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Measurement of parity violation in compound nuclear resonances using epithermal polarized neutrons
NOMB
Nuclear Instruments and Methods in Physics Research B79 (1993) 306-308 North-Holland
Beam Interactions with Materials 8 Atoms
Measurement of parity violation in compound nuclear resonances using epithermal polarized neutrons TRIPLE collaboration C.M. Frankle, J.D. Bowman, J.N. Knudson, S.I. Penttila, S.J. Seestrom, S.H. Yoo and V.W. Yuan Los Alamos National Laboratory, Los Alamos, NM 87545, USA
C.R. Gould, D.G. Haase and G.E. Mitchell North Carolina State University, Raleigh NC 27695 and Triangle Universities Nuclear Laboratory, Durham, NC 27706, USA
N.R. Roberson Duke University, Durham, NC 27706 and Triangle Universities Nuclear Laboratory, Durham, NC 27706, USA
Yu.P. Popov and E.I. Sharapov Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
P.P.J. Delheij TRKJMF, Vancouver, British Columbia, Canada V6T 2A3
H. Postma Universiiy of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands
Polarized neutron transmission experiments show remarkable sensitivity to the presence of small symmetry violating forces in nuclear interactions. Parity-nonconserving (PNC) analyzing powers of a percent or more have been measured on p-wave neutron resonances in nuclei ranging from slBr to =*U. A brief summary is given of the study of PNC effects in neutron resonances. Details of ongoing polarized neutron transmission experiments are discussed and some sample data presented. Several development projects are currently under way, which taken together should improve the sensitivity of these experiments by a factor of - 20. Similar large PNC effects can be observed in (n, y) resonances with much smaller samples. A design for a near 47r gamma ray detector to be constructed of Cshpure) is described.
1. Introduction
cussed in a comprehensive 131.
The first successful observation of parity violation with polarized neutrons was in 1964 by Abov et al. [l]
using thermal neutrons and ‘13Cd as a target. It was not until some 20 years later that Alfimenkov et al. [2] reported the first observation of parity violation in a neutron resonance. They reported an asymmetry of 7.3 k 0.5% for the 0.73 eV 1= 1 (p-wave) resonance in ‘39La. Effects of a similar magnitude were reported for the following p-wave resonances: 0.88 eV (“Br), 4.53 eV (“‘Cd), 1.33 eV (i17Sn). The early history is dis0168-583X/93/$06.00
In recent
years
monograph
two additional
by Krupchitsky
groups
have made
measurements of parity violation in neutron resonances: a group at KEK in Japan and the Time Reversal Invariance and Parity at Low Energies (TRIPLE) collaboration at Los Alamos. The KEK group has reported measurements on the 0.73 eV p-wave resonance in 139La [4]. The TRIPLE collaboration has observed parity violation in multiple resonances per nucleus up to an energy of u 200 eV. Results have been reported on 81Br [5], *17Sn [6], ‘39La [7], 232Th 181,
0 1993 - Elsevier Science Publishers B.V. All rights reserved
301
C.M. Frankle et al. / Parig, violation in compound nuclear resonances and =‘U [9]. Some of these results will be discussed in the following sections.
Table 1 Measured parity violating asymmetries from refs. [5,7-91 with statistical significance greater than two standard deviations Nucleus
2. Neutron transmission
experiments
These experiments are performed at the Manuel Lujan Jr. Neutron Scattering Center (LANSCE) which is part of the Clinton P. Anderson Meson Physics Facility (LAMPF). LANSCE is an extremely intense source of pulsed epithermal neutrons. Up to 100 p,A of 800 MeV protons are delivered to the LANSCE target from the Proton Storage Ring (PSR). The width of the pulse upon exiting the PSR is 250 ns. The PSR currently delivers a 20 Hz pulse train. The beam from the PSR is dumped onto two cylinders of tungsten oriented along the beam axis. Each beam proton produces approximately 20 neutrons. The neutrons are moderated in water to produce an energy spectrum with a l/E energy dependence. The parity violation experiments operate on a flight path of 60 m. Neutrons are polarized by passing them through a polarized proton filter. Since the n-p scattering cross section is highly spin dependent, one spin state is selectively filtered out while the other passes through. We have been using the “Keyworth” cryostat [lo] as the polarized proton filter. This cryostat has a 2.7 T superconducting magnet. The protons in the target material (lanthanum magnesium nitrate (LMN) or butanol) are dynamically pumped into one spin state by - 70 GHz microwaves. The maximum neutron polarization achieved with this cryostat was about 45%, while data was taken with polarizations as low as - 20%. This system is being replaced with a new polarizing cryostat. The new system has a 5 T magnetic field with lop4 homogeneity, pumped 4He at 1 K, 140 GHz microwave pumping, and four times the beam area of the old cryostat. Predicted polarizations using irradiated ammonia are > 90%. The neutron spins are inverted every 10 s using a magnetic spin rotator (“spin flipper”) located just downstream from the cryostat. The spin flipper has a dynamic flipping range from - 0.1 eV to - 1 keV. The spins may also be reversed by changing the microwave pumping frequency, but this process takes - 1 h and therefore is only performed about once per day. Transmission targets are located in the end of the spin flipper. Starting in 1990 targets were cooled in liquid nitrogen to reduce the Doppler broadening of the resonances. Neutrons are detected with an array of seven 6Li glass scintillators. Due to the extremely high intensity of the LANSCE neutron beams, pulse counting with the 6Li glass detectors is not possible. Instead, the tube output current is sampled with a transient digitizer. Details of this technique are given by Bowman et al.
Resonance
p
[%I
energy [eVl ‘lBr ‘39La 232q-h
238~
0.88 0.73 8.3 37.0 38.2 64.5 128.2 167.2 196.2 10.2 63.5 83.7 89.2
1.77*0.33 10.15 + 0.45 1.48kO.25 2.46 f 0.97 10.88 f 2.27 9.78 + 2.08 1.31f0.18 3.45 f 1.19 l.lOkO.46 -0.16*0.08 2.63 + 0.40 1.96kO.86 -0.24&-0.11
[ll]. 6Li glass detectors have the disadvantage that their efficiency decreases inversely with the neutron velocity. A 55 cell l”B loaded liquid scintillator detector is being prepared. Tests with prototype cells indicate an efficiency of about 90% which is approximately constant with neutron energy. This new detector combined with the new polarizing filter should improve the sensitivity of these experiments by a factor of - 20. Results from the study of 41 resonances in four nuclei by neutron transmission have been reported [5,7-g]. A total of 13 resonances showed > 20 statistical significance effects. These are listed in table 1. The largest effect is that of the 38.2 eV resonance in 232Th: P = 10.88 f 2.27%. Data from both neutron helicities for this resonance are shown in fig. 1. The asymmetry is clearly evident.
Neutron
Energy
(eV)
Fig. 1. The 232Th transmission spectrum for the two helicity states: negative (triangles) and positive (squares) in the vicinity of the 38.2 eV resonance. The parity violation is evident by inspection. NoTote that the ordinate zero is suppressed. IV. NUCLEAR PHYSICS
308
CM. Frankle et al. / Parity violation in compound nuclear resonances
3. Neutron capture gamma measurements
4. Summary
One of the drawbacks of neutron transmission measurements is that relatively large quantities ( - kg) of material are required for targets. Only a small number of nearly monoisotopic elements can be obtained in high purity form in this quantity. Separated isotopes are almost never available in large quantities. However, one may obtain the same information as in a neutron transmission experiment by measuring all of the gamma rays from neutron capture. This type of measurement requires only - 10 g of material. We therefore set out to design and construct a near 47~, highly efficient gamma ray detector. The basic requirements for the gamma ray detector are as follows. Because of the intense neutron flux the chosen scintillator should be very fast. A high stopping power is desirable so that nearly all the gamma rays can be detected with a scintillator of reasonable size. Finally, the chosen scintillator should be relatively inexpensive. Initial tests were run with BaF,, BGO, and CsI(pure). Both BaF, and CsI appeared to be fast enough and had sufficiently high stopping powers but CsI(pure) was approximately i the cost of BaF,. A design using two sequential annuli of 12 crystals each was chosen. Together the two annuli form a detector - 30 cm long with a bore of - 20 cm and an outside diameter of - 40 cm. Monte Carlo calculations indicate an overall efficiency (detection efficiency times effective solid angle) of 0.66 for this configuration. Shielding the detector from stray neutrons and gamma rays is an important part of the design. The exterior shield is constructed of - 10 cm of lead surrounded by - 15 cm of 5% boron loaded polyethylene. Calculations indicate this shield configuration will pass - lo-” of 1 keV neutrons, - 4 X 10m3 of 3 MeV gamma rays, and - lo-’ of 478 keV gamma rays. The interior shield, fitted into the detector bore, is used to keep neutrons scattered from the target out of the detector. This material is 10% 6Li loaded polyethylene with an outside diameter of - 20 cm, an inside diameter of - 10 cm, and a length of - 30 cm. This shield transmits only one out of lo3 1 keV neutrons, while emitting only one 2.2 MeV gamma ray per 300 captured neutrons.
For the past several years the TRIPLE collaboration has carried out a program of measuring parity violation in neutron resonances via neutron transmission experiments. This program is continuing with several new nuclei and a much improved system. A neutron capture gamma ray detector is under construction with completion scheduled for mid 1993. Measurements of parity violation in (n, y) resonances on separated isotopes and rare elements should start in earnest upon completion of this new detector.
Acknowledgements
We wish to thank R.N. Mortensen for his technical assistance to the experiment. This work was supported in part by the U.S. Department of Energy, Office of High Energy and Nuclear Physics under Contract No. DE-AC0576ER01067 and No. DE-FG05-88ER40441, and by the U.S. Department of Energy, Office of Energy Research, under Contract No. W-7405ENG-36.
References [l] Y.G. Abov, P.A. Krupchitsky and Y.A. Oratovsky, Phys. Len. 12 (1964) 25. [2] V.P. Alfimenkov et al., Nucl. Phys. A398 (1983) 93. [3] P.A. Krupchitsky, Fundamental Research with Polarized Slow Neutrons (Springer, 1987). [4] Y. Masuda et al., Nucl. Phys. A504 (19891 269. [5] CM. Frankle et al., Phys. Rev. C46 (1992) 1542. [6] EL Sharapov et al., Capture Gamma-Ray Spectroscopy (AIP, 1991) p.756. [7] V.W. Yuan et al., Phys. Rev. C44 (1991) 2187. [8] C.M. Frankle et al., Phys. Rev. L&t. 67 (1991) 564. [9] J.D. Bowman et al., Phys. Rev. Lett. 65 (1990) 1192. [lo] G.A. Keyworth et al., Phys. Rev. C8 (1973) 2352. [ll] J.D. Bowman et al., Nucl. Instr. and Meth. A297 (1990) 183.
Nuclear Instruments and Methods in Physics Research B79 (1993) 306-308 North-Holland
Beam Interactions with Materials 8 Atoms
Measurement of parity violation in compound nuclear resonances using epithermal polarized neutrons TRIPLE collaboration C.M. Frankle, J.D. Bowman, J.N. Knudson, S.I. Penttila, S.J. Seestrom, S.H. Yoo and V.W. Yuan Los Alamos National Laboratory, Los Alamos, NM 87545, USA
C.R. Gould, D.G. Haase and G.E. Mitchell North Carolina State University, Raleigh NC 27695 and Triangle Universities Nuclear Laboratory, Durham, NC 27706, USA
N.R. Roberson Duke University, Durham, NC 27706 and Triangle Universities Nuclear Laboratory, Durham, NC 27706, USA
Yu.P. Popov and E.I. Sharapov Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
P.P.J. Delheij TRKJMF, Vancouver, British Columbia, Canada V6T 2A3
H. Postma Universiiy of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands
Polarized neutron transmission experiments show remarkable sensitivity to the presence of small symmetry violating forces in nuclear interactions. Parity-nonconserving (PNC) analyzing powers of a percent or more have been measured on p-wave neutron resonances in nuclei ranging from slBr to =*U. A brief summary is given of the study of PNC effects in neutron resonances. Details of ongoing polarized neutron transmission experiments are discussed and some sample data presented. Several development projects are currently under way, which taken together should improve the sensitivity of these experiments by a factor of - 20. Similar large PNC effects can be observed in (n, y) resonances with much smaller samples. A design for a near 47r gamma ray detector to be constructed of Cshpure) is described.
1. Introduction
cussed in a comprehensive 131.
The first successful observation of parity violation with polarized neutrons was in 1964 by Abov et al. [l]
using thermal neutrons and ‘13Cd as a target. It was not until some 20 years later that Alfimenkov et al. [2] reported the first observation of parity violation in a neutron resonance. They reported an asymmetry of 7.3 k 0.5% for the 0.73 eV 1= 1 (p-wave) resonance in ‘39La. Effects of a similar magnitude were reported for the following p-wave resonances: 0.88 eV (“Br), 4.53 eV (“‘Cd), 1.33 eV (i17Sn). The early history is dis0168-583X/93/$06.00
In recent
years
monograph
two additional
by Krupchitsky
groups
have made
measurements of parity violation in neutron resonances: a group at KEK in Japan and the Time Reversal Invariance and Parity at Low Energies (TRIPLE) collaboration at Los Alamos. The KEK group has reported measurements on the 0.73 eV p-wave resonance in 139La [4]. The TRIPLE collaboration has observed parity violation in multiple resonances per nucleus up to an energy of u 200 eV. Results have been reported on 81Br [5], *17Sn [6], ‘39La [7], 232Th 181,
0 1993 - Elsevier Science Publishers B.V. All rights reserved
301
C.M. Frankle et al. / Parig, violation in compound nuclear resonances and =‘U [9]. Some of these results will be discussed in the following sections.
Table 1 Measured parity violating asymmetries from refs. [5,7-91 with statistical significance greater than two standard deviations Nucleus
2. Neutron transmission
experiments
These experiments are performed at the Manuel Lujan Jr. Neutron Scattering Center (LANSCE) which is part of the Clinton P. Anderson Meson Physics Facility (LAMPF). LANSCE is an extremely intense source of pulsed epithermal neutrons. Up to 100 p,A of 800 MeV protons are delivered to the LANSCE target from the Proton Storage Ring (PSR). The width of the pulse upon exiting the PSR is 250 ns. The PSR currently delivers a 20 Hz pulse train. The beam from the PSR is dumped onto two cylinders of tungsten oriented along the beam axis. Each beam proton produces approximately 20 neutrons. The neutrons are moderated in water to produce an energy spectrum with a l/E energy dependence. The parity violation experiments operate on a flight path of 60 m. Neutrons are polarized by passing them through a polarized proton filter. Since the n-p scattering cross section is highly spin dependent, one spin state is selectively filtered out while the other passes through. We have been using the “Keyworth” cryostat [lo] as the polarized proton filter. This cryostat has a 2.7 T superconducting magnet. The protons in the target material (lanthanum magnesium nitrate (LMN) or butanol) are dynamically pumped into one spin state by - 70 GHz microwaves. The maximum neutron polarization achieved with this cryostat was about 45%, while data was taken with polarizations as low as - 20%. This system is being replaced with a new polarizing cryostat. The new system has a 5 T magnetic field with lop4 homogeneity, pumped 4He at 1 K, 140 GHz microwave pumping, and four times the beam area of the old cryostat. Predicted polarizations using irradiated ammonia are > 90%. The neutron spins are inverted every 10 s using a magnetic spin rotator (“spin flipper”) located just downstream from the cryostat. The spin flipper has a dynamic flipping range from - 0.1 eV to - 1 keV. The spins may also be reversed by changing the microwave pumping frequency, but this process takes - 1 h and therefore is only performed about once per day. Transmission targets are located in the end of the spin flipper. Starting in 1990 targets were cooled in liquid nitrogen to reduce the Doppler broadening of the resonances. Neutrons are detected with an array of seven 6Li glass scintillators. Due to the extremely high intensity of the LANSCE neutron beams, pulse counting with the 6Li glass detectors is not possible. Instead, the tube output current is sampled with a transient digitizer. Details of this technique are given by Bowman et al.
Resonance
p
[%I
energy [eVl ‘lBr ‘39La 232q-h
238~
0.88 0.73 8.3 37.0 38.2 64.5 128.2 167.2 196.2 10.2 63.5 83.7 89.2
1.77*0.33 10.15 + 0.45 1.48kO.25 2.46 f 0.97 10.88 f 2.27 9.78 + 2.08 1.31f0.18 3.45 f 1.19 l.lOkO.46 -0.16*0.08 2.63 + 0.40 1.96kO.86 -0.24&-0.11
[ll]. 6Li glass detectors have the disadvantage that their efficiency decreases inversely with the neutron velocity. A 55 cell l”B loaded liquid scintillator detector is being prepared. Tests with prototype cells indicate an efficiency of about 90% which is approximately constant with neutron energy. This new detector combined with the new polarizing filter should improve the sensitivity of these experiments by a factor of - 20. Results from the study of 41 resonances in four nuclei by neutron transmission have been reported [5,7-g]. A total of 13 resonances showed > 20 statistical significance effects. These are listed in table 1. The largest effect is that of the 38.2 eV resonance in 232Th: P = 10.88 f 2.27%. Data from both neutron helicities for this resonance are shown in fig. 1. The asymmetry is clearly evident.
Neutron
Energy
(eV)
Fig. 1. The 232Th transmission spectrum for the two helicity states: negative (triangles) and positive (squares) in the vicinity of the 38.2 eV resonance. The parity violation is evident by inspection. NoTote that the ordinate zero is suppressed. IV. NUCLEAR PHYSICS
308
CM. Frankle et al. / Parity violation in compound nuclear resonances
3. Neutron capture gamma measurements
4. Summary
One of the drawbacks of neutron transmission measurements is that relatively large quantities ( - kg) of material are required for targets. Only a small number of nearly monoisotopic elements can be obtained in high purity form in this quantity. Separated isotopes are almost never available in large quantities. However, one may obtain the same information as in a neutron transmission experiment by measuring all of the gamma rays from neutron capture. This type of measurement requires only - 10 g of material. We therefore set out to design and construct a near 47~, highly efficient gamma ray detector. The basic requirements for the gamma ray detector are as follows. Because of the intense neutron flux the chosen scintillator should be very fast. A high stopping power is desirable so that nearly all the gamma rays can be detected with a scintillator of reasonable size. Finally, the chosen scintillator should be relatively inexpensive. Initial tests were run with BaF,, BGO, and CsI(pure). Both BaF, and CsI appeared to be fast enough and had sufficiently high stopping powers but CsI(pure) was approximately i the cost of BaF,. A design using two sequential annuli of 12 crystals each was chosen. Together the two annuli form a detector - 30 cm long with a bore of - 20 cm and an outside diameter of - 40 cm. Monte Carlo calculations indicate an overall efficiency (detection efficiency times effective solid angle) of 0.66 for this configuration. Shielding the detector from stray neutrons and gamma rays is an important part of the design. The exterior shield is constructed of - 10 cm of lead surrounded by - 15 cm of 5% boron loaded polyethylene. Calculations indicate this shield configuration will pass - lo-” of 1 keV neutrons, - 4 X 10m3 of 3 MeV gamma rays, and - lo-’ of 478 keV gamma rays. The interior shield, fitted into the detector bore, is used to keep neutrons scattered from the target out of the detector. This material is 10% 6Li loaded polyethylene with an outside diameter of - 20 cm, an inside diameter of - 10 cm, and a length of - 30 cm. This shield transmits only one out of lo3 1 keV neutrons, while emitting only one 2.2 MeV gamma ray per 300 captured neutrons.
For the past several years the TRIPLE collaboration has carried out a program of measuring parity violation in neutron resonances via neutron transmission experiments. This program is continuing with several new nuclei and a much improved system. A neutron capture gamma ray detector is under construction with completion scheduled for mid 1993. Measurements of parity violation in (n, y) resonances on separated isotopes and rare elements should start in earnest upon completion of this new detector.
Acknowledgements
We wish to thank R.N. Mortensen for his technical assistance to the experiment. This work was supported in part by the U.S. Department of Energy, Office of High Energy and Nuclear Physics under Contract No. DE-AC0576ER01067 and No. DE-FG05-88ER40441, and by the U.S. Department of Energy, Office of Energy Research, under Contract No. W-7405ENG-36.
References [l] Y.G. Abov, P.A. Krupchitsky and Y.A. Oratovsky, Phys. Len. 12 (1964) 25. [2] V.P. Alfimenkov et al., Nucl. Phys. A398 (1983) 93. [3] P.A. Krupchitsky, Fundamental Research with Polarized Slow Neutrons (Springer, 1987). [4] Y. Masuda et al., Nucl. Phys. A504 (19891 269. [5] CM. Frankle et al., Phys. Rev. C46 (1992) 1542. [6] EL Sharapov et al., Capture Gamma-Ray Spectroscopy (AIP, 1991) p.756. [7] V.W. Yuan et al., Phys. Rev. C44 (1991) 2187. [8] C.M. Frankle et al., Phys. Rev. L&t. 67 (1991) 564. [9] J.D. Bowman et al., Phys. Rev. Lett. 65 (1990) 1192. [lo] G.A. Keyworth et al., Phys. Rev. C8 (1973) 2352. [ll] J.D. Bowman et al., Nucl. Instr. and Meth. A297 (1990) 183.