NUCLEAR SPIN POLARIZATION OF BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF TILTED FOIL TECHNIQUE

Abstract

Le mécanisme fondamental de la technique des feuilles inclinées a été étudié expérimentalement pour ses applications aux émetteurs β de courte vie. Le gain de polarisation nucléaire du 12B par des feuilles multiples a été mesuré en fonction du nombre et de l'espacement des feuilles. La technique a ensuite été appliquée à des émetteurs β plus lourds 20F et 41Sc. The fundamental mechanism of the tilted foil technique has been experimentally studied for its effective application to short-lived beta-emitters. The multi-foil enhancement in 12B nuclear polarization was measured as a function of the foil number and the foil spacing. The technique was further applied to heavier beta-emitters, 20F and 41Sc.

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COLLOQUE DE PHYSIQUE
Colloque C6, supplgment au n022, Tome 51, 15 novembre 1990
NUCLEAR SPIN POLARIZATION OF BETA-RADIOACTIVE NUCLEI CREATED BY MEANS OF
TILTED FOIL TECHNIQUE
Y . NOJIRI, S . MOMOTA, A. OZAWA, A. KITAGAWA, M . FUKUDA, K. MATSUTA
and T. MINAMISONO
Department of Physics, Osaka University Toyonaka, Osaka 560, Japan
Le mdcanisme fondamental de la technique des feuilles inclinees a BtB Btudid
expdrimentalement pour ses applications aux 'Bmetteurs P de courte vie. Le
gain de polarisation nuclBaire du "B par des feuilles multiples a 6 t 6 mesurB
en fonction du nombre et de l'espacement des feuilles. La technique a ensuite
dt6 appliqude B des Bmetteurs 6 plus lourds ' O F et 41~c.
ustract - The fundamental mechanism of the tilted foil technique has been
experimentally studied for its effective application to short-lived beta-
emitters. The multi-foil enhancement in 12B nuclear polarization was
measured as a function of the foil number and the foil spacing. The
technique was further applied to heavier beta-emitters, 2 0 ~
and 41~c.
Since the first discovery of atomic polarization produced through beam-tilted
foil interactions/l/, the tilted foil technique has been applied successfully in
a series of various measurements on nuclear physics/2/. In fact, the technique
has proved its effectiveness for experimental studies on short-lived radioactive
nuclei, particularly in NMR measurements on beta-radioactive nuclei,
27~i (~1/2=4.2 sec) /3/ and 3 3 ~ 1 ( ~ 1 1 2 ' 2 . 5 1 sec) /4/. In these experiments, it has
played a very crucial role to create nuclear polarizations of the beta emitters
produced through nuclear reactions. In order to expand its applicability to a
wide range of beta-emitters, the technique was recently applied to the creation
of polarized projectile fragments, 39~a
(T1i2=0. 86 sec) and 4 3 ~ i
produced in high-energy heavy-ion collisions/5,6/.
Essential mechanisms constituting the technique are 1) atomic polarization
creation through beam-tilted foil interactions and 2) the atomic polarization
transfer to nuclei through possible hyperfine interactions. The mechanism 1)
has been well studied experimentally through measurement of atomic polarization,
and it is concluded from the studies that the polarization creation is resulted
from asymmetric surface collisions between outgoing atoms and and lattice ones.
For the mechanism 2), quantum oscillation of atomic polarization was confirmed.
Direct evidence of the creation of the nuclear polarization by the mechanisfi has
been shown in the measurement of the nuclear polarization in short lived beta
emitter 12~/7/. Enhancement in the nuclear polarization by use of multi-foil
stack was also confirmed in Ref. 7.
Although the mechanism of the polarization transfer and the multi-foil
enhancement have been proposed theoretically, it should be confirmed
experimentally, in order to use the technique effectively for the wide range of
application. In particular, we should know more about the optimum condition for
the inter-foil spacing of the multi-foil stack, which is defined as the distance
between foils in the direction of beam path, and the foil number, in addition to
the tilt angle.
In the present experiment, the fundamental mechanism in the polarization
enhancement achieved by use of multi-tilted foils was studied. As a proof of
the proposed polarization mechanism, the multi-foil enhancement effect of
nuclear polarization in short-lived beta-emitter l2B(1& l+, T1/2= 20 msec) was
measured as a function of distances between two foils and also of numbers of
multi-foils. In the following, the technique was further applied to heavier
(T1,2=0. 51 sec)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990670

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C6-550
COLLOQUE DE PHYSIQUE
short-lived beta emitters, 2 0 ~
T1/2= 596.3 (17) msec) , t o prove the effectiveness of the technique.
(I& 2+, T1/2= 11.03 (6) sec) and 4 1 ~ c
(I*= 7/2-,
The essential part of the present measurement is similar t o the previous
one/8/. The experimental setup is shown in Fig. 1. Beta emitters, 1 2 ~ , 2 0 ~ 1
and 4 1 ~ ~ ,
were produced through the nuclear reactions 1 1 ~
and 4 0 ~ a (d, n) 41~c, respectively. Deuteron beams (Ed- 4 MeV) were extracted from
the Van de Graaff accelerator a t Osaka University.
the target into the recoil angle 3 0 ' were selected by a collimator. Nuclear
polarization was induced in the nuclei by passing through a multi-foil stack.
The stack consists of 1-6 carbon foils of 5 pg/cm2 thick tilted 60' relative t o
the beam direction. The polarized nuclei were focussed by use of an
electrostatic lens and implanted into a catcher.
Oe) was used t o maintain the atomic polarization during the flight of the recoil
nuclei while a strong magnetic field (- 2 kOe) was applied t o preserve the
nuclear spin polarization after the implantation. The stopping materials were a
metallic Pt foil, a single crystal CaF2, and fine granular crystals T i c for 1 2 ~ ,
2 0 ~ , and 41~c, respectively. The nuclear polarization was determined through
measurements of asymmetries in beta-ray emissions from the nuclei by means of
beta-NMR technique.
(d, p) 1 2 ~ ,
19F (d, p) 2 0 ~ 1
Recoil nuclei emerged from
A week magnetic field (- 10
Beam (d) f
Fig. 1. Schematic view of experimental setup.
The nuclear spin polarization of 1 2 ~
a function of inter-foil spacing.
exponential increase as the inter-foil spacing increased as shown in Fig. 2.
The polarization P (X) was well described by a form,
produced with two foils was measured as
The induced nuclear polarization showed an
where X is the inter-foil spacing, PO is a saturated value of induced
polarization, PO-P1 i s the polarization created by single foil, and X0 is a free
distance parameter.
From the least x2 analysis, the distance parameter was

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~nter-Foil Distance (mm)
Foil Number
Fig. 2.
polarization of 12B as a function
of inter-foil spacing.
Enhancement of nuclear Fig. 3. Multi-foil enhancement
of l2B nuclear polarization.
deduced t o be Xo= 1.26(46) mm.
z of t h e polarization from atomic spin t o the nuclear spin, as Xo= vt, where v
is the velocity of 1 2 ~
after passing through a foil. Using the average velocity
v= 3.1 mm/nsec of 1 2 ~ ,
the transfer time was deduced t o be %= 0.41(15) nsec.
O n the other hand, the transfer time Z can be calculated from the known
hyperfine interaction strength. Here, we assume that the atomic polarization
can only be induced in atomic states with non-zero orbital-angular momentum.
Because the boron atoms are populated i n a large part i n the ground state after
passing through foils, the ground state of neutral boron, 2~1/2 is naturally
assumed t o take large part in producing the nuclear polarization.
known hyperfine interaction strength %f-
value for the transfer time was deduced t o be 2-
agreement between the present observation and the calculation shows above
assumption t o be plausible from the view point of fundamental mechanisms of the
polarization production i n the beam-tilted f o i l interaction.
Polarization enhancement of the 12B by use of multi-tilted foils was
observed by changing the f o i l number from 1 t o 6.
inter-foil spacing was kept t o be 1 mm.
as shown in Fig. 3. According t o the discussion in Ref. 10, the data was
analyzed using the relation,
The parameter X0 is related t o the transfer time
From the
1.24 X 109 (l/sec)/9/, the calculated
0.8 nsec.
A good
In t h i s observation, the
The result showed an exponential rise
where P(N) is induced nuclear polarization by use of N foils, and P(-) is the
saturated value of the polarization. From the least x2 analysis, the optimum
value for the parameter k was deduced t o be k= 0.42(7). The parameter k is
related t o the atomic spin J and nuclear spin I as,
k= P(I,J)(PJ+ (1- Pj)J/I),
p (I, J) = (11412) {21+ (h2- 1) ln (I+ h) / (l- 1) 1,
h= ~ I J /
( I ~ +
J ~ ) .
(3)
where PJ is the atomic polarization created a t the exit surface of the foils.
So, the k is determined by the ratio J/I. Considering the nuclear spin 1=1 of
12B, the average atomic spin <J> which effectively participates i n the
polarization creation process was deduced t o be <J>= 0.88(20) from the
experimental value of k. This indicates that the excited state 2 ~ 3 / 2
another contribution t o the polarization creation together with the ground state
2 ~ 1 / 2 of the neutral boron atom.
has

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COLLOQUE DE PHYSIQUE
In the succeeding experiments, the t i l t e d f o i l technique has been further
applied t o heavier beta-emitters, 2 0 ~
and 4 1 ~ ~ .
nuclei were mainly neutral and plus one out of t h e f o i l s i n the present
experimental conditions. The results are shown i n Fig. 4.
with 1 mm inter-foil spacing were used for 20F. The observed polarization for
2 0 ~
induced with t i l t e d f o i l s were 0.7f0.2 %. In the 4
f o r single f o i l . The observed polarization of 4 1 ~ c was 0.8f0.3 %.
f o i l enhancement could not be applied i n the 4
energy.
As a conclusion, t h e mechanism of polarization transfer became rather clear
from present experiment.
Further applications of t h e t i l t e d f o i l technique t o
wider variety of radioactive nuclei are expected t o achieve new developments i n
nuclear physics.
Atomic charge states of both
Six carbon f o i l s
1~~
case, the result is
The multi-
case due t o small kinetic
1~~
b)
1.5 -
Average
1.0
0.5 -
0.0 '
T
123
Runs
Runs
Fig. 4.
technique.
Open circles 0 and an open circle with dot O denote the data with carbon f o i l s
of 5 pg/cm2 thick and 10 pg/cm2 thick, respectively.
averages of the separate runs. Six carbon f o i l s w e r e used f o r 2 0 ~ . In the 4
case, t h e data a r e f o r single carbon f o i l .
Nuclear spin polarization of 2 0 ~
and 41~c
created by the t i l t e d f o i l
Closed circles
denote
1~~
REFERENCES
/l/ Berry, H.G. et a l . , Phys. Rev. Lett. 2 (1974) 751.
/2/ Berry, H.G. and M. Hass, Ann. Rev. Nucl. Part. Sci. 32 (1982) 1.
Nojiri, Y. e t al., J. Phys. Soc. Japan
/3/ Nojiri, Y. et al., J. Phys. Soc. Japan
/4/ Rogers, W.F. et al., Phys. Lett. JJB (1986) 293.
/5/ Nojiri, Y. e t al., Nucl. Inst. Meth. Phys. Res., m (1988) 193.
/6/ Matsuta, K. e t al., Hyperfine Interactions, 1990.
/7/ Nojiri, Y. and Deutch, B.I., Phys. Rev. Lett. a (1983) 180.
/8/ Ozawa, A. e t al., Hyperfine Interactions, 1990.
/9/ Fraga, S. e t al., Hand book of Atomic Data, Elsevier Scientific Pub., 1976.
/10/ Goldring, G. and Niv, Y., Hyperfine Interactions, 21 (1985) 209.
(1986) Suppl. 391.
(1986) Suppl. 1086.