- Email: [email protected]
The persistence of nuclear polarization produced by the capture of polarized thermal neutrons
Nuclear Physics 22 (1961) 336-338 ;©North-Holland Publishing Co., Amsterdam
Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE PERSISTENCE OF NUCLEAR POLARIZATION PRODUCED BY THE CAPTURE OF POLARIZED THERMAL NEUTRONS A. H. WAPSTRA t and D. W. CONNOR Argonne National Laboratory, Lemont, Illinois Received 30 September 1960 Abstract : The relation between observed beta asymmetry and the static magnetic guide field is investigated for the capture of neutrons by 7Li, 19 F and $TAI.
Among the early demonstrations of parity nonconservation in beta decay was the observation by Burgy et al. 1) of a directional asymmetry in the beta decay of the 8Li nuclei produced when polarized neutrons were captured by 7Li. The effect, whose observability requires that the nuclear polarization persist for a time comparable to the 8Li half-life, 0.8 sec, was detectable only with fresh samples of the LieCO. and LiF powders used. The complete attenuation of the observed asymmetry in the course of about 30 minutes irradiation is probably to be explained by the rapid build-up of paramagnetic centres in the sample, due to the fast neutrons and pile gammas with which the polarized beam was contaminated 2) . It is well known from the work of Bloembergen 8 ) that paramagnetic impurities can produce relaxation times much less than 1 sec . Recently Connor has been able to perform a magnetic resonance measurement of the 8Li nuclear magnetic moment a) by using the beta asymmetry effect as an indicator of the nuclear polarization. Using a neutron beam which had been polarized by reflection from a magnetized cobalt mirror s) and consequently was quite free of contaminating hard radiations, he did not observe the sample aging remarked above. In both observations of the 8Li decay a static magnetic field at the sample served to decouple the nuclear moments of the polarized 8Li nuclei from those of the surrounding nuclei, and to provide a guide field for the polarized neutrons to prevent their depolarization. We thought it interesting to investigate the relation between the observed beta asymmetry and this magnetic field; also, we tried to find similar effects in other substances that looked promising, especially in 19F and 27A1. The data of interest for these cases are given in table 1. . This table shows that in investigating these two nuclei one should not measure the beta rays during the irradiation as, contrary to the 7Li case, the capture t On leave from the Institute of Nuclear Physics Research in Amsterdam and the Technological University, Delft. 336
THE PERSISTENCE OF NUCLEAR POLARIZATION
337
TABLE 1 Relevant data about neutrons capture in 7Li, 19F and 27A1 Nuclear spin Target
Initial
Final
Reaction ion energy (keV)
Beta decay energy (keV)
TLi
1
2033
16002
19F
1
6599
7028
1
7724
4640
27A1
Half life (see)
0.8
Computed
Asymmetry
Upper channel spin (%)
Loner channel spin
M
11
8
11
15
0
140
11
8
gamma rays will give a large background in the beta scintillation counters at any discriminator setting. An, automatic shutter system was therefore used to provide alternating irradiation and counting periods of the order of the decay halflives. The alternate irradiations were made with full (> 90 %) polarization and with an unpolarized neutron beam. 27A1 was investigated as powdered cubic oxyde. No effect was found . Probably, therefore, the relaxation time of 28Al in this material is somewhat shorter than its halflife . Fluorine was used as a monocrystal of Car2 , and Li in 7LiF as before 4) ; the effect of the beta rays of the unwanted activities (47Ca, "Ca, 2°F) was eliminated by proper setting of the discriminator, as checked by following the halflives . The results of the experiments are shown in fig. 1 . The lines drawn through 126
Fig. 1. Relation between the experimental asymmetry in the direction of the nuclear electrons emitted by Lie and s°F samples formed by capture of polarized neutrons, and the magnetic field strength at the samples.
338
A . H . WAPSTRA AND D . W. CONNOR
the experimental points are theoretical ones, computed from the assumption that the decrease in the asymmetry observed at lower magnetic fields is due to depolarization of the nuclei with a relaxation tirie proportional to the square of the magnetic field, Zrelax = CH2,
as it should be in first approximation if it is determined mainly by dipole interaction. The best values of C following from the above experiments are (1 .1 ±0.1) X 10-4 sec/Gauss2 in LiF a .,4 (7 ± 3) X 10-4 seclGauss2 for CaF2 . Since there is a possibility that part of the decrease may be due to depolarization of the neutron beam at lower guide fields, the above values should be considered as lower limits . The maximum asymmetry in 8 Li, 10.4±0.3 %, is somewhat larger than the values obtained before 1, 4) since, due to a better adjustment of the neutron slits, the neutron beam was less contaminated with depolarized or fast neutrons and gamma rays. The above value, compared with the theoretical data in table 1 computed from data given by Shapiro e), confirms earlier conclusions s. 7)
that the neutron capture in 7Li occurs mainly, perhaps even exclusively, with channel spin 2. The maximum asymmetry in 2OF is 2.2±0.3 %. From this, one can only conclude that channel spin 1 does occur ; it may even occur exclusively. A large fraction of the 2OF nuclei may have so short a relaxation time that the asymmetry is not detectable, the observed effect being due entirely to a small fraction ( > 15 %) of the nuclei for which it is relatively long . Plausibility is lent s- o this supposition by the following considerations. Recoil from the capture gamma imparts kinetic energy to the 2OF nucleus, 2 keV for the ground state transition and several hundred eV for most cascades . Consequently the typical nucleus is certainly dislodged from its lattice site . A small fraction of the displaced nuclei may come to rest at normal sites, either by occupying vacancies or "bumping" 19F nuclei . The remainder may come to rest in interstitial positions where, since the symmetry is not cubic, the relaxation by quadrupole interaction may be rapid. In the case of 8Li, quadrupole relaxation may be negligible because of the probably small quadrupole moment Q(gLi) - -0.002. Clearly, further experiment will be necessary before the relative importance of the two explanations can be assessed . It is a pleasure to acknowledge valuable discussions with Dr. T. B. Novey .
1) 2) 3) 4) 5) 6) 7)
References
Burgy, Davidon, Novey, Perlow and Ringo, Bull . Am . Phys . Soc. 2 (1957) 206 T. B. Novey, private communication N. Bloembergen, Physica, 15 (1949) 386 D. Connor, Phys. Rev. Lett . 3 (1959) 429 D. J. Hughes and M. T. Burgy, Phys . Rev. 81 (1959) 498 F. L. Shapiro, Uspekhi Fiz. Nauk 65 (1958) 133 D. Kurath, Bull . Am . Phys . Soc . 2 (1957) 206
Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE PERSISTENCE OF NUCLEAR POLARIZATION PRODUCED BY THE CAPTURE OF POLARIZED THERMAL NEUTRONS A. H. WAPSTRA t and D. W. CONNOR Argonne National Laboratory, Lemont, Illinois Received 30 September 1960 Abstract : The relation between observed beta asymmetry and the static magnetic guide field is investigated for the capture of neutrons by 7Li, 19 F and $TAI.
Among the early demonstrations of parity nonconservation in beta decay was the observation by Burgy et al. 1) of a directional asymmetry in the beta decay of the 8Li nuclei produced when polarized neutrons were captured by 7Li. The effect, whose observability requires that the nuclear polarization persist for a time comparable to the 8Li half-life, 0.8 sec, was detectable only with fresh samples of the LieCO. and LiF powders used. The complete attenuation of the observed asymmetry in the course of about 30 minutes irradiation is probably to be explained by the rapid build-up of paramagnetic centres in the sample, due to the fast neutrons and pile gammas with which the polarized beam was contaminated 2) . It is well known from the work of Bloembergen 8 ) that paramagnetic impurities can produce relaxation times much less than 1 sec . Recently Connor has been able to perform a magnetic resonance measurement of the 8Li nuclear magnetic moment a) by using the beta asymmetry effect as an indicator of the nuclear polarization. Using a neutron beam which had been polarized by reflection from a magnetized cobalt mirror s) and consequently was quite free of contaminating hard radiations, he did not observe the sample aging remarked above. In both observations of the 8Li decay a static magnetic field at the sample served to decouple the nuclear moments of the polarized 8Li nuclei from those of the surrounding nuclei, and to provide a guide field for the polarized neutrons to prevent their depolarization. We thought it interesting to investigate the relation between the observed beta asymmetry and this magnetic field; also, we tried to find similar effects in other substances that looked promising, especially in 19F and 27A1. The data of interest for these cases are given in table 1. . This table shows that in investigating these two nuclei one should not measure the beta rays during the irradiation as, contrary to the 7Li case, the capture t On leave from the Institute of Nuclear Physics Research in Amsterdam and the Technological University, Delft. 336
THE PERSISTENCE OF NUCLEAR POLARIZATION
337
TABLE 1 Relevant data about neutrons capture in 7Li, 19F and 27A1 Nuclear spin Target
Initial
Final
Reaction ion energy (keV)
Beta decay energy (keV)
TLi
1
2033
16002
19F
1
6599
7028
1
7724
4640
27A1
Half life (see)
0.8
Computed
Asymmetry
Upper channel spin (%)
Loner channel spin
M
11
8
11
15
0
140
11
8
gamma rays will give a large background in the beta scintillation counters at any discriminator setting. An, automatic shutter system was therefore used to provide alternating irradiation and counting periods of the order of the decay halflives. The alternate irradiations were made with full (> 90 %) polarization and with an unpolarized neutron beam. 27A1 was investigated as powdered cubic oxyde. No effect was found . Probably, therefore, the relaxation time of 28Al in this material is somewhat shorter than its halflife . Fluorine was used as a monocrystal of Car2 , and Li in 7LiF as before 4) ; the effect of the beta rays of the unwanted activities (47Ca, "Ca, 2°F) was eliminated by proper setting of the discriminator, as checked by following the halflives . The results of the experiments are shown in fig. 1 . The lines drawn through 126
Fig. 1. Relation between the experimental asymmetry in the direction of the nuclear electrons emitted by Lie and s°F samples formed by capture of polarized neutrons, and the magnetic field strength at the samples.
338
A . H . WAPSTRA AND D . W. CONNOR
the experimental points are theoretical ones, computed from the assumption that the decrease in the asymmetry observed at lower magnetic fields is due to depolarization of the nuclei with a relaxation tirie proportional to the square of the magnetic field, Zrelax = CH2,
as it should be in first approximation if it is determined mainly by dipole interaction. The best values of C following from the above experiments are (1 .1 ±0.1) X 10-4 sec/Gauss2 in LiF a .,4 (7 ± 3) X 10-4 seclGauss2 for CaF2 . Since there is a possibility that part of the decrease may be due to depolarization of the neutron beam at lower guide fields, the above values should be considered as lower limits . The maximum asymmetry in 8 Li, 10.4±0.3 %, is somewhat larger than the values obtained before 1, 4) since, due to a better adjustment of the neutron slits, the neutron beam was less contaminated with depolarized or fast neutrons and gamma rays. The above value, compared with the theoretical data in table 1 computed from data given by Shapiro e), confirms earlier conclusions s. 7)
that the neutron capture in 7Li occurs mainly, perhaps even exclusively, with channel spin 2. The maximum asymmetry in 2OF is 2.2±0.3 %. From this, one can only conclude that channel spin 1 does occur ; it may even occur exclusively. A large fraction of the 2OF nuclei may have so short a relaxation time that the asymmetry is not detectable, the observed effect being due entirely to a small fraction ( > 15 %) of the nuclei for which it is relatively long . Plausibility is lent s- o this supposition by the following considerations. Recoil from the capture gamma imparts kinetic energy to the 2OF nucleus, 2 keV for the ground state transition and several hundred eV for most cascades . Consequently the typical nucleus is certainly dislodged from its lattice site . A small fraction of the displaced nuclei may come to rest at normal sites, either by occupying vacancies or "bumping" 19F nuclei . The remainder may come to rest in interstitial positions where, since the symmetry is not cubic, the relaxation by quadrupole interaction may be rapid. In the case of 8Li, quadrupole relaxation may be negligible because of the probably small quadrupole moment Q(gLi) - -0.002. Clearly, further experiment will be necessary before the relative importance of the two explanations can be assessed . It is a pleasure to acknowledge valuable discussions with Dr. T. B. Novey .
1) 2) 3) 4) 5) 6) 7)
References
Burgy, Davidon, Novey, Perlow and Ringo, Bull . Am . Phys . Soc. 2 (1957) 206 T. B. Novey, private communication N. Bloembergen, Physica, 15 (1949) 386 D. Connor, Phys. Rev. Lett . 3 (1959) 429 D. J. Hughes and M. T. Burgy, Phys . Rev. 81 (1959) 498 F. L. Shapiro, Uspekhi Fiz. Nauk 65 (1958) 133 D. Kurath, Bull . Am . Phys . Soc . 2 (1957) 206