- Email: [email protected]
Test of time reversal invariance with TRINE
Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
Test of time reversal invariance with TRINE T. Soldner!,*, L. Beck!, K. Schreckenbach!, A. Bussie`re", R. Kossakowski", P. Liaud#, O. Zimmer$ !Physik-Department E21, Technische UniversitaK t Mu( nchen, 85747 Garching, Germany "Laboratoire d'Annecy-le-Vieux de Physique Particules, 74941 Annecy-le-Vieux, France #Institut des Sciences Nucle& aires, 38026 Grenoble, France $Institut Laue Langevin, 38042 Grenoble, France
Abstract The new detector TRINE (time reversal invariance neutron experiment) was developed to test the time reversal invariance in the neutron decay. The precision of former experiments can be improved by one order of magnitude with an improved proton detection, a better background suppression and an angular resolving measurement using multiwire proportional chambers in coincidence with plastic scintillators, and the higher neutron #ux and polarization available today. The concept of the detector and the status of the project is discussed. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 11.30.Er; 23.20.En; 23.40.!s Keywords: Discrete symmetries; Standard Model; Beta decay; Time reversal; Weak interaction
1. Introduction The discovery of parity violation in 1957 triggered a variety of searches for violations of the three fundamental discrete symmetries P (parity), C (charge), and T (time reversal). Whereas in the case of P and C a class of violating processes was discovered (b-decay), up to now a violation of time reversal invariance could be found in the decay of neutral kaons only, and the origin of this violation is not understood. In the decay of polarized, free neutrons two coef"cients D and R are de"ned, for which a non-zero
* Corresponding author.
value indicates T violation [1]. D is the coe$cient of the T-odd and P-even triple correlation d= r ) (p ]p 6 ) l #2 &1#D / % dE dX dX 6 E E6 % % l % l
(1)
of the neutron spin r and the momenta p and / % p 6 of the emitted electron and antineutrino. = is l the decay probability, X are the solid angles, and i E are the energies of the emitted particles. Api plying T to this triple product is equivalent to spin inversion. Hence, a spin-dependent correlation between electron and antineutrino momenta would indicate T violation. At present the value for D is limited to 3]10~3 by experiments with free neutrons [2,3] and to 1]10~3 by an experiment on 19Ne [4]. It should
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 5 5 - 4
SECTION 4.
644
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
be noted that "nal state e!ects simulate a value of 2]10~4 for 19Ne but only of 2]10~5 for neutrons. Since a number of theories predict values for D in the range of 10~4 (see e.g. Refs. [5}7]) more precise direct measurements of D are of great interest. 2. Detector set-up 2.1. Requirements To determine D the angular correlation between electron and antineutrino with respect to the neutron spin has to be measured. Since the antineutrino cannot be detected directly, its momentum is derived from the proton using momentum conservation. The triple product shows its maximum and highest sensitivity to D at right angles between electron and proton momenta. Hence, in the previous experiments an arrangement of two proton and two electron detectors around the longitudinally polarized neutron beam and perpendicular to each other was used. However, larger angles between proton and electron are preferred due to phase space arguments [8]. For a detailed investigation of systematic errors an angular resolving measurement is of interest. 2.2. The TRINE set-up The set-up used in the TRINE detector is given in Fig. 1. It permits the resolved measurement of angles between 453 and 1353 averaged on the solid angle of one proton detector. The neutrons are longitudinally polarized. The polarization is guided by a weak magnetic "eld in beam direction. The electrons are detected by two plastic scintillators (SCI) in coincidence with multiwire proportional chambers (MWPC). The MWPCs allow an angular resolved measurement and improve the background suppression due to their low c e$ciency. The previous experiments used only scintillators for the electron detection. Four MWPCs with a total of 240 anodes and 100 cathodes are used. CF is applied as counting gas 4 allowing a low operating pressure of 100 mbar only. This low pressure is essential since it permits to use very thin mylar foils (6 lm) for the separation of the
Fig. 1. Cross-section of the TRINE detector. The neutron beam passes the detector perpendicular to the plane of the drawing.
counting gas and the evacuated neutron beam chamber. The protons are detected after acceleration in a focussing electrostatic "eld of about 30 kV by four proton detectors in each of the thus de"ned detector planes. The energies of the particles are determined by the pulse height of the SCI signal for the electrons and by time-of-#ight for the protons, respectively. The set-up is symmetric with respect to the neutron beam, thus suppressing asymmetries caused by parity violation in neutron decay (A and B coe$cients) in "rst order. Dividing for instance the SCIs into their lower and upper halfs using the coincidence with the MWPCs four electron detectors in one detector plane are de"ned. This gives four combinations with an average angle of 603, eight combinations with right angles, and four combinations with 1203 between p and p . This high num% 1 ber of di!erent coincidences allows cross-checks and a precise determination of the systematic effects. In the "nal experiment 16 detector planes will be used to improve the statistics covering a neutron decay volume of 40 cm length. The neutron beam diameter will be about 3 cm.
3. Proton detection Due to kinematics, the endpoint of the proton recoil energy spectrum is only E "751 eV. Hence, 1
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
645
Fig. 2. Spectra of background (a) and 27 keV protons (b) with a phosphor screen and of 20 keV protons with a PIN diode (c).
the protons cannot be detected directly but have to be accelerated. This is typically done by an electrostatic potential of about 30 kV. The former experiments detected the protons by thin scintillating layers (NaI, CsI) which were deposited directly on a photomultiplier window. This method o!ers a poor separation of signal to noise. Since these scintillators are hygroscopic they are di$cult to handle. For these reasons, alternative methods were investigated. Two alternatives were checked: phosphor screens and ion implanted PIN diodes with very thin dead layers. The parameters of a phosphor screen can be controlled via its composition. Due to the high light output (1 photon per 11 eV electron energy) and the short light decay time (55 ns), P47 (YSiO:Ce) was tested. The smallest available single crystals (1 lm diameter) were used giving a layer thickness of about 3 lm only. This thin layer reduces the sensitivity to the c-background. Test spectra for background and 27 keV protons obtained at the Munich accelerator are shown in Fig. 2.
The tested PIN diodes were developed for highresolution a spectroscopy. In the recent years, PIN diodes with very thin entrance windows have become available. This is essential since the range of 20 keV protons in Si is 200 nm only. PIN diodes with dead layers of 25 and 50 nm, an active area of 100 mm2, and an intrinsic layer of 300 lm (Canberra PD/TW-100-13-100-AM and Eurisys PLUS 100-12 EBF) were tested. A spectrum of 20 keV protons is given in Fig. 2. The weak signal of the PIN diodes (about 7]10~4 pC) requires preampli"ers with a very low noise. In the test measurements, the noise of the used preampli"er (Canberra 2003) connected to the PIN diode was found to be equivalent to 4.2 keV. For the TRINE detector special preampli"ers were developed due to space limitations. The spectra show that the PIN diodes are the best choice for the proton detection. They exhibit a good separation of signal to noise and an e$ciency of almost 100%. In the TRINE detector, 64 PIN diodes with 25 nm entrance windows from Canberra will be used. SECTION 4.
646
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
4. Data acquisition A simpli"ed #ow chart of the data acquisition system is given in Fig. 3. The SCI signal is split into a fast signal and a slow energy signal by a modi"ed main ampli"er (Ortec 855). If the fast signal exceeds the threshold, the MWPCs are gated. The MWPCs are read out using the RMH (receiver memory hybrid) system developed at CERN [9]. This system provides separate monitor pulses for each MWPC. Only coincident pulses from the SCI and the two MWPCs belonging to this SCI give a valid event, start the time to amplitude converter (TAC) for the time of #ight measurement and open the gates for the readout of the RMH system and the other data acquisition. This prevents events with an incomplete electron detection from loading the data acquisition system and thus reduces the dead time. The slow energy signal from the SCI is connected to an ADC (CAEN V556). The main ampli"ers of the PIN diodes (developed at ISN) give a logic pulse if the signal exceeds the threshold. The logic signals are connected to two gated 32-bit pattern units (Struck 7137). They allow the identi"cation of the triggering PIN diode and provide a fast OR of the signals which stops the TAC. The analogue signals of the PIN diodes are multiplexed and connected to the ADC for control purposes only. The TAC output signal is connected to the ADC, too. To summarize, for each event the following signals have to be registered: the analogue signals of the SCI, the triggering PIN diode and the TAC, the number of the triggering PIN diode (pattern units),
and the numbers of the hit wires (RMH). All read out is done by a VME system. For control purposes all hits of the SCIs, the wire chambers, and the individual PIN diodes are counted independently of coincidence conditions using three 32 channel 32-bit counters (SIS 3801). This gives the possibility to identify hardware problems very fast. Furthermore, all (incomplete) events with a complete electron detection but without a proton signal are registered which permits to test the electron part of the detector with a higher statistics and to "nd related detector asymmetries.
5. Comparison The main advantages of the TRINE detector compared with the prior D measurements are the better suppression of the c background by the MWPCs, the e$cient proton detection with PIN diodes, the better angular resolution and additional possibilities for cross-checks. These improvements in the detector are combined with the progress in neutron technique: the higher #ux and the almost perfect neutron polarization available today using a bender-type supermirror polarizer [10]. Some important data of the latest D measurements and TRINE are given in Table 1. The smaller beam size of TRINE allows a better angular resolution and a good homogeneity of the neutron beam and the guiding "eld. In spite of this smaller beam size the estimated total count rate is higher due to the higher neutron #ux. The polarized neutron capture #ux of 109 n s~1 cm~2 in the
Table 1 Comparison of the latest D measurements. For TRINE the estimated data for the complete detector are given
Fig. 3. Simpli"ed #ow chart of data acquisition.
Ref. [2]
Ref. [3]
TRINE
Beam size (cm2) Flux (pol.) (n s~1 cm~2) Polarization (%) Count rate (s~1) Total events (106)
5]8 1.6]107 70$7 1.5 5.9
14]2 3.6]107 68$3 0.8 2.5
2]4 +109 +98 +30 ??
D (10~3)
!1.1$1.7
2.2$3.0
??
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
decay volume is presently available at the ILL employing an existing focussing supermirror polarizer manufactured at PNPI, Gatchina.
647
Acknowledgements This work is supported by BMBF (grant 06TM879) and associated with the SFB 375.
6. Status and outlook References In the "rst stage, only 12 PIN diodes in 3 detector planes will be used. All components of the detector are available and have been tested separately. The proton detection su!ered from high background caused by hydrogen from the surfaces of the focussing electrode. An improved electrode with graphite covered surfaces will be available in the near future. Presently, the components are plugged together. In the next months test measurements will be carried out at the Munich research reactor FRM. The "rst measurement with polarized neutrons is scheduled for March 1999 at the ILL cold polarized neutron beam facility. The improvements in the detector and neutron technique should allow to improve the sensitivity for D down to the range of 10~4 with the "nal detector version (16 detector planes).
[1] J.D. Jackson, S.B. Treiman, H.W. Wyld, Phys. Rev 106 (1957) 517. [2] R.I. Steinberg, P. Liaud, B. Vignon, V.W. Hughes, Phys. Rev. D 13 (1976) 2469. [3] B.G. Erozolimskii, Yu.A. Mostovoi, V.P. Fedunin, A.I. Frank, O.V. Khakhan, Sov. J. Nucl. Phys. 28 (1978) 48. [4] A.L. Hallin, F.P. Calaprice, D.W. MacArthur, L.E. Piilonen, M.B. Schneider, D.F. Schreiber, Phys. Rev. Lett. 52 (1984) 337. [5] R.N. Mohapatra, J.C. Pati, Phys. Rev. D 11 (1975) 566. [6] P. Herczeg, Phys. Rev. D 28 (1983) 200. [7] P. Herczeg, Beta-decay beyond the standard model, in: H. Henrikson, P. Vogel (Eds.), Fundamental Symmetries in Nuclei and Particles, World Scienti"c, Singapore, 1989, p. 46. [8] E.G. Wasserman, Time reversal invariance in polarized neutron decay, Thesis, Los Alamos National Laboratory, 1994. [9] J.B. Lindsay, C. Millerin, J.C. TarleH , H. Verweij, H. Wendler, Nucl. Instr. Meth. 156 (1978) 329. [10] O. SchaK rpf, Physica B 156 (1989) 639.
SECTION 4.
Test of time reversal invariance with TRINE T. Soldner!,*, L. Beck!, K. Schreckenbach!, A. Bussie`re", R. Kossakowski", P. Liaud#, O. Zimmer$ !Physik-Department E21, Technische UniversitaK t Mu( nchen, 85747 Garching, Germany "Laboratoire d'Annecy-le-Vieux de Physique Particules, 74941 Annecy-le-Vieux, France #Institut des Sciences Nucle& aires, 38026 Grenoble, France $Institut Laue Langevin, 38042 Grenoble, France
Abstract The new detector TRINE (time reversal invariance neutron experiment) was developed to test the time reversal invariance in the neutron decay. The precision of former experiments can be improved by one order of magnitude with an improved proton detection, a better background suppression and an angular resolving measurement using multiwire proportional chambers in coincidence with plastic scintillators, and the higher neutron #ux and polarization available today. The concept of the detector and the status of the project is discussed. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 11.30.Er; 23.20.En; 23.40.!s Keywords: Discrete symmetries; Standard Model; Beta decay; Time reversal; Weak interaction
1. Introduction The discovery of parity violation in 1957 triggered a variety of searches for violations of the three fundamental discrete symmetries P (parity), C (charge), and T (time reversal). Whereas in the case of P and C a class of violating processes was discovered (b-decay), up to now a violation of time reversal invariance could be found in the decay of neutral kaons only, and the origin of this violation is not understood. In the decay of polarized, free neutrons two coef"cients D and R are de"ned, for which a non-zero
* Corresponding author.
value indicates T violation [1]. D is the coe$cient of the T-odd and P-even triple correlation d= r ) (p ]p 6 ) l #2 &1#D / % dE dX dX 6 E E6 % % l % l
(1)
of the neutron spin r and the momenta p and / % p 6 of the emitted electron and antineutrino. = is l the decay probability, X are the solid angles, and i E are the energies of the emitted particles. Api plying T to this triple product is equivalent to spin inversion. Hence, a spin-dependent correlation between electron and antineutrino momenta would indicate T violation. At present the value for D is limited to 3]10~3 by experiments with free neutrons [2,3] and to 1]10~3 by an experiment on 19Ne [4]. It should
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 5 5 - 4
SECTION 4.
644
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
be noted that "nal state e!ects simulate a value of 2]10~4 for 19Ne but only of 2]10~5 for neutrons. Since a number of theories predict values for D in the range of 10~4 (see e.g. Refs. [5}7]) more precise direct measurements of D are of great interest. 2. Detector set-up 2.1. Requirements To determine D the angular correlation between electron and antineutrino with respect to the neutron spin has to be measured. Since the antineutrino cannot be detected directly, its momentum is derived from the proton using momentum conservation. The triple product shows its maximum and highest sensitivity to D at right angles between electron and proton momenta. Hence, in the previous experiments an arrangement of two proton and two electron detectors around the longitudinally polarized neutron beam and perpendicular to each other was used. However, larger angles between proton and electron are preferred due to phase space arguments [8]. For a detailed investigation of systematic errors an angular resolving measurement is of interest. 2.2. The TRINE set-up The set-up used in the TRINE detector is given in Fig. 1. It permits the resolved measurement of angles between 453 and 1353 averaged on the solid angle of one proton detector. The neutrons are longitudinally polarized. The polarization is guided by a weak magnetic "eld in beam direction. The electrons are detected by two plastic scintillators (SCI) in coincidence with multiwire proportional chambers (MWPC). The MWPCs allow an angular resolved measurement and improve the background suppression due to their low c e$ciency. The previous experiments used only scintillators for the electron detection. Four MWPCs with a total of 240 anodes and 100 cathodes are used. CF is applied as counting gas 4 allowing a low operating pressure of 100 mbar only. This low pressure is essential since it permits to use very thin mylar foils (6 lm) for the separation of the
Fig. 1. Cross-section of the TRINE detector. The neutron beam passes the detector perpendicular to the plane of the drawing.
counting gas and the evacuated neutron beam chamber. The protons are detected after acceleration in a focussing electrostatic "eld of about 30 kV by four proton detectors in each of the thus de"ned detector planes. The energies of the particles are determined by the pulse height of the SCI signal for the electrons and by time-of-#ight for the protons, respectively. The set-up is symmetric with respect to the neutron beam, thus suppressing asymmetries caused by parity violation in neutron decay (A and B coe$cients) in "rst order. Dividing for instance the SCIs into their lower and upper halfs using the coincidence with the MWPCs four electron detectors in one detector plane are de"ned. This gives four combinations with an average angle of 603, eight combinations with right angles, and four combinations with 1203 between p and p . This high num% 1 ber of di!erent coincidences allows cross-checks and a precise determination of the systematic effects. In the "nal experiment 16 detector planes will be used to improve the statistics covering a neutron decay volume of 40 cm length. The neutron beam diameter will be about 3 cm.
3. Proton detection Due to kinematics, the endpoint of the proton recoil energy spectrum is only E "751 eV. Hence, 1
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
645
Fig. 2. Spectra of background (a) and 27 keV protons (b) with a phosphor screen and of 20 keV protons with a PIN diode (c).
the protons cannot be detected directly but have to be accelerated. This is typically done by an electrostatic potential of about 30 kV. The former experiments detected the protons by thin scintillating layers (NaI, CsI) which were deposited directly on a photomultiplier window. This method o!ers a poor separation of signal to noise. Since these scintillators are hygroscopic they are di$cult to handle. For these reasons, alternative methods were investigated. Two alternatives were checked: phosphor screens and ion implanted PIN diodes with very thin dead layers. The parameters of a phosphor screen can be controlled via its composition. Due to the high light output (1 photon per 11 eV electron energy) and the short light decay time (55 ns), P47 (YSiO:Ce) was tested. The smallest available single crystals (1 lm diameter) were used giving a layer thickness of about 3 lm only. This thin layer reduces the sensitivity to the c-background. Test spectra for background and 27 keV protons obtained at the Munich accelerator are shown in Fig. 2.
The tested PIN diodes were developed for highresolution a spectroscopy. In the recent years, PIN diodes with very thin entrance windows have become available. This is essential since the range of 20 keV protons in Si is 200 nm only. PIN diodes with dead layers of 25 and 50 nm, an active area of 100 mm2, and an intrinsic layer of 300 lm (Canberra PD/TW-100-13-100-AM and Eurisys PLUS 100-12 EBF) were tested. A spectrum of 20 keV protons is given in Fig. 2. The weak signal of the PIN diodes (about 7]10~4 pC) requires preampli"ers with a very low noise. In the test measurements, the noise of the used preampli"er (Canberra 2003) connected to the PIN diode was found to be equivalent to 4.2 keV. For the TRINE detector special preampli"ers were developed due to space limitations. The spectra show that the PIN diodes are the best choice for the proton detection. They exhibit a good separation of signal to noise and an e$ciency of almost 100%. In the TRINE detector, 64 PIN diodes with 25 nm entrance windows from Canberra will be used. SECTION 4.
646
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
4. Data acquisition A simpli"ed #ow chart of the data acquisition system is given in Fig. 3. The SCI signal is split into a fast signal and a slow energy signal by a modi"ed main ampli"er (Ortec 855). If the fast signal exceeds the threshold, the MWPCs are gated. The MWPCs are read out using the RMH (receiver memory hybrid) system developed at CERN [9]. This system provides separate monitor pulses for each MWPC. Only coincident pulses from the SCI and the two MWPCs belonging to this SCI give a valid event, start the time to amplitude converter (TAC) for the time of #ight measurement and open the gates for the readout of the RMH system and the other data acquisition. This prevents events with an incomplete electron detection from loading the data acquisition system and thus reduces the dead time. The slow energy signal from the SCI is connected to an ADC (CAEN V556). The main ampli"ers of the PIN diodes (developed at ISN) give a logic pulse if the signal exceeds the threshold. The logic signals are connected to two gated 32-bit pattern units (Struck 7137). They allow the identi"cation of the triggering PIN diode and provide a fast OR of the signals which stops the TAC. The analogue signals of the PIN diodes are multiplexed and connected to the ADC for control purposes only. The TAC output signal is connected to the ADC, too. To summarize, for each event the following signals have to be registered: the analogue signals of the SCI, the triggering PIN diode and the TAC, the number of the triggering PIN diode (pattern units),
and the numbers of the hit wires (RMH). All read out is done by a VME system. For control purposes all hits of the SCIs, the wire chambers, and the individual PIN diodes are counted independently of coincidence conditions using three 32 channel 32-bit counters (SIS 3801). This gives the possibility to identify hardware problems very fast. Furthermore, all (incomplete) events with a complete electron detection but without a proton signal are registered which permits to test the electron part of the detector with a higher statistics and to "nd related detector asymmetries.
5. Comparison The main advantages of the TRINE detector compared with the prior D measurements are the better suppression of the c background by the MWPCs, the e$cient proton detection with PIN diodes, the better angular resolution and additional possibilities for cross-checks. These improvements in the detector are combined with the progress in neutron technique: the higher #ux and the almost perfect neutron polarization available today using a bender-type supermirror polarizer [10]. Some important data of the latest D measurements and TRINE are given in Table 1. The smaller beam size of TRINE allows a better angular resolution and a good homogeneity of the neutron beam and the guiding "eld. In spite of this smaller beam size the estimated total count rate is higher due to the higher neutron #ux. The polarized neutron capture #ux of 109 n s~1 cm~2 in the
Table 1 Comparison of the latest D measurements. For TRINE the estimated data for the complete detector are given
Fig. 3. Simpli"ed #ow chart of data acquisition.
Ref. [2]
Ref. [3]
TRINE
Beam size (cm2) Flux (pol.) (n s~1 cm~2) Polarization (%) Count rate (s~1) Total events (106)
5]8 1.6]107 70$7 1.5 5.9
14]2 3.6]107 68$3 0.8 2.5
2]4 +109 +98 +30 ??
D (10~3)
!1.1$1.7
2.2$3.0
??
T. Soldner et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 643}647
decay volume is presently available at the ILL employing an existing focussing supermirror polarizer manufactured at PNPI, Gatchina.
647
Acknowledgements This work is supported by BMBF (grant 06TM879) and associated with the SFB 375.
6. Status and outlook References In the "rst stage, only 12 PIN diodes in 3 detector planes will be used. All components of the detector are available and have been tested separately. The proton detection su!ered from high background caused by hydrogen from the surfaces of the focussing electrode. An improved electrode with graphite covered surfaces will be available in the near future. Presently, the components are plugged together. In the next months test measurements will be carried out at the Munich research reactor FRM. The "rst measurement with polarized neutrons is scheduled for March 1999 at the ILL cold polarized neutron beam facility. The improvements in the detector and neutron technique should allow to improve the sensitivity for D down to the range of 10~4 with the "nal detector version (16 detector planes).
[1] J.D. Jackson, S.B. Treiman, H.W. Wyld, Phys. Rev 106 (1957) 517. [2] R.I. Steinberg, P. Liaud, B. Vignon, V.W. Hughes, Phys. Rev. D 13 (1976) 2469. [3] B.G. Erozolimskii, Yu.A. Mostovoi, V.P. Fedunin, A.I. Frank, O.V. Khakhan, Sov. J. Nucl. Phys. 28 (1978) 48. [4] A.L. Hallin, F.P. Calaprice, D.W. MacArthur, L.E. Piilonen, M.B. Schneider, D.F. Schreiber, Phys. Rev. Lett. 52 (1984) 337. [5] R.N. Mohapatra, J.C. Pati, Phys. Rev. D 11 (1975) 566. [6] P. Herczeg, Phys. Rev. D 28 (1983) 200. [7] P. Herczeg, Beta-decay beyond the standard model, in: H. Henrikson, P. Vogel (Eds.), Fundamental Symmetries in Nuclei and Particles, World Scienti"c, Singapore, 1989, p. 46. [8] E.G. Wasserman, Time reversal invariance in polarized neutron decay, Thesis, Los Alamos National Laboratory, 1994. [9] J.B. Lindsay, C. Millerin, J.C. TarleH , H. Verweij, H. Wendler, Nucl. Instr. Meth. 156 (1978) 329. [10] O. SchaK rpf, Physica B 156 (1989) 639.
SECTION 4.