This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Bowman, J.D.

Cain, B.

Jamriska, D.

Knott, J.E.

Komives, A.

Leuschner, M.

Szymanski, J.J.

English

IUScholarWorks

A STUDY OF SINGLE-PARTICLE PARITY-NONCONSERVING 
NUCLEAR MATRIX ELEMENTS 
B. Cain, J.E. Knott, A. Komives, M. Leuschner, and J.J. Szvmanski 
I nd iana  Univers i ty  Cyc lo t ron  Facili ty,  B looming ton ,  I n d i a n a  4 7 4 0 8  
J.D. Bowman and D. Jamriska 
Los A l a m o s  Na t iona l  Laboratory, Los A lamos ,  N e w  Mexico  87545  
An IUCF-LANL-Princeton collaboration has been formed to measure the weak mix- 
ing between single-particle states in 207~b.132 The experiment was originally motivated 
by recent measurements at the Los Alamos Neutron Scattering Center by the TRIPLE 
c~l labora t ion .~ '~  As discussed below, it is now understood that the experiment has the po- 
tential of measuring the isoscaler weak amplitude in n ~ c l e i , ~  in addition to any knowledge 
gained about the neutron-nucleus experiments. 
The interpretation of the data from this experiment is straightforward. Indeed, one 
of the important features of 2 0 7 ~ b  is that it is a good shell-model nucleus. The states of 
2 0 7 ~ b  can be well described as single-particle shell-model states. 
A low-sensitivity (first-generation) version of the 207Pb experiment is sensitive to large 
single-particle weak mixing matrix elements. Large single-particle weak matrix elements 
are an important requirement of several theoretical calculations of the TRIPLE experi- 
mental An improved 207Pb experiment, with sensitivity in the range of a few 
eV, is expected to see a non-zero parity non-conserving (PNC) e f f e ~ t . ~  An experiment with 
this sensitivity will be able to measure the isoscaler weak amplitude in nuclei. 
The weak interaction of two nucleons in a nucleus is described by two dominant am- 
plitudes, isoscalar and isovector, which correspond to rho-exchange and pion-exchange 
terms. The experimental limits on these amplitudes are shown in Fig. 1 (see Ref. 12). 
Figure 1. Plotted are the ranges in h: (isoscalar) - f, (isovector) space allowed by various 
nuclear parity violation experiments (see Ref. 9). 
Bi (Ni) 
//' 
Superconducting Magnet Coil 
I.==J 1-1 Pb Polarization Direction 
Ge Detector 
Figure 2. Experimental setup for the forward-backward asymmetry measurement. 
A strong limit on the isovector amplitude is given by the measurements of the polarization 
of the 1081-keV gamma line in 18F. Other measurements are sensitive to different linear 
combinations of the isoscalar and isovector amplitudes. The extraction of the isoscalar 
amplitude necessarily involves a nuclear structure calculation, which will be different for 
2 0 7 ~ b  than in other cases. The fact that 207Pb is one nucleon removed from a doubly- 
magic nucleus leads us to believe that modern shell model calculations will be able to 
predict the relevant nuclear structure reliably. In many previous measurements of the 
isoscalar amplitude, nuclear structure uncertainties dominate the error in the extraction 
of the amplitude. 
The weak mixing of single-particle states is measured by detecting either a circular 
polarization or a forward-backward asymmetry of the 1.063 MeV gamma ray emitted from 
the decay of 207Bi down through states of 207Pb. The circular polarization of the gamma 
rays is measured for unpolarized 207Pb and the forward-backward asymmetry requires 
polarized 207 pb  nuclei. Either PNC effect is proportional to the single-particle weak matrix 
element and is a consequence of transitions between parity-mixed states of 2 0 7 ~ b .  
The experimental setup for the circular polarization experiment consists of a 2 0 7 ~ i  
source, gamma polarimeter, intrinsic CsI gamma detectors and a fast data-acquisition 
system. The setup details were described in a previous annual report, and will not be 
repeated here. The setup for the asymmetry measurement is considerably different and is 
shown in Fig. 2. The major components of this setup are a dilution refrigerator, a source 
consisting of 207Bi diffused into a Ni lattice, and two intrinsic Ge detectors. A holding field 
is set up inside the dilution refrigerator by a superconducting coil; the direction of this 
field is flipped by 180" every 10 min during data taking. The 207Bi nuclei are polarized 
Electronics for High-speed Pulse Counting 
Discriminator Histogramming Memory 
8 bits of data 
I 
I Transient Digitizer I 
I I 
L,,,--,,-,------------------I 
- 
Figure 3. The modified data acquisition system. 
and control 
lines 
by the large internal field of the Ni lattice and by the low (7-10 mK) temperature of the 
dilution refrigerator. We are using the dilution refrigerator located at Princeton University 
to measure the forward-backward asymmetry of the 1.063-MeV line. 
The progress made during the last year on each experiment follows. 
One of the major technical hurdles of the circular polarization experiment is the high 
counting rates needed to get sufficient sensitivity. The analyzing power of the polarimeter 
is 1 %, SO the expected experimental asymmetry is small. Thus, the experiment must 
be capable of high data rates in the detectors and data-acquisition system. The gamma 
detectors use intrinsic CsI, which has a decay lifetime of 30 ns and can be used for gamma 
rates of N 1 MHz. The data-acquisition system is very fast and is discussed below. The 
expected sensitivity of the experiment is 50 eV for a 1-2 month run. 
The main components of the data acquisition system are shown in Fig. 3. The tran- 
sient digitizer (TD) (DSP model 2030s) samples, holds and converts the output of the 
photomultiplier tube. The routing box is a device built at IUCF and is used to inter- 
face the TD with a his t ogramming memory. Originally, we used a commercially-available 
histogramming memory module (Ortec model HM413), but we have later found that the 
HM413 induces false asymmetries in the data. Thus, we have redesigned the routing box 
to include memory and a CAMAC interface. The data-acquisition system now contains 
just the TD and the routing box, with the routing box doing the task of the histogramming 
Amplifier phototube 
memory in addition to controlling the TD and routing the detector signals. An energy 
spectrum taken with our CsI crystals and the data-acquisition system is shown in Fig. 4. 
This spectrum was taken with a 137Cs source at a counting rate of 100 kHz. 
+ 
Since the last annual report, we have run the polarimeter, and done systematic error 
studies. We have particularly worked on systematic errors arising from electronics effects 
I i I v r""-""----"'---'--- --' I 
--+ 
I 
I 
! I 
Memory 
I 
I 
tC 
I 
CAMAC 

The polarization in our last run was only 13%, and work is continuing to improve the 
polarization. In particular, new sources have been fabricated and measurements using an 
X-ray fluoresence spectrometer and an electron microscope have been made to determine 
the crystal properties of the sources. 
1. J.J. Szymanski, B. Cain, J.E. Knott, A. Komives, M. Leuschner, J.D. Bowman, and 
D. Jamriska, in Proc. of the Int. Conf. on Particles and Nuclei, Perugia, Italy, June 
1993. 
2. M. Leuschner, J.J. Szymanski, B. Cain, J.E. Knott, A. Komives, J.D. Bowman, I.C. 
Girit, M. Petrov, and A. Andalkar, in Proc. of the 5th Conf. on the Intersections of 
Particle and Nuclear Physics, St. Petersburg, Florida, June 1994. 
3. J.D. Bowman, et al., Phys. Rev. Lett. 65, 1192 (1990); X. Zhu, et al., Phys. Rev. C 
46, 768 (1992). 
4. C.M. Frankle, et al., Phys. Rev. Lett. 67, 564 (1991); C.M. Frankle, et al., Phys. Rev. 
C 46, 778 (1992). 
~ 5. J.J. Szymanski, J.D. Bowman, M. Leuschner, B.A. Brown, and I.C. Girit, accepted 
by Phys. Rev. C. 
I 6. N. Auerbach, Phys. Rev. C 45, R514 (1992). 
I 7. N. Auerbach and J.D. Bowman, Phys. Rev. C 46, 2582 (1992). 
8. J.D. Bowman, et al., Phys. Rev. Lett. 68, 780 (1992). 
9. V.V. Flambaum, Phys. Rev. C 45, 437 (1992). 
10. S.E. Koonin, C.W. Johnson, and P. Vogel, Phys. Rev. Lett. 69, 1163 (1992). 
11. C.H. Lewenkopf and H.A. Weidenmuller, Phys. Rev. C 46, 2601 (1992). 
12. E. Adelberger, in Intersections between Particle and Nucleon Physics, ed. D.F. 
Geesaman (AIP Conf. Proc. No. 150, New York, 1986) p. 1177. 
A MEASUREMENT OF PARITY-VIOLATING 
NEUTRON TRANSMISSION IN XENON 
M. Leuschner, W. M. Snow, and J.  J .  Szymanski 
Indiana University, Bloomington, Indiana 4 7408 
and the TRIPLE Collaboration 
A series of measurements of parity violating neutron transmission asymmetries in 
heavy nuclei carried out by the TRIPLE collaboration have demonstrated a number of 
cases of large amplifications of parity violating effects.' We are preparing an experiment to 
measure the parity violating asymmetry in polarized neutron transmission through xenon. 
There are two separate motivations for carrying out such a measurement. First, such a 
measurement would be a useful addition to understanding the A-dependence of the effect. 


A Study of Single-Particle Parity-Nonconserving Nuclear Matrix Elements

https://scholarworks.iu.edu/dspace/bitstream/2022/514/1/iucf1993-1994_67-71.pdf

A Study of Single-Particle Parity-Nonconserving Nuclear Matrix Elements

Abstract

Similar works

Full text

Available Versions

IUScholarWorks