from in-beam and off-beam measurements yielding
consistent results, have been utilized in our attempt to resolve the ambiguity.
The γ-rays produced were detected by the Indian National Gamma Array (INGA) consisting of 18 Compton
suppressed Clover Germanium detectors placed around the target[11] at the INGA-HYRA beam line of the IUAC,
New Delhi. Four Clover detectors were placed at 57◦, six at 90◦, four at 123◦, and four at 148◦for facilitating
measurement of directional correlation from oriented states (DCO) ratios. The linear signals, along with the anti-
coincidence logic and the trigger signals, were processed through the indigenously developed Clover electronics
modules dedicated for the INGA set-up[12]. Since the measurement of half lives of the isomeric states is crucial for
2

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Figure 2: Excitation functions measured by tagging on the strong gamma transitions.
the experiment, time to digital converters (TDC) were used with stop signals from the individual Clover units and
common start signal from the master trigger which can be chosen to select 2-fold or higher fold events. Range of
the TDC was set to 400 ns for the exclusion of delayed γ-rays possibly coming from α- and β decays. Altogether
315×106two-fold and 48×106three-fold coincidences were recorded in ∼50 hours at 100 MeV beam energy, and
∼ 20% of above numbers were obtained at 88 and 94 MeV. In order to identify and measure the yield of the ER
nuclei from their known α− and β−decay modes, data were taken in the multiscaling mode during the beam-off
condition between the runs. All the on-line and off-line data were collected using the CAMAC-based CANDLE data
acquisition system[13], and were analysed off-line using CANDLE, INGASORT[14] and RADWARE[15] analysis
softwares.
Data analyses were done in several steps. For the excitation function measurements, γγ coincidence matrices
were generated from the 2-fold data collected at the three different energies. Yields for the intense γ-rays, normalized
by the current integrator readings recorded at the beam dump during the excitation function runs, were obtained
from the Francium Kαand Kβ x-ray gated projections of the symmetrized matrices for each energy. The gated
spectra are shown in the Fig. 1, where the intense γ-rays of208Fr and209Fr are indicated. The 724 keV γ-ray is
present in both the nuclei, and hence could not be resolved in the experiment. Since its intensity is stronger in
the decay of209Fr levels, the excitation function shows the energy dependence typical of209Fr. The trends of the
excitation functions graphs for208Fr and209Fr, shown in the Fig. 2 are comparable to that predicted from the
PACE calculations, though it overestimates the208Fr yield and underestimates the209Fr yield at 100 MeV. The
210Fr yield could not be measured even at the two lower energies as the corresponding characteristic γ-rays are not
known.
The relative yields of the Fr isotopes (A = 208 − 210) at the three different energies were also obtained from
the radioactive decay runs taken in the beam-off condition. The time stamps in the list data blocks were used to
generate the time marker. The Eγ vs. time matrices were generated using the CANDLE analysis software from
which, the Eγ spectra at different time windows were generated. The decay curves for the intense characteristic
γ−rays belonging to the decay branches of these nuclei were obtained. A few decay curves are shown in the Fig. 3.
The ERs decay via their well known α− and β−decay branches, with half lives which are well documented for
these nuclei. The half lives T1/2, obtained by fitting the equation: n = n0exp(−0.693t/T1/2) for different decay
branches observed in this experiment, are enlisted in the Table 1. The results show reasonably good agreement with
the reference values quoted in the NNDC database[16, 17, 18, 19, 20, 21]. Relative yields of different ERs at the
beginning of beam-off runs, were extracted from the fitted n0values, measured half lives and the branching fractions
of the different decay branches. The relative yields, obtained from our data for the Fr isotopes, are shown in the
Table 1. The major contribution to the uncertainties (? 5%) quoted in the relative yields is from the fluctuation
3

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Figure 3: Offline decay plots of various isotopes used for extracting the yield ratio of the Francium evaporation residues.
in the beam current before the beginning of the beam-off runs, which were monitored during the analysis, and the
rest from the exponential fit to the data. The results are in reasonable agreement with the measured excitation
functions of Fig. 2, which agrees with the assignment of the 826 keV isomeric level and the corresponding gamma
transitions to208Fr, rather than to209Fr[2], in agreement with the conclusions of Refs. [3, 8].
From the online data taken at 100 MeV beam energy, the γ − γ matrices, Francium X-ray gated γγ matrices,
the prompt and the delayed γγ matrices and the γ-gated γ∆T matrices were constructed for establishing the
level scheme and resolving the isomeric transitions. From the first two sets of matrices and by gating on the
strongest 632 keV ground state transition and the intense 194 keV transition, the γ-transitions belonging to208Fr
were clearly established. These transitions, observed at 100 MeV beam energy, are shown in the Table 2. The
relative intensities given in the table were obtained from the 632 keV gated spectra. Because of the existence of
low lying isomer with half lives ∼ 200− 400 ns, and also due to the large internal conversion of some of the levels,
the intensity balance across the isomeric levels could only approximately be done. The observed γ-rays and their
relative intensities match reasonably well with those obtained recently by Dracoulis et al.[8]. However, quite a
few additional γ-transitions were observed and indicated in the table. A directional correlation of oriented nuclei
Table 1: Half lives and relative yields of Fr isotopes from the off-line decay runs.
the branching fractions. The characteristic γ-transitions Eγ (col. 5), monitored for decay analysis, are the strongest lines in the
corresponding daughter nuclei. The yields given are the extracted relative yields at each beam energy.
ER Decay branchT1/2
T1/2
(This expt)
208Fr59.1(3) sec[16]63(20) sec
208Fr →204At (89%)
204At →204Po (96%)9.2(2) min[17]9.6(2) min
209Fr50.0(3) sec[18]
209Fr →205At (89%)
205At →205Po (90%)26.9(8) min[19]33.8(2) min
210Fr3.18(6) min[20] 3.4(2) min
210Fr →206At (60%)
206At →206Po (99%) 30.0(8) min[21]29.9(6) min
Percent figures within brackets in col. 2 are
Eγ
Yield
(88 MeV)
Yield
(94 MeV)
Yield
(keV)
635.8
(100 MeV)
208Fr →208Rn (11%)
516.300.14(1)0.46(8)
209Fr →209Rn (11%)
719.3
643.8
1.0(2)1.00(5)1.0(3)
210Fr →210Rn (40%)
700.7 0.35(5)0.18(1)0.37(7)
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Table 2: Intensities and DCO ratios of the γ-rays of208Fr at 100 MeV beam energy. Intensities are normalized relative to the 632 keV
ground state transition. Newly found γ-transitions in this experiment are indicated bya. For the DCO ratios, the gating transitions
are indicated in col. 6. Multipolarity of the gating transitions are indicated in brackets and the multipolarity assignments are shown
in col. 7.
Eγ
Iγ
Ei
Ef
RDCO
(keV) (keV)(keV)
110.0(20)a
11.00(45)40773967
115.8(20)a
7.58(31)39673851
122.7(18)a
17.9(13)38513728
123.2(14)a
8.6(10)22622139
150.2(17)a
8.3(17)38563706
156.4(17)a
13.4(18)21391983
160.1(17)9.9(25)407039101.26(59)
181.8(18)a
11.6(19)391037281.20(39)
194.1(18) 818(12)8266320.99(6)
209.7(19)a
8.70(79)27632553
∼ 0.5
232.4(18)15.9(13)432540931.93(80)
238.2(18)38.8(16)40933855 1.74(32)
268.5(14)a
15.4(20) 19831715
284.6(15)a
5.0(16)30482763
∼ 0.5
288.1(18)11.39(4) 22461958
∼ 0.5
290.9(15)a
3.8(13)2553 2262
∼ 0.5
298.8(19)17.1(28) 21331834 1.04(13)
303.1(19)26.8(35)15121209 2.1(6)
313.1(17)a
3.9(12)37283415
∼ 0.5
321.8(22) 24.8(12)18341512 1.02(18)
359.3(21) 67.6(41) 171513561.03(21)
365.4(16)a
4.5(12) 26332268
382.9(18)a
7.13(90)1209826 0.60(17)
404.8(19)64.9(39) 18001395 0.92(12)
410.7(13)a
8.45(76)34153004
∼ 0.5
428.3(15)a
7.0(13)226218340.50(34)
467.8(21)a
9.61(41)226818000.8(4)405(M1/E1)
498.6(13)a
4.5(12)37063207
510.2(22)40.6(33)31902680
∼ 2
563.5(24)88.4(45)195813951.85(44)
566.2(24)a
27.1(38)548749210.44(17)
569.4(24)197.7(62)13958260.89(9)
596.2(24)a
38.5(47) 492143252.0(8)
603.6(26)a
12.3(26)32072603
632.2(19)1000 6320
645.3(19)a
8.9(31)26031958 0.6(4)
665.1(20)a
11.7(47)385531901.05(37)
721.6(23)a
21.0(38) 268019581.0(6)
723.9(23) 160.30(46)13566321.14(11)
742.0(23)32.2(13)300422620.56(23)
774.6(18)5.28(2)21691395
∼ 1
880.2(16)7.26(2)26801800
∼ 1
955.8(16)a
3.6(11)400430480.49(24)
Eγ(gate)
(keV)
Multipolarity
303(M1)
303(M1)
632(E2)
303(M1)
569(E2)
569(E2)
M1
M1
E1 (9−→ 9+)
E2
M1
M1
303(M1)
569(E2)
303(M1)
303(M1)
632(E2)
303(M1)
303(M1)
632(E2)
E2
M1
E2
M1
M1
(E2)
M1
E2
322(M1)
569(E2)
303(M1)
303(M1)
E2
M1/E1 (11 → 11)
(E2)
E2
M1/E1 (11 → 12)
569(E2)
632(E2)
563(M1)
632(E2)
632(E2)
M1
M1
E2
E2
M1
E2
(E1/M1) (12 → 12)
M1
M1
E2
E2
(E2)
(E2)
(E2)
563 (M1)
563(M1)
563(M1)
632(E2)
303(M1)
569(E2)
569(E2)
303(M1)
5

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Figure 4: Level scheme of208Fr obtained from the present work. Transitions which were not observed but necessary to fit the level
scheme are indicated by ?. The dotted transitions are observed in the previous work[8], but not in this work.
(DCO) analysis was also performed with the data taken at (90◦,148◦) and (90◦,123◦) angle pairs. The gating
transitions and their multipolarities used for DCO ratio calculations are shown in the Table 2. These assignments
match with those obtained in Ref. [8].
Based on the intensity correlations obtained from our gated spectra, and also from the DCO ratio measurements,
the level scheme for208Fr was established as shown in the Figure 4. Apart from one sequence of transitions (I)
passing through 359 and 724 keV, which directly feeds the 632 keV first excited state, two major sequences (III
and V) and three minor sequences (II, IV and VI) of transitions, which pass through the isomeric 826 keV level,
have been observed. A few relevant gated spectra are shown in the Figs. 5 and 6. Out of these, three sequences (II,
III and IV) of transitions pass through the strong 569 keV transition, and two (V and VI) through the sequence of
303 and 322 keV transitions. In all the major sequences, ordering of the transitions are cross-checked by intensity
correlations and also by reverse gating.
About 25 new transitions, over and above those observed by Dracoulis et al.[8], were found, as noted in the
Table 2. The 722 keV transition in sequence III falls on the tail of the much stronger 724 keV transition of sequence
I, and could only be observed in the 238 and 569 keV gates, as shown in the Fig. 6, except for a small contamination
at 725 keV in the 569 keV gate, possibly coming from198Au produced by neutron transfer reaction. The 725 keV
contamination was also present in the 194 keV gated spectrum for the same reason mentioned above. The 724
keV contamination line was not found in the 563 keV gate. The contamination from the 724 keV transition is
much more severe as it also belongs to209Fr. Similarly, the 510 keV γ-ray is also a new line, which was observed
consistently in the coincident gates of sequence III transitions. It was placed in the level scheme just above the
722 keV transition, based on the observed intensities in various forward and reverse gates. The 665-510-722 keV
sequence in III is bypassed by the 150-499-604-645 keV sequence II transitions which is evident from the 645 keV
gated spectrum of Fig. 5 b. The 665 keV and the 150 keV transitions are weak in intensity and allow only an
approximate intensity balance. The sequence I was known up to 359 keV[8], which was extended with the 123-156-
268 keV transitions observed by overlapping the 632,724 and 359 keV gated spectra (see Fig. 5a). The sequence
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Figure 5: (a) Overlap of the 359, 632 and 724 keV gated spectra manifesting the sequence I transitions above the 1750 keV level. (b)
The 645 keV gated spectrum for the sequence II transitions.
IV transitions pass through 569 keV but they bypass the 563 keV and 722 keV transitions and hence were placed
accordingly. The 880 keV transition, which was also shown in Ref. [8], was absent in 365 and 468 keV gates, but
present in the 405 keV gate. Hence it was placed to directly feed the 1800 keV level. By matching the sum energies
of the relevant transitions in IV, a 47 keV transition was placed just below the 2680 keV level. This, perhaps, could
not be identified due to the absorbers placed in front of the Clover detectors.
For the sequence V and VI, several new transitions were found and the relative ordering was modified over
the level scheme shown in Ref. [8]. 210, 284, 291 and 956 keV lines were observed for the first time in the 299,
303 and 322 keV gates (see Fig. 6a), but absent from the 742 keV gate. This makes the placement of 742 keV
above the 299-303-322 keV sequence. Further, the 428 keV line was present in the 303, 322 and 742 keV gates but
absent in the 299 keV gate. This was also confirmed by reverse gating on 428 keV. The 129 keV line, though weak
possibly because of internal conversion, was present in 299, 303 and 322 keV gates but absent in the 428 keV gate,
which makes its placement sequential to 299 keV but parallel to 428 keV. The γ-rays coming from the levels above
3004 keV in sequence V were observed for the first time, except for the 160 keV transition which has already been
reported[8]. A new 383 keV feeding transition to 826 keV level along the sequence V was found. Justification for
its placement is given below.
One of the major point of controversy is the life time of the 826 keV isomeric level, which has been measured
by several workers[2, 3, 8], and the existence of other isomers in208Fr. A systematic search for isomeric transitions
were done from our data and the half lives were extracted. The ∆T spectral measurements covered a ±400 ns
TDC range, but an useful range of ∼ 500 ns could be utilized due to delay matching of the detector array. Typical
∆T spectra gated by 632 keV are shown in the Fig. 7. From the 194 keV and 632 keV γ-gated γ∆T coincidence
matrices, we have projected the gated ∆T spectra for the 569 keV, 563 keV, 299 keV, 303 keV, 322 keV and
742 keV gated transitions. The ∆T spectra for the first two gated transitions are found to be similar in nature.
This clearly establishes the fact that 563 and 569 keV transitions are in sequence and above the 826 keV isomeric
level. The combined ∆T spectrum for the 563 + 569 keV as start and 632 keV as stop are shown in the Fig. 7(a).
However, the ∆T spectra for the 299, 303, 322 and 742 keV as start and 194 or 632 keV as stop, though similar in
nature among themselves, differ significantly from the previous ones (see Fig. 7(a,d)) in that the exponential decay
is much faster indicating the existence of another faster isomeric transition above the 826 keV level. The gated
∆T spectra obtained for the gates between the pairs of γ-rays among the 299, 303, 322 and 742 keV, the 563 and
569 keV pair of γ-rays showed their prompt nature. By comparing the prompt and delayed gated spectra, taken
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Figure 6: A few relevant gated spectra of208Fr. Gating transitions are indicated in the figures. Contamination lines are indicated by
∗.Plots in (c) to (e) are zoomed plots of the spectra shown in (f), to bring out the weak transitions noted in the experiment.
for the sequence V transitions mentioned above, a new but weak 383 keV isomeric transition was found and it was
placed just above the 826 keV level along the sequence V.
The half life of the 826 keV isomeric level was extracted by fitting exponential decay function to the ∆T spectra,
shown in the Fig. 7(c). The results are given in the Table 3. Half life of 233±18 ns was obtained, which is consistent
with the result ∼ 200 ns quoted in Ref. [3]. However, the result differs from that obtained recently by Dracoulis
et al.[8]. Half life of the 1209 keV isomeric level was extracted from the ∆T spectra of Fig. 7(b), in a similar way.
The result obtained is: 27.6±3.4 ns. The same half life was extracted independently from the ∆T spectrum shown
in Fig. 7(d), using two exponential decay fit and the measured half life of the 826 keV isomer mentioned above.
The results were mutually consistent and the average of the two measurements is quoted in Table 3.
3. Interpretation of results
The difference in the measured values of the 826 keV isomer half life could not be explained. One of the draw
back of the present measurement is the restricted range of ∆T. This could not be avoided in the experimental
set up of the INGA array, which was optimized for prompt γ-spectroscopy. To find out the correctness of the
present technique, half lives have been extracted for a number of isomeric transitions already known in several
nuclei produced in our in-beam experiment. These are listed in the Table 3. The isomer half lives spanned from
∼ 150 ns to ∼ 600 ns. The results are found to be in good agreement with the earlier measurements, within the
quoted uncertainties. The half life for206At was also reported in Ref. [8] as 813 ± 21 ns, which is almost twice
that obtained earlier[23]. While the probable cause of this difference was cited as due to the restricted TDC range
±300 ns in the previous work, our result does not seem to agree with this conclusion. We get results which are
consistent with earlier measurements, though the uncertainties are larger in case of larger half lives (eg.208Rn[22]).
The restricted range of TDC would result in poor statistics for fitting data, with consequent larger error bars, as
manifested in our results.
The results of DCO ratio measurements are enlisted in the Table 2. These measurements are pivoted on the E2
assignment of the 632 keV ground state transition, which follows from the systematics in the neighbouring nuclei,
including Francium isotopes. Based on our DCO results, ∆J = 0 E1 character was assigned to the 194 keV isomeric
8

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Figure 7: Different ∆T spectra used to search for and measure the half lives of the isomeric transitions in208Fr.
Table 3: Half lives of isomeric levels of different nuclei produced in the experiment.
MLT1/2(ns)T1/2(ns)
(keV) (This expt)(Earlier)
194.1E1233(18)432(11)[8]
∼ 200[3]
382.9E233(7)
88.7E2590(144) 509(14)[22]
121.6E1 377(44)410(80)[23]
813(21)[8]
12.1E2161(4) 158(2)[24]
Nucl. Level
(keV)
826
Eγ
208Fr
208Fr
208Rn
206At
1209
1828
807
204Po1639
9

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transition. The same transition was assigned E1(10−→ 9+) in Ref. [8]. However, both the Jπassignments arise
from the same π(1h9/2)5⊗ ν(1i13/2)−1configuration, with the only difference that the 9−isomeric level would be
placed higher than a 10−state, as evident from the spin multiplets observed in208Bi from the neutron pick-up
reactions[25]. The 11−state, arising out of the same configuration, is assigned to the next 1209 keV level, which
is also an isomeric level decaying by 383 keV E2 transition. Based on the measured half life and the estimated
internal conversion coefficient of 0.0723(11)[26], single particle strength of 0.0264(12) W. u. was obtained. For the
isomeric transition of the 826 keV 9−level, internal conversion coefficient of 0.0947(14) was estimated, and the
corresponding single particle strength was obtained as 1.06(8)×10−7W. u. Similar results for the E1, M1, E2 and
M2 isomeric transitions are obtained in trans-Lead nuclei(eg.208Rn[22]) in this mass region.
In the level scheme shown in Fig. 4, the series of levels from 632 keV (7+) to 1983 keV (15+) along sequence
I arise from the π(1h9/2)5⊗ ν(2f5/2)−1with proton excitations of higher seniority leading to the generation of
angular momenta. This is clear from the connecting sequence of E2 transitions. The maximum angular momentum
generated from proton excitation in this case is 25/2, which leads to maximum Jπ= 15+for the given configuration.
The 123 and 156 keV transitions above the 1983 keV level could be of E2 nature, though our data is inadequate
to make a definite conclusion.
The sequence II transitions, observed for the first time, are weak in intensity but they are fitted into the level
scheme from the matched sequence of γ-ray energies, coincidence conditions and intensities. However, the spin
assignments of only the 2603 keV level could be done from the DCO ratio for the 645 keV transition by gating
on 563 keV M1 transition. This could be a stretched E2 transition or a ∆J = 0 M1 transition, but the latter
assignment was adopted to fix the spins of the level sequence. The 604 keV, 499 keV and 150 keV transitions
appear to be M1 though it cannot be confirmed from our data. Sequence III transitions extend to the highest
excitation energy of208Fr in this experiment. It starts from the 1395 keV 11−level which is connected by 569
keV E2 transition to the 826 keV 9−level. This sequence is likely to arise from the π(1h9/2)4(2f7/2)⊗ν(1i13/2)−1
configuration which leads to the highest spin of 22−, and are connected by a series of M1 transitions. Though we
could extend up to 20−level, the absence of 19−level is possibly because of the fact that it is pushed down by the
residual proton particle neutron hole repulsive interaction, from where gamma transitions are hindered. A detailed
shell model calculation will be needed to make a definite conclusion in this regard.
The low lying transitions of the sequences V and VI, extending up to 2763 keV J = 19 level can be formed
by π(1h9/2)5⊗ν(1i13/2)−1configuration through proton excitation. However, levels above 19−along sequence VI,
built on proton excitation to the maximum proton spin of 25/2, are due to neutron hole excitation arising from
ν(2f5/2−21i−1
keV 15−level along the sequence V can be between levels generated by π(1h9/241i13/2) ⊗ ν(2f5/2)−1, which leads
to a maximum Jπ= 21−. The levels above it are connected by M1 transitions which are probably generated by a
different configuration. A better statistics and / or higher resolving power of the array will be needed to extend
the level scheme further.
13/2), leading up to the maximum Jπ= 23−level observed. The series of E2 transitions above the 2262
4. Conclusion
The level scheme of208Fr was modified over the existing level scheme, and extended up to ∼ 23? and ∼ 5.5 MeV
excitation energy using a high resolving power Clover detector array. A number of new γ-transitions were observed
and their DCO ratios were measured. Based on search for isomeric transitions from the data using the tagged γγ
time difference technique, the half lives of several isomeric transitions in208Fr and in a few neighbouring nuclei
produced as ER in the experiment were measured. The results agree reasonably well with the previously known
half lives. The half life of the known 194 keV isomeric transition in208Fr was found to differ from the previously
reported value. A new isomeric E2 transition was obtained and its half life was measured. The Weisskopf estimate
of the single particle strength of the associated isomeric levels reveal similarity with the previous estimates in the
neighbouring nuclei. From the shell model based interpretation of the level scheme, it is clear that the majority
of the excited states are caused by 1h9/2proton excitations, and neutron hole excitations predominantly in 2f5/2
and 1i13/2shells. The importance of 1i13/2neutron hole in generating the isomeric levels is clear from the present
data. It may be noted that though we have observed only two isomer levels, there can be a few more such levels
which could not be observed due to limted statistics in our data. A pulsed beam based experiment, coupled with
such a high resolving power array will be needed to extend the study further.
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5. Acknowledgement
We are grateful to all the colleagues of the INGA collaboration for their help during the experiment. Smooth
running of the 16UD Pelletron accelerator and the INGA detector array at the IUAC, New Delhi by the staff
therein are gratefully acknowledged.
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