- Email: [email protected]
Parity non-conservation in the γ-decay of 41K
Volume
30B.
number 1
PHYSICS
PARITY V. M. LOBASHOV,
NON-CONSERVATION
N. A. LOZOVOY, A.F.
Yoffe
LETTERS
IN
V. A. NAZARENKO,
Physico-Technical Received
Institute.
1 September
THE
y-DECAY
L. M. SMOTRIZKY Leningrad.
OF
1969
41K
and G. I. KHARKEVITSH
USSR
22 May 1969
Parity non-conservation in nuclear forces was observed by measuring the circular polarization of the 1290 keV y-transition in 41K. The measured value of the circular polarization is P = + (1.9 i0.3) x 10m5.
The study of parity non-conservation effects in nuclear processes may give information on the existence and properties of the weak nucleonnucleon interaction [l], predicted by the theory of universal weak interaction. However, the interpretation of the effects observed in complex nuclei is complicated due to the uncertainties in nuclear model calculations. From this latter point of view the investigation of such effects in spherical near-magic nuclei seems to have some advantages as compared with those in deformed nuclei. In this work the circular polarization of the y-quanta emitted by unpolarized nuclei was measured. The 1290 eV M2 transition in 41K excited & Ar (tl = 109 min) was invesafter p-decay of tigated. The circular polarization (c.p.) of this transition may be caused by the interference between regular M2 and irregular E2 radiation. As the ratio of the reduced amplitudes (E2)/{M2) is usually of the order 10 - 100, the circular polarization of this transition may be enhanced. The c.p. was measured with a method described earlier in ref. 2. A feature of the method is that the integral detector current is measured instead of the individual pulses produced in the y-quanta detector. A periodic change of the y-ray intensity upon reversal of magnetization of the polarimeter is measured by a resonant device tuned to the reversal frequency. As a resonant device a pendulum filter was used. In fig. 1 a part of the experimental arrangement including the polarimeter and the 41Ar source is shown. The electronic equipment and the procedure of measurement were similar to those used in ref. 2. In contradistinction to ref. 2 a “transmission type” polarimeter was used, because the “forward scattering” type plarimeter was found to be unacceptable for 41Ar. The 41Ar P-decay has two
100 mm CI
Fig. 1. Experimental arrangement. polarimeter+and 41Ar source. 1 - Tank with argon in active zone. >41Ar liquid source, 3 - tubes connecting the tank with the source volume, 4 - liquefaction device. 5 -heater. 6 - vacuum jacket, 7 - liquid nitrogen automatic feed. 8 reactor active zone, 9 - magnetized absorber. 10 - polarimeter coils. 11 -magnetic yoke of polarimeter. 12 scintillation crystal CsI (TI). 13 - light pipe, 14 - scintillations detector-photodiode (surface barrier counter). 39
Volume 30B, number 1
PHYSICS
branches with endpoint energies of 1200 keV (99.2%) and 2490 keV (0.8%). It can be shown that the external bremsstrahlung effect from electrons stopping in the liquid argon must be small in comparison with the internal bremsstrahlung (i.b.) effect, which can be calculated. The calculations carried out for a “forward scattering” method in a geometry similar to that used in ref.2 gave the effective c.p. from internal bremsstrahlung from 4IAr P-decay as (3 - 15) X 10m5 (depending on the lead filter thickness). This is the reason why the “transmission” method in spite of the smaller polarization efficiency proved to be better. Due to the change of the sign of the polarization efficiency at a y-ray energy of 600 keV, and due to the absorption of the soft part of the i.b. spectrum, the i.b. effect proved to be an order of magnitude smaller than for the scattering method. It depends on the length of the magnetized absorber such that at 60 mm the i.b. effect becomes equal to zero. The polarimeter shown in fig. 1 has a magnetized obsorber of 60 mm effective length. The polarization efficiency for 1290 keV y-quanta was determined as 3.3%. The 41Ar source was produced by means of neutron irradiation of argon circulating in a high pressure gas loop with liquefaction in the source volume as shown in fig. 1. During the experiment the source volume was filled with liquefied argon, which was then evaporated in the output tube. The circulation period of the gas was about 15 min. This method allows to obtain a high stability of the source activity owing to the inertness of the liquefaction process and to the large ratio of the 41Ar decay period to the fast reactor power fluctuation period. The activity of the 41Ar source was about 700 Ci. Control experiments were carried out with a 46~~ source (activity 2000 Ci) which shows an unhindered E2 transition, which can be expected to be practically unpolarized, of about the same energy (1120 keV) as for the 41Ar source (1290 keV). In order to check the calculation of the i.b. effect, a measurement was also made with a 188Re source (endpoint energy 2116 keV). The experimental and calculated results proved to be in good agreement. The results of measurements with the 41Ar and 46Sc sources are given in table 1. The quantity 6 is, as usually, defined as the relative difference of the y-ray intensity for the two magnetization directions. Run 1 with 4IAr was performed without filters in front of the detector. Runs 2 and 3 were car40
LETTERS
1 September 1969
Table 1 Experimental results. 46sc, 6 x 107
41Ar, 6 x 107 Run 1
Run2 Pb-filter 8mm
Run 3 Pb-filter 4mm
+6.5 +1.0
+ 10.0 f 3.0
+4.6 f 2.2
Weighted Run 1 Run 2 average +6.5
l 0.85
-0.4 +0.6
+0.4 *to.7
ried out with lead filters, 8 mm and 4 mm thick, respectively. Run 1 with 46Sc was made before the 41Ar measurement. Run 2 was carr’ d out in the intervals between the runs with “Ar. Both runs showed the absence of false effects. In addition to the 46Sc measurements, the polarimeter and the switching device were also checked with a 24Na source (2.75 MeV E2 y-rays). In these measurements no false effects were observed within an accuracy of 6 of about 0.3 X lo-?. The results of the measurements with 41Ar demonstrate the existence of an effect 6 = =+(6.5*0.8)x 10-T corresponding to a circular polarization P = + (1.9OkO.25) x 10-5. In general, an effect of the same sign may be caused by internal bremsstrahlung from highenergy (> 1 MeV) K-capture or low-energy 0 decay. Low-energy p-activity may be eliminated both by the independence of the effect on the lead filter thickness and by the purity of the argon used. Besides, in order to cause the measured c.p. of about 10m5, the corresponding activity must be much larger than that of 41Ar. The absence of the high-energy K-capture activity was checked by studying the Y -spectrum from the source after 10 - 20 hours following the switching off of the irradiation. All the known isotopes decaying through K-capture and produced by reactor neutron irradiation have a decay period longer than that of 41Ar. The c.p. measurements with the decayed 41Ar source performed after a day following the switching off of the irradiation showed the absence of any effect (equivalent to 6 < 0.3 X 10-T). It confirmed the small influence of the i.b. from longlived 37Ar decaying through K-capture (endpoint 814 keV) and produced by irradiation of the 36Ar in the natural argon. Thus the observed circular polarization can only be ascribed to the 1290 keV transition in 41K. Taking into account the errors of the control experiments the c.p. value must be written as P = = + (1.90* 0.30) x 10-5. The comparison with the theory, which can
Volume
30B,
number
1
PHYSICS
only be qualitative by nature, may be performed with preliminary results of Gaponov and Fursov [3], who obtained for the 1290 keV transition in 41K an estimate of P s(O.6 - 2.0) x 10-5. Their estimate is based on the theory of finite Fermi systems similar to that described in ref. 4. The present result is consistent with this estimate. At present the main result of the experiment described is the observation of one more case of parity non-conservation in nuclear forces in addition to the known cases of 181Ta, 175Lu [2] and 114Cd [5]. We are very grateful to the staff of the WWFtM reactor of the A. F. Yoffe Physico-T ethnical Institute for the arrangement of good experi-
LETTERS
1 September
1969
mental conditions. We wish to thank Professor D. M. Kaminker for his interest and support of this work, and V. A. Knjazkov and L. F. Sayenko for help during the measurements.
References 1. M. Gell-Mann 2. 3.
4. 5.
and R. Feynman, Phys. Rev. 109 (1958) 193. V. M. Lobashov et al., Phys. Letters 25B (1967) 104. Y. Gaponov and Y. Fursov, Report on the Conference on nuclear Spectroscopy and nuclear Structure, Erevan (USSR). Y. Gaponov and Y. Fursov, Journal of Nuclear Physics (USSR) 9 (1969) 963. Y. G. Abov, P.A. Krupchitsky and Y. A.Oratovsky, Phys. Letters 12 (1964) 25.
41
30B.
number 1
PHYSICS
PARITY V. M. LOBASHOV,
NON-CONSERVATION
N. A. LOZOVOY, A.F.
Yoffe
LETTERS
IN
V. A. NAZARENKO,
Physico-Technical Received
Institute.
1 September
THE
y-DECAY
L. M. SMOTRIZKY Leningrad.
OF
1969
41K
and G. I. KHARKEVITSH
USSR
22 May 1969
Parity non-conservation in nuclear forces was observed by measuring the circular polarization of the 1290 keV y-transition in 41K. The measured value of the circular polarization is P = + (1.9 i0.3) x 10m5.
The study of parity non-conservation effects in nuclear processes may give information on the existence and properties of the weak nucleonnucleon interaction [l], predicted by the theory of universal weak interaction. However, the interpretation of the effects observed in complex nuclei is complicated due to the uncertainties in nuclear model calculations. From this latter point of view the investigation of such effects in spherical near-magic nuclei seems to have some advantages as compared with those in deformed nuclei. In this work the circular polarization of the y-quanta emitted by unpolarized nuclei was measured. The 1290 eV M2 transition in 41K excited & Ar (tl = 109 min) was invesafter p-decay of tigated. The circular polarization (c.p.) of this transition may be caused by the interference between regular M2 and irregular E2 radiation. As the ratio of the reduced amplitudes (E2)/{M2) is usually of the order 10 - 100, the circular polarization of this transition may be enhanced. The c.p. was measured with a method described earlier in ref. 2. A feature of the method is that the integral detector current is measured instead of the individual pulses produced in the y-quanta detector. A periodic change of the y-ray intensity upon reversal of magnetization of the polarimeter is measured by a resonant device tuned to the reversal frequency. As a resonant device a pendulum filter was used. In fig. 1 a part of the experimental arrangement including the polarimeter and the 41Ar source is shown. The electronic equipment and the procedure of measurement were similar to those used in ref. 2. In contradistinction to ref. 2 a “transmission type” polarimeter was used, because the “forward scattering” type plarimeter was found to be unacceptable for 41Ar. The 41Ar P-decay has two
100 mm CI
Fig. 1. Experimental arrangement. polarimeter+and 41Ar source. 1 - Tank with argon in active zone. >41Ar liquid source, 3 - tubes connecting the tank with the source volume, 4 - liquefaction device. 5 -heater. 6 - vacuum jacket, 7 - liquid nitrogen automatic feed. 8 reactor active zone, 9 - magnetized absorber. 10 - polarimeter coils. 11 -magnetic yoke of polarimeter. 12 scintillation crystal CsI (TI). 13 - light pipe, 14 - scintillations detector-photodiode (surface barrier counter). 39
Volume 30B, number 1
PHYSICS
branches with endpoint energies of 1200 keV (99.2%) and 2490 keV (0.8%). It can be shown that the external bremsstrahlung effect from electrons stopping in the liquid argon must be small in comparison with the internal bremsstrahlung (i.b.) effect, which can be calculated. The calculations carried out for a “forward scattering” method in a geometry similar to that used in ref.2 gave the effective c.p. from internal bremsstrahlung from 4IAr P-decay as (3 - 15) X 10m5 (depending on the lead filter thickness). This is the reason why the “transmission” method in spite of the smaller polarization efficiency proved to be better. Due to the change of the sign of the polarization efficiency at a y-ray energy of 600 keV, and due to the absorption of the soft part of the i.b. spectrum, the i.b. effect proved to be an order of magnitude smaller than for the scattering method. It depends on the length of the magnetized absorber such that at 60 mm the i.b. effect becomes equal to zero. The polarimeter shown in fig. 1 has a magnetized obsorber of 60 mm effective length. The polarization efficiency for 1290 keV y-quanta was determined as 3.3%. The 41Ar source was produced by means of neutron irradiation of argon circulating in a high pressure gas loop with liquefaction in the source volume as shown in fig. 1. During the experiment the source volume was filled with liquefied argon, which was then evaporated in the output tube. The circulation period of the gas was about 15 min. This method allows to obtain a high stability of the source activity owing to the inertness of the liquefaction process and to the large ratio of the 41Ar decay period to the fast reactor power fluctuation period. The activity of the 41Ar source was about 700 Ci. Control experiments were carried out with a 46~~ source (activity 2000 Ci) which shows an unhindered E2 transition, which can be expected to be practically unpolarized, of about the same energy (1120 keV) as for the 41Ar source (1290 keV). In order to check the calculation of the i.b. effect, a measurement was also made with a 188Re source (endpoint energy 2116 keV). The experimental and calculated results proved to be in good agreement. The results of measurements with the 41Ar and 46Sc sources are given in table 1. The quantity 6 is, as usually, defined as the relative difference of the y-ray intensity for the two magnetization directions. Run 1 with 4IAr was performed without filters in front of the detector. Runs 2 and 3 were car40
LETTERS
1 September 1969
Table 1 Experimental results. 46sc, 6 x 107
41Ar, 6 x 107 Run 1
Run2 Pb-filter 8mm
Run 3 Pb-filter 4mm
+6.5 +1.0
+ 10.0 f 3.0
+4.6 f 2.2
Weighted Run 1 Run 2 average +6.5
l 0.85
-0.4 +0.6
+0.4 *to.7
ried out with lead filters, 8 mm and 4 mm thick, respectively. Run 1 with 46Sc was made before the 41Ar measurement. Run 2 was carr’ d out in the intervals between the runs with “Ar. Both runs showed the absence of false effects. In addition to the 46Sc measurements, the polarimeter and the switching device were also checked with a 24Na source (2.75 MeV E2 y-rays). In these measurements no false effects were observed within an accuracy of 6 of about 0.3 X lo-?. The results of the measurements with 41Ar demonstrate the existence of an effect 6 = =+(6.5*0.8)x 10-T corresponding to a circular polarization P = + (1.9OkO.25) x 10-5. In general, an effect of the same sign may be caused by internal bremsstrahlung from highenergy (> 1 MeV) K-capture or low-energy 0 decay. Low-energy p-activity may be eliminated both by the independence of the effect on the lead filter thickness and by the purity of the argon used. Besides, in order to cause the measured c.p. of about 10m5, the corresponding activity must be much larger than that of 41Ar. The absence of the high-energy K-capture activity was checked by studying the Y -spectrum from the source after 10 - 20 hours following the switching off of the irradiation. All the known isotopes decaying through K-capture and produced by reactor neutron irradiation have a decay period longer than that of 41Ar. The c.p. measurements with the decayed 41Ar source performed after a day following the switching off of the irradiation showed the absence of any effect (equivalent to 6 < 0.3 X 10-T). It confirmed the small influence of the i.b. from longlived 37Ar decaying through K-capture (endpoint 814 keV) and produced by irradiation of the 36Ar in the natural argon. Thus the observed circular polarization can only be ascribed to the 1290 keV transition in 41K. Taking into account the errors of the control experiments the c.p. value must be written as P = = + (1.90* 0.30) x 10-5. The comparison with the theory, which can
Volume
30B,
number
1
PHYSICS
only be qualitative by nature, may be performed with preliminary results of Gaponov and Fursov [3], who obtained for the 1290 keV transition in 41K an estimate of P s(O.6 - 2.0) x 10-5. Their estimate is based on the theory of finite Fermi systems similar to that described in ref. 4. The present result is consistent with this estimate. At present the main result of the experiment described is the observation of one more case of parity non-conservation in nuclear forces in addition to the known cases of 181Ta, 175Lu [2] and 114Cd [5]. We are very grateful to the staff of the WWFtM reactor of the A. F. Yoffe Physico-T ethnical Institute for the arrangement of good experi-
LETTERS
1 September
1969
mental conditions. We wish to thank Professor D. M. Kaminker for his interest and support of this work, and V. A. Knjazkov and L. F. Sayenko for help during the measurements.
References 1. M. Gell-Mann 2. 3.
4. 5.
and R. Feynman, Phys. Rev. 109 (1958) 193. V. M. Lobashov et al., Phys. Letters 25B (1967) 104. Y. Gaponov and Y. Fursov, Report on the Conference on nuclear Spectroscopy and nuclear Structure, Erevan (USSR). Y. Gaponov and Y. Fursov, Journal of Nuclear Physics (USSR) 9 (1969) 963. Y. G. Abov, P.A. Krupchitsky and Y. A.Oratovsky, Phys. Letters 12 (1964) 25.
41