- Email: [email protected]
A search for time-parity violation in the beta decay of polarized neutrons
Volume 27B, number
PHYSICS
9
A SEARCH
FOR
30 September
LETTERS
TIME-PARITY VIOLATION IN OF POLARIZED NEUTRONS
B. G. EROZOLIMSKY,
THE
L. N. BONDARENKO, YU. A. MOSTOVOY, V. P. ZACHAROVA and V. A. TITOV
I. V. Kurchatov
Institute
of Atomic
Received
Energy,
BETA
1968
DECAY
B. A. OBINYAKOV,
Moscow, USSR
26 July 1968
The angular correlation between the directions of escape of an electron and neutrino and neutron spin in the beta decay of polarized neutrons is measured accurately. It is shown that time parity is not violated within the experimental error. The corresponding phase difference between the beta-decay constants gA andgv is 8 = 178.7 * 1.3 degrees.
The discovery of the CP-parity violation effect in K-meson decays [l] intensified the experimenter’s interest to verify time-parity in weak interactions and, in particular, to search in beta decay for the triple correlation between the vectors of electron momentum pe, neutrino momentum pv and the spin of the decaying nucleus Q [2-41. The above correlation is characterized by the coefficient D, which is largely determined by the imaginary parts of the weak interaction constants so that its deviation from zero would mean timeparity violation in the process under consideration. Burgy et al. [3] have obtained the value D = = -0.04 f 0.05 for the triple correlation in the beta decay of neutrons, which corresponds to a phase angle 8 of 175 * 6 degrees between the complex values gA and g,. In the present study an attempt is made to attain an essentially higher level of accuracy in measurements of this type. The experiment reduces to counting coincidences between electron and recoil protons for two opposite spin orientations of the decaying polarized neutrons. The arrangement of the detectors and the direction of polarization is chosen so as to make the vectors pe, pv and amutually orthogonal. The coincidence counting rates (N.+ and N_) for opposite spin directions are related as N+ - N_
N, + N_
=KPD
tual experimental conditions is taken into account). A neutron beam from a reactor is reflected off magnetized cobalt mirrors. The neutron intensity is 3 X 107 s-l, and the polarization is = 85 %. The spin direction is changed by 180° by non-adiabatic inversion with the aid of current foils without changing the guide magnetic field direction in the chamber [5]. Recoil protons are detected by two CsI crystal scintillation counters 20pm thick fixed opposite each other on both sides of the beam (see fig. 1). The P-detectors, each consisting of five plastic scintillators with multipliers, are mounted normal to the proton counters. IY
,
where P is the beam polarization, and K an instrumental coefficient including averaging over the values of v/c of the electrons and over the angles between the vectors Pe, pv and u (i.e. , the deviation of these angles from 90° under ac-
Fig. 1. Experimental arrangement; cylinder, 2: external grid cylinder, trode, 4: spherical grid, 5: vacuum (Tl) crystal 20 pm thick. A magnetic applied along the beam
1: internal grid 3: spherical elecchamber, 6: CsI field of 2 Oe is axis.
557
Volume 27B, number 9
PHYSICS
LETTERS
30 September 1968
counter and each beta detector are registered by four separate gate-type circuits opening a time to after the P-electron signal and waiting for the signal from the corresponding recoil proton till t max. It can be readily shown that a system of this type automatically selects those decay events for which a neutrino pulse lies within a cone of angles the axis of which is parallel to the axis of cylinder 1 no matter where the neutron decays. About 60 neutron beta decays per hour are registered by the four coincidence circuits, with 15 to 20 accidental coincidences per hour. The following numbers are summed up to calculte the investigated effect N+ = Ni31Pl”
Fig. 2. Coincidence electron and recoil (a) electron energy Ee = 150-550 keV; (b) electron energy (c) no accelerating
counting rate versus delay between proton signals; interval registered by P-detector interval Ee = 350-550 keV; voltage on the electrode 3 (fig. 1).
Neutron decays are detected inside a grid cylinder 1 to make sure that no electric fields change the recoil proton velocity. The space inside the cylinder, free from electric fields, is used as the flight interval for recoil protons, so that decay events with neutrino emission in a definite angular cone can be identified by flight times. A potential difference of 800 V is applied between grid cylinder 1 and external cylinder 2 to avoid losing protons reaching the grid cylinder. The cylindrical mirror formed by the two electrodes repulses the protons back into cylinder 1 without changing their axial velocity component. The protons reaching the butt ends of cylinder 1 are caught by accelerating spherical field of electrodes 3 and 4 focussing them on counter crystal 6. The coincidence pulses from each proton 558
r”;32P2 + N’I3,p2
+ N’L32P1
where the unprimed values Npp correspond to the coincidences stored for all four combinations of proton and electron counters with one neutron spin direction while the primed numbers correspond to the same with the opposite neutron spin direction. A system of this kind, consisting of two pairs of counters, strongly symmetrizes the experimental arrangement and makes it much less sensitive to all kinds of guid magnetic field distortions and detector efficiency changes. The dependence of the coincidence counting rate versus delay time t between electron and proton counter pulses is used to test the operation of the setup. The results obtained for two intervals of electron energies E, are represented in figs. 3a and 3b. The dotted lines represent the calculated curves normalized over the area according to experimental data in the upper part of fig. 3 (Ee = 150 - 550 keV). The results are in good agreement with the calculations. It is clear from the figure that the peak of spurious coincidences near a zero-delay t can be reliably separted from the real events corresponding to the decay of neutrons. The operating time interval in the gates of the coincidence circuits is 0.6 - 1.4 ps. The data presented in fig. 3c have been obtained without applying a proton accelerating voltage of 25 kV. A similar result has been obtained by placing a cadmium foil inside the beam. These data confirm the fact that the events detected within the operating time delay interval are practically entirely due to neutron decays. About 80000 decay events have been registered so far.
PHYSICS
Volume 27B, number 9 The asymmetry
observed
KPD
= +0.004
LETTERS
is f 0.004
The authors wish to thank Professor P. Ye. Spivak for his sponsorship of, and constant interest in, the present study.
.
The measurement of the neutron polarization coefficient yields P = 0.85 * 0.10. The instrumental coefficient is K = 0.47 k 0.03. Hence the value of the correlation constant D is D = +O.Ol ix 0.01
30 September 1968
.
This corresponds to a phase angle between the axial and vector constant 0 = 178.7 f 1.3 degrees. It should be noted that the error is statistical and corresponds to one standard deviation.
References 1. J. H.Christenson, J. W. Cronin, V. L. Fitch and R. Turlay, Phys. Rev. Letters 13 (1964) 138. 2. M.A.CIark et al., Phys.Rev. Letters 1 (1958) 100. 3. M.T.Burgy et al., Phys. Rev. 120 (1960) 1829. 4. F. P. Calaprice et al., Phys. Rev. Letters 18 (1967) 918. 5. M.H.Beil, P.Carlos and J.Matuszek, J.de Physique 24 (1963) 359.
*****
559
PHYSICS
9
A SEARCH
FOR
30 September
LETTERS
TIME-PARITY VIOLATION IN OF POLARIZED NEUTRONS
B. G. EROZOLIMSKY,
THE
L. N. BONDARENKO, YU. A. MOSTOVOY, V. P. ZACHAROVA and V. A. TITOV
I. V. Kurchatov
Institute
of Atomic
Received
Energy,
BETA
1968
DECAY
B. A. OBINYAKOV,
Moscow, USSR
26 July 1968
The angular correlation between the directions of escape of an electron and neutrino and neutron spin in the beta decay of polarized neutrons is measured accurately. It is shown that time parity is not violated within the experimental error. The corresponding phase difference between the beta-decay constants gA andgv is 8 = 178.7 * 1.3 degrees.
The discovery of the CP-parity violation effect in K-meson decays [l] intensified the experimenter’s interest to verify time-parity in weak interactions and, in particular, to search in beta decay for the triple correlation between the vectors of electron momentum pe, neutrino momentum pv and the spin of the decaying nucleus Q [2-41. The above correlation is characterized by the coefficient D, which is largely determined by the imaginary parts of the weak interaction constants so that its deviation from zero would mean timeparity violation in the process under consideration. Burgy et al. [3] have obtained the value D = = -0.04 f 0.05 for the triple correlation in the beta decay of neutrons, which corresponds to a phase angle 8 of 175 * 6 degrees between the complex values gA and g,. In the present study an attempt is made to attain an essentially higher level of accuracy in measurements of this type. The experiment reduces to counting coincidences between electron and recoil protons for two opposite spin orientations of the decaying polarized neutrons. The arrangement of the detectors and the direction of polarization is chosen so as to make the vectors pe, pv and amutually orthogonal. The coincidence counting rates (N.+ and N_) for opposite spin directions are related as N+ - N_
N, + N_
=KPD
tual experimental conditions is taken into account). A neutron beam from a reactor is reflected off magnetized cobalt mirrors. The neutron intensity is 3 X 107 s-l, and the polarization is = 85 %. The spin direction is changed by 180° by non-adiabatic inversion with the aid of current foils without changing the guide magnetic field direction in the chamber [5]. Recoil protons are detected by two CsI crystal scintillation counters 20pm thick fixed opposite each other on both sides of the beam (see fig. 1). The P-detectors, each consisting of five plastic scintillators with multipliers, are mounted normal to the proton counters. IY
,
where P is the beam polarization, and K an instrumental coefficient including averaging over the values of v/c of the electrons and over the angles between the vectors Pe, pv and u (i.e. , the deviation of these angles from 90° under ac-
Fig. 1. Experimental arrangement; cylinder, 2: external grid cylinder, trode, 4: spherical grid, 5: vacuum (Tl) crystal 20 pm thick. A magnetic applied along the beam
1: internal grid 3: spherical elecchamber, 6: CsI field of 2 Oe is axis.
557
Volume 27B, number 9
PHYSICS
LETTERS
30 September 1968
counter and each beta detector are registered by four separate gate-type circuits opening a time to after the P-electron signal and waiting for the signal from the corresponding recoil proton till t max. It can be readily shown that a system of this type automatically selects those decay events for which a neutrino pulse lies within a cone of angles the axis of which is parallel to the axis of cylinder 1 no matter where the neutron decays. About 60 neutron beta decays per hour are registered by the four coincidence circuits, with 15 to 20 accidental coincidences per hour. The following numbers are summed up to calculte the investigated effect N+ = Ni31Pl”
Fig. 2. Coincidence electron and recoil (a) electron energy Ee = 150-550 keV; (b) electron energy (c) no accelerating
counting rate versus delay between proton signals; interval registered by P-detector interval Ee = 350-550 keV; voltage on the electrode 3 (fig. 1).
Neutron decays are detected inside a grid cylinder 1 to make sure that no electric fields change the recoil proton velocity. The space inside the cylinder, free from electric fields, is used as the flight interval for recoil protons, so that decay events with neutrino emission in a definite angular cone can be identified by flight times. A potential difference of 800 V is applied between grid cylinder 1 and external cylinder 2 to avoid losing protons reaching the grid cylinder. The cylindrical mirror formed by the two electrodes repulses the protons back into cylinder 1 without changing their axial velocity component. The protons reaching the butt ends of cylinder 1 are caught by accelerating spherical field of electrodes 3 and 4 focussing them on counter crystal 6. The coincidence pulses from each proton 558
r”;32P2 + N’I3,p2
+ N’L32P1
where the unprimed values Npp correspond to the coincidences stored for all four combinations of proton and electron counters with one neutron spin direction while the primed numbers correspond to the same with the opposite neutron spin direction. A system of this kind, consisting of two pairs of counters, strongly symmetrizes the experimental arrangement and makes it much less sensitive to all kinds of guid magnetic field distortions and detector efficiency changes. The dependence of the coincidence counting rate versus delay time t between electron and proton counter pulses is used to test the operation of the setup. The results obtained for two intervals of electron energies E, are represented in figs. 3a and 3b. The dotted lines represent the calculated curves normalized over the area according to experimental data in the upper part of fig. 3 (Ee = 150 - 550 keV). The results are in good agreement with the calculations. It is clear from the figure that the peak of spurious coincidences near a zero-delay t can be reliably separted from the real events corresponding to the decay of neutrons. The operating time interval in the gates of the coincidence circuits is 0.6 - 1.4 ps. The data presented in fig. 3c have been obtained without applying a proton accelerating voltage of 25 kV. A similar result has been obtained by placing a cadmium foil inside the beam. These data confirm the fact that the events detected within the operating time delay interval are practically entirely due to neutron decays. About 80000 decay events have been registered so far.
PHYSICS
Volume 27B, number 9 The asymmetry
observed
KPD
= +0.004
LETTERS
is f 0.004
The authors wish to thank Professor P. Ye. Spivak for his sponsorship of, and constant interest in, the present study.
.
The measurement of the neutron polarization coefficient yields P = 0.85 * 0.10. The instrumental coefficient is K = 0.47 k 0.03. Hence the value of the correlation constant D is D = +O.Ol ix 0.01
30 September 1968
.
This corresponds to a phase angle between the axial and vector constant 0 = 178.7 f 1.3 degrees. It should be noted that the error is statistical and corresponds to one standard deviation.
References 1. J. H.Christenson, J. W. Cronin, V. L. Fitch and R. Turlay, Phys. Rev. Letters 13 (1964) 138. 2. M.A.CIark et al., Phys.Rev. Letters 1 (1958) 100. 3. M.T.Burgy et al., Phys. Rev. 120 (1960) 1829. 4. F. P. Calaprice et al., Phys. Rev. Letters 18 (1967) 918. 5. M.H.Beil, P.Carlos and J.Matuszek, J.de Physique 24 (1963) 359.
*****
559