68 Measuring β Lifetimes with the Michigan State University Recoil Product Mass Separator
Downloaded via UNIV OF ROCHESTER on August 2, 2018 at 09:20:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
L. H. Harwood, D. Mikolas, J. A. Nolen, Jr., B. Sherrill, J. Stevenson, and Z. Q. Xie National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 Beta-decay half-lives were measured for eight neutron rich isotopes produced by fragmentation ofE/A=30MeV Oions.The first measurements of the half -lives of Be(4.2±0.7 ms.)and C(202±17 ms.) have been made along with the half-lives of Li, Li, Be, B, B. The lifetime of Be is the shortest known beta lifetime.This is the first experiment to use the MSU Reaction Product Mass Separator 18
14
17
9
11
12
14
15
14
We report here measurements of the beta-decay half-lives of eight neutron rich isotopes.This experiment was the first to be performed using the Reaction Product Mass Separator(RPMS)at Michigan State University.The MSU RPMS is designed to separate exotic nuclei produced in intermediate energy heavy-ion collisions. The RPMS separates fragments of different m/q at the focal plane making it possible to study neutron rich isotopes with m/q=3 without contamination from stability line m/q=2 nuclei which are 10 times more abundent.The RPMS,shown in figure 1,is a triple focusing device(in x,y,and velocity) with dispersion in m/q. 6
Fig.1. Pictorial drawing of the RPMS. The beam inflector magnet is the first component at the left,followed by the bellows system and target chamber,aperature holder, quadrupole doublet,Wein filter(5 m. long),magnetic dipole and pivoting "tail" with quadrupole doublet and focal plane detectors at the right end. The RPMS is designed[No184 ] to separate heavy ion reaction products with energies in the range E/A=10-100 MeV. By eliminating interfering m/q species with defining slits and concentrating the isotopes under investigation into small detectors the RPMS provides a clean environment for decay studies. Using a 30 MeV/A 0 beam we made the first measurements of the lifetimes of **Be and C along with the half-lives of Li , L i , Be, B,and B during a single 48 hour run.Beta lifetimes were measured by.measuring the time interval between detection of a heavy ion and its subsequent beta decay. 18
1
l7
9
1l
1 2
l5
0097-6156/ 86/ 0324-0454$06.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
1 2
68.
HARWOOD ET AL.
455
Measunng β Lifetimes
T h i s t e c h n i q u e has b e e n p r e v i o u s l y d e m o n s t r a t e d by a g r o u p a t LBL[Mur82], B e r y l l i u m and tantalum targets were used with thicknesses chosen to stop the 30 MeV/A 0 beam but allow lower charge n e u t r o n r i c h fragments to e x i t the target i n t o the RPMS.The tantalum target was found to give better y i e l d s of neutron r i c h isotopes and was used f o r the measurements. The f o c a l p l a n e d e t e c t o r c o n s i s t e d of a two d i m e n s i o n a l p o s i t i o n s e n s i t i v e proportional counter followed by a s i l i c o n ΔΕ-Ε t e l e s c o p e u s i n g a 300 mm x100ym.(Si)AE and 500mm x5mm.(Si-Li)E detectors. Lifetime measurements were performed by turning the beam o f f whenever an ion with z£3 was d e t e c t e d i n the t e l e s c o p e .The beam was l e f t o f f f o r a preset time equal to several h a l f - l i v e s of the longest l i v e d isotope being c o l l e c t e d , a n d a l l b e t a decays i n the i n t e r v a l were recorded.The beam i s t u r n e d o f f i n about 40ys. by changing the phase of the r f on one of the dees of the K500 cyclotron.Isotope i d e n t i f i c a t i o n was acomplished by p l o t t i n g ( F i g . 2.) the d e f l e c t i o n i n the RPMS (proportional to m/q)vs the s i l i c o n t e l e s c o p e p a r t i c l e i d e n t i f i c a t i o n function. le
2
2
m/q
PARTICLE ID
PARAMETER—>
F i g . 2 . P l o t of m/q determined from d e f l e c t i o n i n the RPMS vs detector p a r t i c l e i d e n t i f i c a t i o n parameter.
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
silicon
456
NUCLEI OFF THE LINE OF STABILITY 6
For m/q»3 data He i s a l o n g l i v e d beta background which i s eliminated by s e t t i n g the beta t h r e s h o l d at 2 . 5 MeV. He i s eliminated from the m/q»3.5 d a t a by s t o p p i n g them i n f r o n t of the f o c a l plane d e t e c t o r s w i t h b r a s s defining s l i t s . The decay curves are shown i n f i g u r e 3.The curves are best f i t s to an exponential plus a constant background. 6
MSU-85-354 μ
ι
I
,
,
I
I
ι
.
I
j
TIME (msec) Figure 3. Beta l i f e t i m e p l o t s , t h e curves exponential plus constant background form.
are best f i t s
to an
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
68.
Measuring β Lifetimes
HARWOOD ET AL.
l
457
1 2
The a n a l y s i s of **Be a n d B e i s c o m p l i c a t e d by being rather shortlived.For these two isotopes events second beta w i t h i n two daughter h a l f - l i v e s i n o r d e r l i f e t i m e spectrum. The r e s u l t s of t h i s experiment are compared with i n t a b l e I. .
Table I
the daughter n u c l e u s were required to have a t o be i n c l u d e d i n the p r e v i o u s measurements
L i f e t i m e measurements and p r e d i c t i o n s This
Previous f
Isotope
9
Li
1 l
12
ms
173±14
7.7±0.6 ms
Li Be
l 4
21,3±2.2 ms
Be
Measurement
175±1 8.5±0.2
a
ms
24.4±3.0 ms
b
4.2±0.7 ms 20.41±.06 ms
B
12.8±0.8 ms
16.1±1 .2 ms
1 7
76 ms
a
20.0±1.5 ms
1 5
Prediction
ms
B
1 2
U
Measurement
B
8.8±0.6 ms
c
202±17
11.0±1.0 ms
c·
2.3
ms
8.8
ms
3.7
ms
13.6 ms
d e
11.1
ms
6.2
ms
(4l4,292,238)ms
ms
Beta-decay h a l f - l i v e s for the neutron-rich i s o t o p e s measured experiment
are
compared
w i t h p r e v i o u s measurements when they
T h e o r e t i c a l p r e d i c t i o n s for Gamow-Teller beta decay
are a l s o
in this exist.
provided for
17 comparison. assumption
The
three t h e o r e t i c a l h a l f - l i v e s
of (1/2*,
+
3/2 ,
5/2*)
ground-state J
for 1 1
C correspond to the
values for
1 7
C
calculation.
a) [Roe74] b) [Alb78] c) [Ajz75]
d) [Alb74] e) [Duf] f ) [Bro85]
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
i n the
458
NUCLEI OFF THE LINE OF STABILITY
There are two cases of discrepancies outside of one s t a n d a r d d e v i a t i o n uncertainties B and B e . In both cases t h e r e was o n l y one p r e v i o u s measurement and t h e r e i s no o b v i o u s reason f o r the d i s c r e p a n c i e s . l t i s p o s s i b l e t h a t the background under the **B decay curve of ref.[Alb74] was underestimated l e a d i n g to the larger value of the e x t r a c t e d l i f e t i m e . I n the case o f the B l i f e t i m e of r e f .[Duf] no decay curve was shown so i t i s hard to compare q u a l i t y of f i t s . For the n u c l e i s t u d i e d i n t h i s experiment the dominant decay mode i s Gamow-Teller beta decay.Partial h a l f l i v e s corresponding t o Gamow-Teller beta decay c a l c u l a t e d i n a s p h e r i c a l s h e l l model formalism [Bro85] are shown i n Table I.The c a l c u l a t e d l i f e t i m e s are with one e x c e p t i o n ( C ) a l l shorter than the measured l i f e t i m e s . In conclusion thre f i r s t h a l f - l i f e measurements of l i g h t neutron r i c h n u c l e i u s i n g the MSU R e a c t i o n Product Mass S e p a r a t o r has r e s u l t e d i n the measurement o f e i g h t h a l f - 1 i v e s , t w o o f w h i c h r e p r e s e n t f i r s t time measurements and three of which are second measurements.The RPMS coupled with f a s t beam switching has proven to provide a very clean environment i n which to study the decays of neutron r i c h n u c l e i . 1 W
I 5
1
l 5
17
References [Ajz75] F.Ajzenberg-Selove,Nucl. Phys. A248,1(1975). [Alb74] D.E. Alburger and D.R. Goosman,Phys. Rev. C10,912(1974). [Alb78]D.E. Alburger,et. al.,Phys. Rev. C17,1525(1978). [Bro85]B.A. Brown private communication [Duf]J.P.Dufour,et.al.,Preprint CENBG 8430. [Mur82]M.J. Murphy,Τ.J.M. Symons,G.D. Westfall and H . J . Crawford,Phys.Rev.Lett.49,455(1982). [Nol84]J.A.Nolen,Jr.,L.H.Harwood,M.S. Curtin,Ε. Ormand and S. Bricker,Proceedings of the Conference on Instrumentation for Heavy-Ion Nuclear Research,Oak Ridge National Laboratory,Oct. 1984,to be published. [Roc74]E.Roeckl,P.F. Dittner,C.Detraz,R.Klapisch,C.Thibault,and C.Rigaud,Phys. Rev. C10,1181(1974). RECEIVED July 15, 1986
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
