This is the first measurement of the transverse polarization of beta particles from the decay of polarized nuclei. For many years, the weak interaction has been the focus of a large part of the physics community. Recently, progress has been made toward understanding and unifying the fundamental interactions.
However, in contrast to the case of parity violation, little progress has been made toward understanding time-shift violation. Thus, contributions to nuclear beta decay from time-reversal-violating mergers cannot be ruled out. A well-known test is the experimental limit on the presence of a static neutron electric dipole moment.
The tightest constraint on time-reversal violating contributions to nuclear reactions is in Example 14. For allowed nuclear beta decay, finding time-reversal violation is analogous to measuring the phase angle between the amplitudes for the couplings described above.
LIMIT
NUCLEAR POLARIZATION AT LOW TEMPERATURE
The quantization of angular momentum determines states with different projections of angular momentum on this axis. Nuclear polarization occurs when the states with a positive angular momentum projection are unequally populated with the states with a negative angular momentum projection. The orientation of a system of nuclei with rotational symmetry about any axis can be fully characterized by the relative populations p(m) of the states with an angular momentum projection ,m , on this axis.
The interaction between the nuclear magnetic dipole moment, ~ , and the hypertensive magnetic field , · H, will facilitate orientation of the nuclei by removing the degeneracy of the different m states. Besides removing the degeneracy of the spin states, the entropy of the nuclear system must be reduced. Corrections for the finite solid angles subtended by the source and detector are by the Q >-.
The nuclear polarization obtained in this experiment was found by considering the spatial distribution of gamma-ray emission from the nuclei. The core orientation parameters B2 and B4 for the polarized cores at mK temperature were determined by the measure-.
BETA PARTICLE DISTRIBUTION FROM POLARIZED NUCLEI
As discussed in section 1.1, the weak interaction would violate time-reversal symmetry if one of the coupling phase angles is not zero or pi. The ultimate goal of these experiments with oriented nuclei is to measure the possible contributions to beta-decay correlations of scalar couplings with a phase violating the time reversal. The correlations most sensitive to a time reversal violating scalar coupling contain terms with SA sin(¢ . 5- However, the correlation determined by R has a contribution proportional to the SA sin( < Ps .. lt;P A) and which is not reduced by the electromagnetic factor a Z. The polarization of beta particles due to the decay of polarized nuclei can be calculated using the decay distribution equation 2.2.1. The transverse polarization of beta particles emitted perpendicular to the nuclear polarization has two projections, shown in Figure 1.2.2. For a time-reversing symmetric weak interaction with only vector and axial vector couplings, the N and R coefficients become A time reversal that violates scalar coupling would modify R tor mixed Fermi-Gamow-Teller beta transitions with a term containing SA sin(¢s' A I is generally a result of unequal solid angles or unequal detection efficiency for the two scattering directions, L and R. In many experiments, separate contributions to As and A I can be found by measuring AM while reversing the P direction. Also, exchanging the role of detectors R and L by rotating the detector array around the initial pulse direction by 180 degrees was used to eliminate A I effects. To measure the transverse polarization of the electrons, the scattering asymmetry of the polarized electrons is measured in this experiment. The optimal design of a polarimeter for transverse polarization measurement must take into account the choice of scattering angles, solid angles and foil thickness. To test the theory for the transverse polarization of beta particles from polarized nuclei, a transition with a large value for N in equation 2.2.1 is desired. In order to achieve high polarization sensitivity with the Mott scattering technique, the beta decay must be an electron decay with a maximum kinetic energy of several hundred keV. To simplify the analysis of the experimental results, a nucleus with only one allowed decay in the beta energy range of interest is preferred. Nuclear polarization is enabled by large values of the nuclear magnetic moment and the magnetic hyperfine field in the ferromagnetic host. Furthermore, to measure the nuclear polarization during the experiment, a subsequent gamma decay with known properties in the nuclear decay scheme is required. In order to minimize the deflection of the electron momentum and spin due to the external magnetic field used to saturate the source film, permendura, an alloy of 49% Fe, 49% Co, and 2% V, was used as the host material. Permendura, properly heat treated in external magnetic field, it is highly anisotropic and can be magnetically saturated along the desired axis by less than one Oersted. To avoid multiple scattering of the observed beta particles, 60-Co should be used in the reference plug as close to the sheet surface as possible. The springs used in the experiment were made of said 13 micron thick untreated heat treated sheet obtained from Arnold Engineering Co. The magnetic properties of the sheet were achieved by heat treating it in a pure H2 atmosphere with an external 140 Orsted field applied. as shown. A droplet with a diameter of 3 mm and 1.5 microliters of CoCl solution was applied to the surface of the foil. Even in an argon atmosphere, the foil tabs were electroplated with nickel. In addition, a calculation of the hyperfine field saturation from the measured gamma-ray anisotropy as a function of the applied external magnetic field is shown for the first source at 45 m K degrees. The apparent saturation of these foils at approx. 10 Ørsted instead of the aforementioned less than 1 Ørsted can be understood by considering the demagnetization field for the particular foil geometry. An estimate of the depolarization due to the multiple scattering in the foil of the observed beta particles will require insight into the depth of the foil of 60-Co d.i!Iused through the surface. After heat treatment, the source's tab is gold coated in an argon atmosphere. The tabs are pressed into the cold finger with the sharp edges of the cold finger clamps. A side view, figure 3.3.1, looking south into the laboratory along the source polarization axis shows the polarimeter geometry in cross-section. Neglecting the effects of multiple scattering in the foil, the maximum scattering asymmetry for the 150 kY electrons is at 125 degrees as seen in Figure 2.3.1. The two most discussed, the transmission and the symmetrical designs, are shown in Figure 3.3.3 together with the refection geometry. The choice of foil thickness is determined by the signal to the background and the change in scattering asymmetry with foil thickness. The proportional counter was used to create a gate for the scattered electrons. The timing resolution of the proportional counter and Si(Li) detector systems was about 150 nanoseconds. Due to the small gamma ray efficiency of the proportional counters, the gamma ray background in the Si(Li) detectors was reduced by a factor of 100. The polarimeter was attached to the outside of the room temperature vacuum chamber of the dilution refrigerator. The nuclear polarization of the 6Q-Co source in this experiment was determined from the change in the gamma-ray distribution. Using the nuclear magnetic moment of and assuming that all the nuclei see the hyperfine saturation field of -282 ± 4 KG, the nuclear orientation parameters are uniquely related to delta. The gamma ray energy spectrum of a germanium detector placed perpendicular to the nuclear polarization axis of the source was continuously measured to know the nuclear polarization. Cold, warm refers to the temperature of the source in millikelvin or LHe temperature, respectively. There was no significant deviation of the data with time between all experiments. The predicted transverse polarization and energy spectrum of the electrons as they enter the polarimeter must be known. For this spectrum, mylar windows of the same thickness as in the refrigerator were placed over the source. This predicted cross-polarization must be corrected for the effects of scattering in the source foil and the windows of the refrigerator. In addition, the effects of the applied magnetic field used to saturate the source foil must be included. In this analysis, we will not consider the depolarization of electrons entering the polarimeter due to scattering in the cooler windows. The determination of the electron polarization reduction due to scattering in the source film was divided into two parts. The transverse polarization of beta particles entering the polarimeter can be reduced in another way. Many of the electrons that eventually enter the polarimeter were initially emitted from the polarized nuclei in other directions. The net result can be a reduction in the average polarization of the electrons reaching the polarimeter. Thus, the expected transverse polarization of the beta particles entering the polarimeter is that determined by the scattering of electrons from polarized 60-Co corrected by the simplified effects of scattering in the source foil. Mott scattering is sensitive to the transverse polarization of the electrons perpendicular to the scattering plane. The energy spectrum of electrons scattered from the gold foil of the polarimeter detected in Si(Li) detector 1 can be calculated as. P S (E) is the predicted transverse polarization of the electrons perpendicular to the scattering plane as a function of electron energy. The goal of this experiment is to confirm the predictions of the beta decay theory. The results are consistent with the expected effect based on the theory of the transverse polarization of beta· particles from the decay of oriented nuclei.KF..ASUREMENT OF TRANSVERSE POLARIZATION BY MO'IT SCATIERING Several methods are available for measuring the polarization of elec-
H curve jt calculated B
SOURCE STRENGTH 65 micro Ci
