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A proposal for experiments in low-energy antineutrino physics
8.C
[
Nuclear Physics 70 (1965) 574--576; (~) North-Holland Pubhshing Co., Amsterdam
[ N o t to be reproduced by photoprint or m i c r o f i l m without written permission from the p u b h s h e r
A P R O P O S A L FOR E X P E R I M E N T S IN LOW-ENERGY ANTINEUTRINO PHYSICS L. A. MIKAELIAN, P. E. SPIVAK and V. G. TSINOYEV
I. V. Kurchatov Atomic Energy Institute, Moscow, USSR Recewed 7 October 1964 Abstract: A programme for studying interactions of low-energy antmeutrmos generated by a special nuclear pulse reactor is proposed. Apart from the experiments m ~e--e scattering, the programme includes the tnvestlgatton of electromagnetic properties of ~e and the reverse fl decay of proton and other nucleL
Neutrino (v) and antineutrino (,5) are known to occur in the decay of different elementary particles. Theoretically, there is no doubt that there must also exist the absorption processes inverse to those involving v and ,3. Two of them have already been observed in experiment, the reverse beta-decay of a nucleon 1, 3) and the process inverse to the absorption of a muon by a nucleon 2, 3). It is not known whether there are direct interactions of v and ~ with other elementary partacles. One of the most urgent and complicated problems is probably the detection of the corresponding scattering. We have considered the possibility of observing the scattering of low energy ~e on the electrons. According to our estimation, the minimum cross section value which can be detected in this experiment is 2 - 3 . 10 -4.6 c m z. A special reactor is proposed to carry out these experiments. In a series of experiments in an ear-reactor flux 1.3 • 1013 ~e/Cm2. sec, Reines and Cowan i) detected the inverse beta-decay of a nucleon v e + P -~ n + e+
(1)
and measured its cross section. This reaction is convenient for observation. Using multiple coincidences the authors reduced the background to a fraction of the effect. The ~ e - e scattering ~e+e + -~ V + e - .
(2)
is much more difficult to detect. Here the particle under observation is only a recoil electron. The cross section of the process calculated by the universal weak interaction theory 4) at low energies is smaller by about two orders of magnitude than the cross section of the reaction (1). Most of the recoil electrons are in the energy region below 2 MeV where natural radioactivity is quite high. Under these circumstances the effect-to-background ratio is expected to be about 10-5. 574
ANTINEUTRINO PHYSICS
575
These difficulties can be surmounted by building a special nuclear reactor which will have the following two main features. The reactor will generate ~o of higher energies than those resulting from the usual decay of fission fragments. This can be achieved if some neutrons are absorbed in Li 7 capturing thermal neutrons with a cross section of 33 mb. The Li 8 produced in this process decays with a lifetime of 0.8 sec and emits ~e with an endpomt energy 13 MeV. The ~ - e scattering cross section in the region under study increases linearly with the ~ neutrino energy. Furthermore, the number of fast recoil electrons will increase, and this will considerably decrease the detector background. The short lifetime of L18 makes it possible to use the reactor under pulsed operating conditions. This idea, suggested by S. M. Feinberg, will help to improve greatly the effect-to-background ratio at a relatively small average power of the reactor. Feinberg and Shevelev have estimated that the average power of the reactor must be 5.104-105 kW in order to have a flux of 1.5 × 3.1015 ~Jcm 2 -sec (behind sufficient shielding) during a one second pulse (the pulse frequency repetition being 10 times in 24 hours). The averaging of the cross section of ~ o - e scattering in the spectrum of ~ of the decay of Li 8 yields 36 • 10 - 4 5 c m 2, and the cross section of the process leading to recoil electrons with an energy above 2 MeV is 13 • 10 - 4 5 c m 2. When choosing scintillating material for the target in this experament, the following consideration should be taken into account: simultaneously with scattering on electrons neutrinos may cause inverse beta decay processes in the detector which will induce positons and gamma-quanta. To decrease this background, it is desirable to choose a scintillator with a high atomic number. This is desirable also because the inverse beta-decay cross section for most not too light nuclei as 1-3 orders of magnitude smaller than that for hydrogen in the energy region considered. Estimates show that the reverse beta-decay probability for NaI is not larger than 30 ~ of the probability for the appearance of recoil electrons from ~ - e scattering. To decrease further this background as well as other ones it is advisable to construct the detector as a set of cylindrical scintillators placed close to one another. Each scintillator can be seen by an individual photomultiplier in anticoincidence with all the others. The idea of sectioning a neutrino detector was suggested in a different form by Reines a) who proposed the use of organic scintillators. Now let us consider the effect and background values in the ~ fluxes given above. In a detector containing 2.6 x 1029 electrons ( ~ 1000 kg of NaI) 40 to 80 recoil electrons are observed in the range from 2 to 5 MeV during 24 h (i.e., in l0 sec of actual operation). Calculations show that unless special background suppression measures are taken, the number of background counts in this energy interval is 250 per sec. The sectioning of the detector will probably decrease this value to 20 to 40 counts per sec. In 24 h of operation the effect may be measured with a statistical accuracy of 15 to 30 ~. It is relevant that the above reactor offers wide opportunities in antineutrino
576
L.A. MIKAELIANet aL
physics. With its aid it will be possible, for example, to begin a systematic study of the inverse beta decay processes of nuclei. It is particularly interesting to carry out precise measurements of the cross section of the inverse beta decay of the nucleon, which will permit a more accurate determination of the absolute value of the axial vector interaction constant. The inverse beta-decay experiments can be carried out not only as Reines and Cowan ~) &d, but also by determining the radioactive nuclei produced (e.g. in the transitions Na 23 -+ Ne 23 and Li 6 --* He6). As far as is known, the antineutrino does not cause the transformation of a neutron into a proton. The detection of the transformatmns C 13 ~ Y 13, F 19 --~ Ne 19 and some others will permit (as estimations show) checking this circumstance more accurately than Davis 5) did. The study of the processes ~e+Z ~ Z+ve+7, ~e+Z ~ Z+~e+e ++e-,
whose amplitudes contain the product of the constants of electromagnetic and weak interactions is of great interest. The contribution of these amplitudes to ~ - e scattering is also important. It should be pointed out that it is not only as a source of antineutrinos that the above reactor may be used. I f the reactor is charged with Cu 63, for example, fluxes of monochromatic neutrinos (constant in time) can be obtained with an energy of 1.7 MeV, their intensity reaching 3 to 5.10 x2 ve/cm2 • sec. This circumstance may be of interest for investigations of solar neutrinos coming to the earth. The authors are grateful to S. M. Feinberg, Ya. V. Shevelev, B. M. Pontecorvo, V. P. Dzhelepov, L. B. Okun, I. S. Shapiro, I. Ya. Pomeranchuk and J. V. Gaponov for numerous discussions. The authors are highly grateful to A. P. Alexandrov for valuable discussions and his attention to the work.
References 1) F. Remes, Ann. Rev. Nucl. Sci. 10 (1960) 2) G. Danby et al., Phys. Rev. Lett. 9 (1962) 36 3) H. H. Blgham, et al., Proc. Int. Conf. Elem. Particles, Siena (1963) Vol. 1, p. 555; G. Bernardmi et al., ibid p. 571 4) R. Feynman and M. Gell-Mann, Phys. Rev. 109 (1958) 193 5) R. Davis, Bull. Am. Phys. Soc. 1 (1956)
[
Nuclear Physics 70 (1965) 574--576; (~) North-Holland Pubhshing Co., Amsterdam
[ N o t to be reproduced by photoprint or m i c r o f i l m without written permission from the p u b h s h e r
A P R O P O S A L FOR E X P E R I M E N T S IN LOW-ENERGY ANTINEUTRINO PHYSICS L. A. MIKAELIAN, P. E. SPIVAK and V. G. TSINOYEV
I. V. Kurchatov Atomic Energy Institute, Moscow, USSR Recewed 7 October 1964 Abstract: A programme for studying interactions of low-energy antmeutrmos generated by a special nuclear pulse reactor is proposed. Apart from the experiments m ~e--e scattering, the programme includes the tnvestlgatton of electromagnetic properties of ~e and the reverse fl decay of proton and other nucleL
Neutrino (v) and antineutrino (,5) are known to occur in the decay of different elementary particles. Theoretically, there is no doubt that there must also exist the absorption processes inverse to those involving v and ,3. Two of them have already been observed in experiment, the reverse beta-decay of a nucleon 1, 3) and the process inverse to the absorption of a muon by a nucleon 2, 3). It is not known whether there are direct interactions of v and ~ with other elementary partacles. One of the most urgent and complicated problems is probably the detection of the corresponding scattering. We have considered the possibility of observing the scattering of low energy ~e on the electrons. According to our estimation, the minimum cross section value which can be detected in this experiment is 2 - 3 . 10 -4.6 c m z. A special reactor is proposed to carry out these experiments. In a series of experiments in an ear-reactor flux 1.3 • 1013 ~e/Cm2. sec, Reines and Cowan i) detected the inverse beta-decay of a nucleon v e + P -~ n + e+
(1)
and measured its cross section. This reaction is convenient for observation. Using multiple coincidences the authors reduced the background to a fraction of the effect. The ~ e - e scattering ~e+e + -~ V + e - .
(2)
is much more difficult to detect. Here the particle under observation is only a recoil electron. The cross section of the process calculated by the universal weak interaction theory 4) at low energies is smaller by about two orders of magnitude than the cross section of the reaction (1). Most of the recoil electrons are in the energy region below 2 MeV where natural radioactivity is quite high. Under these circumstances the effect-to-background ratio is expected to be about 10-5. 574
ANTINEUTRINO PHYSICS
575
These difficulties can be surmounted by building a special nuclear reactor which will have the following two main features. The reactor will generate ~o of higher energies than those resulting from the usual decay of fission fragments. This can be achieved if some neutrons are absorbed in Li 7 capturing thermal neutrons with a cross section of 33 mb. The Li 8 produced in this process decays with a lifetime of 0.8 sec and emits ~e with an endpomt energy 13 MeV. The ~ - e scattering cross section in the region under study increases linearly with the ~ neutrino energy. Furthermore, the number of fast recoil electrons will increase, and this will considerably decrease the detector background. The short lifetime of L18 makes it possible to use the reactor under pulsed operating conditions. This idea, suggested by S. M. Feinberg, will help to improve greatly the effect-to-background ratio at a relatively small average power of the reactor. Feinberg and Shevelev have estimated that the average power of the reactor must be 5.104-105 kW in order to have a flux of 1.5 × 3.1015 ~Jcm 2 -sec (behind sufficient shielding) during a one second pulse (the pulse frequency repetition being 10 times in 24 hours). The averaging of the cross section of ~ o - e scattering in the spectrum of ~ of the decay of Li 8 yields 36 • 10 - 4 5 c m 2, and the cross section of the process leading to recoil electrons with an energy above 2 MeV is 13 • 10 - 4 5 c m 2. When choosing scintillating material for the target in this experament, the following consideration should be taken into account: simultaneously with scattering on electrons neutrinos may cause inverse beta decay processes in the detector which will induce positons and gamma-quanta. To decrease this background, it is desirable to choose a scintillator with a high atomic number. This is desirable also because the inverse beta-decay cross section for most not too light nuclei as 1-3 orders of magnitude smaller than that for hydrogen in the energy region considered. Estimates show that the reverse beta-decay probability for NaI is not larger than 30 ~ of the probability for the appearance of recoil electrons from ~ - e scattering. To decrease further this background as well as other ones it is advisable to construct the detector as a set of cylindrical scintillators placed close to one another. Each scintillator can be seen by an individual photomultiplier in anticoincidence with all the others. The idea of sectioning a neutrino detector was suggested in a different form by Reines a) who proposed the use of organic scintillators. Now let us consider the effect and background values in the ~ fluxes given above. In a detector containing 2.6 x 1029 electrons ( ~ 1000 kg of NaI) 40 to 80 recoil electrons are observed in the range from 2 to 5 MeV during 24 h (i.e., in l0 sec of actual operation). Calculations show that unless special background suppression measures are taken, the number of background counts in this energy interval is 250 per sec. The sectioning of the detector will probably decrease this value to 20 to 40 counts per sec. In 24 h of operation the effect may be measured with a statistical accuracy of 15 to 30 ~. It is relevant that the above reactor offers wide opportunities in antineutrino
576
L.A. MIKAELIANet aL
physics. With its aid it will be possible, for example, to begin a systematic study of the inverse beta decay processes of nuclei. It is particularly interesting to carry out precise measurements of the cross section of the inverse beta decay of the nucleon, which will permit a more accurate determination of the absolute value of the axial vector interaction constant. The inverse beta-decay experiments can be carried out not only as Reines and Cowan ~) &d, but also by determining the radioactive nuclei produced (e.g. in the transitions Na 23 -+ Ne 23 and Li 6 --* He6). As far as is known, the antineutrino does not cause the transformation of a neutron into a proton. The detection of the transformatmns C 13 ~ Y 13, F 19 --~ Ne 19 and some others will permit (as estimations show) checking this circumstance more accurately than Davis 5) did. The study of the processes ~e+Z ~ Z+ve+7, ~e+Z ~ Z+~e+e ++e-,
whose amplitudes contain the product of the constants of electromagnetic and weak interactions is of great interest. The contribution of these amplitudes to ~ - e scattering is also important. It should be pointed out that it is not only as a source of antineutrinos that the above reactor may be used. I f the reactor is charged with Cu 63, for example, fluxes of monochromatic neutrinos (constant in time) can be obtained with an energy of 1.7 MeV, their intensity reaching 3 to 5.10 x2 ve/cm2 • sec. This circumstance may be of interest for investigations of solar neutrinos coming to the earth. The authors are grateful to S. M. Feinberg, Ya. V. Shevelev, B. M. Pontecorvo, V. P. Dzhelepov, L. B. Okun, I. S. Shapiro, I. Ya. Pomeranchuk and J. V. Gaponov for numerous discussions. The authors are highly grateful to A. P. Alexandrov for valuable discussions and his attention to the work.
References 1) F. Remes, Ann. Rev. Nucl. Sci. 10 (1960) 2) G. Danby et al., Phys. Rev. Lett. 9 (1962) 36 3) H. H. Blgham, et al., Proc. Int. Conf. Elem. Particles, Siena (1963) Vol. 1, p. 555; G. Bernardmi et al., ibid p. 571 4) R. Feynman and M. Gell-Mann, Phys. Rev. 109 (1958) 193 5) R. Davis, Bull. Am. Phys. Soc. 1 (1956)