DESIGN OF A WIEN FILTER AND MEASUREMENT OF LONGITUDINAL POLARIZATION OF BETA PARTICLES BY S. S. ABHYANKAR AND M. R. BHIDAY

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1 DESIGN OF A WIEN FILTER AND MEASUREMENT OF LONGITUDINAL POLARIZATION OF BETA PARTICLES BY S. S. ABHYANKAR AND M. R. BHIDAY (Department of Physics, University of Poona, Poona-7, India) Received January 18, 1971 (Communicated by Prof. B. V. Thosar, F.A.SC.) ABSTRACT Measurement of longitudinal polarization of beta particles is carried out for a pure beta source Th ß --> Pb 204 (2 -#0 unique first forbidden) for v/c = 0.6 using a Wien-filter for transformation of polarization and Mott-scattering for the analysis of transverse polarization. The Wienfilter is most versatile for measurements to study the dependance of polarization with (1) the atomic No. Z, and (2) E the incident energy. The resolution of the Wien-filter arrangement reported here is about 10% with sufficient transmission. The value of polarization is P t (1.09 ± 0.083) vlc which is in agreement with the theoretical value predicted for this transition. INTRODUCTION THE theory of beta decay predicts that the degree of polarization should be proportional to v/c. An accurate knowledge of the energy dependance in continuation with the spectrum shape allows the relative magnitude of several matrix elements to be fixed. The values of these matrix elements are expected to show the effects of a possible violation of time reversal invariance in beta decay. Deviations from full polarization have been reported by several authors. 1-4 Sosnowski et al.5 have also reported such deviations of polarization with Z value of the source. Since definite information on the Z dependance of polarization was lacking it was thought necessary to study this aspect of helicity in greater detail. This paper describes mainly the design and performance of the Wien-filter constructed for this study. Al 53

2 S 4 S. S. ABHYANKAR AND M. R. BHIDAY EXPERIMENTAL For the measurement of longitudinal polarization of beta particles Mott-scattering method has been used. This method of measurement requires a transformation of polarization of beta particles, from longitudinal to the transverse form. Out of the several available methods for polarization transformation Wien-filter arrangement has been selected. Such a system was found to be the most advantageous because (a) it also acts as a velocity selector, (b) any degree of transformation of polarization for different energies is possible, (c) such a system is axially symmetric and (d) the instrumental asymmetry effects as well as the efficiency of the counters can be tested by suitable reversal of either the electric field E or the magnetic field B. A Wien-filter consists of a crossed electric and magnetic field arrangement (Fig. 1). A beta particle with velocity v travels undeflected through the crossed electric and magnetic fields if JEJv = ß jbj c (1) FiG. 1. Schematic diagram of the experimental set up. whereas, the polarization vector is turned through an angles eg BL 2mcvy 6 (2) where L is the length of the cross-field and g is the gyro magnetic ratio (for electrons g = 2). Hence such a system acts as a polarization transformer besides being a velocity selector. The required values of B (magnetic field) for various selected ß-values and for different degrees of transformation of polarization 4 Q rh) are shown in Table I.

3 Measurement of Longitudinal Polarization of Beta Particles 55 TABLE I A 0 ß= (v/c) Corresponding B in Gauss E in K.E. of kv/cm electrons (KeV) ir/2= Full transformation A0_vr/3= In the present design of the Wien-filter, the length L of the crossed field region was limited to 32 cm, as determined by the size of the magnetic field. The necessary magnetic field was obtained in between the pole-pieces (diameter 32 cm) of a ß-ray spectrometer. The field can be varied uniformly and is stable to within 1%. However, the field structure was not uniform over the full diameter, and showed a slight gradient near the circumference. To counteract the fringing field of the magnet, the plates of the electric condenser were tapered at the ends, producing the same gradient in the electric field as that of the fringing magnetic field. High voltages upto 30 K.V. derived from a Cockroft-Walton voltage doubler circuit could be applied to the plates and measured to an accuracy of 1% by a 300 M Q resistance voltmeter. The magnetic field of the order of 100 gauss could be maintained constant and measured to an accuracy of about 1%. This measurement was standardised with 624 KeV conversion electrons from C+S137

4 56 S. S. ABHYANKAR AND M. R. BHIDAY The Wien-filter was coupled to the scattering chamber through a second collimating slit. The electrons scattered at 90 0 by the gold foils were detected by two plastic scintillator counters placed symmetrically about the beam axis. The two detectors consisted of identical NE102A plastic scintillators of 5 cm. diameter and of 4 mm. thickness wrapped in aluminium foil ( ") and optically coupled to E M I 9714 photomultipliers. 0, RO% 1200, BOO 244 4'I _ CENT/H6TR6-^ FiG. 2. Horizontal beam profile. The foil was fixed in a perspex foil holder inserted from the top of the scattering chamber. The foil could be rotated about a vertical axis and also could be raised or lowered without breaking the vacuum. The whole of the scattering chamber containing the scintillation counters and the foil could be rotated about the beam axis so that the L-R asymmetry could be detected in any azimuthal plane. Therefore, this arrangement affords a direct measurement of instrumental or geometrical asymmetry,

5 Measurement of Longitudinal Polarization of Beta Particles 57 The beta-ray sources Th 204 and Cs 137, prepared on thin mylar by evaporation over a spot of diameter 1 cm had a thickness of 3 mg/sq. cm. PERFORMANCE The selected beam was studied for its horizontal spread by a scanning arrangement, which consists of a rectangular scintillator coupled to a photomultiplier, which could be moved transversely to the beam in steps of 1 mm A typical horizontal scan is shown in Fig. 2 which indicates that the beam was symmetrical about the axis and of nearly 2 cm width. Resolution of the system was studied by scanning the Cs137 conversion speak. Figure 3 gives the pulse height spectrum of the conversion electrons. The resolution is about 10% which is quite comparable with that reported by the previous workers.' 2, 5, 7 _ CHANNEL NUMBER. FIG. 3. Pulse height spectrum of Cs 337 Figures 4 and 5 show the complete spectra of Th 204 and Cs 137 as seen by this instrument. The linearity of the system was tested as follows. The position of Cs 137 conversion peak was obtained by varying the electric field E for a fixed value of the magnetic field B. The magnetic field was then

6 58 S. S. ABHYANKAR AND M. R. BHIDAY FiG. 4. Beta ray spectrum of Cs FiG 5. Beta spectrum of TI 204 by wicn-filter.

7 Measurement of Longitudinal Polarization of.eta Particles 5 altered by a suitable step and the corresponding position of the conversion peak was again determined. The ratio of the two field values in both the cases was found to be the same within the experimental error as shown in Fig. 6. Before proceeding to carry out the left-right scattering asymmetry measurements, the two detecting systems were made identical in t heir performance within 0.8 volts.!lgcrponsj ^ FIG Key conversion-line spectrum for two values of magnetic field showing the linearity of wien filter. The pulse height spectrum of Th 204 for the selected energy 128 KeV (ß v/c = 0.6) was obtained for incident and scattered electrons. The scattered pulse height peak was found to be displaced by about 2 volts towards the low energy channel. The two detectors were then provided with the appropriate bias to exclude the possibility of recording inelastically scattered electrons (Moller e-e scattering). RESULTS AND DDISCUSSION The value of longitudinal polarization PI is calculated from the relations Pl = (v/c) S (B) ; and L^ = IL -^- Ix where p is the observed asymmetry and S (0) is the asymmetry function.

8 60 S. S. ABHYANKAR AND M. R. BHIDAY The left-right scattered intensities were recorded in position 0 over a long interval of time to provide a good statistics. A number of runs were taken with two thin gold foils. The foils were prepared by vacuum deposition of 99.9% pure gold at N.C.L., Poona. The foils were kept inclined at 60 to the beam axis to minimise multiple and plural scattering. 0 The reflection, transmission effects and instrumental asymmetry effects (0-180 ) were ascertained. 0 The left-right scattered intensities were studied over different aximuthal planes and the maximum asymmetry plane ( ) was found out. The observed IL-IR intensities were corrected for instrumental asymmetry effects and for background. Several scattering runs were taken and a typical one is reported in Table 1I. The observed values of asymmetry A = 0.18 (from Table II) TABLE II L-R Asymmetry measurements for Th 204 electrons of v/c = 0.6 for transition Th 204. ß Pb204 --* (2 p- 0) (unique first forbidden) E = 14.4 Key/cm B = 76 oersteds Source thickness T = 3 mg/sq. cm. ist Foil t = 0.5 mg/sq. cm Ik-Counts 1.-Counts I,; Counts Is-Counts Reflection Transmission Mean (After subtracting the geometrical asymmetry) D= I= IR =0.18% L IR II Foil t = 1 mg/sq. cm I, -Counts IR Counts In-Counts IR Counts Reflection Transmission Mean (After subtracting the geometrical asymmetry) Í=0.5% 18 p= Í R L R

9 Measurement of Longitudinal Polarization of $eta Particles 61 The value of S (0) for gold for ß = 0.6 and for scattering angle 90 0 from data provided by Nelson and Pidds 9 which also takes screening into consideration is S(0) = 0.27 ± The calculated values of the longitudinal polarization Pl for Th 204 -* ß - Pó 204 (2._0, unique first forbidden) is Pl = ( )v/c, which is in close agreement with theoretical value predicted for this transition. The whole system, made of 'Alkathene' and plexiglass which is advantageous as being of low Z, reduces the back-scattering from the walls. The source used was a pure beta source; hence the gamma background effects and secondary electron effects were negligible. The thickness of the source and the scattering foils were small enough to limit the errors due to multiple and plural scattering within experimental limits. The accuracy of these measurements can be further improved by increasing signal to noise ratio and one can then carry out such measurements under identical conditions for a greater number of pure ß emitting isotopes of widely varying values of Z and for different energies in greater detail. REFERENCES 1. Spivak,, P. E., Nuclear Phys., 1961, 23, 169. Mikaelyan, L. A. et al.; 2. Avakyan, R. O., Soviet Phy. JETP, 1962, 14, 491. Pushkin, V. E., et al. 3. Ladage, A. 4. Kulkarni, V. G. Panat, C. V. and Thosar, B. V. 5. Sosnowski, R. and Wilhelmi, Z. 6. Bargman, V. Michel, L. and Telegdi, V. L. 7. Cavanag, P. E., Turner, J. F. et al. 8. Tolhoek, H. A. and De Groot, R. S. 9. Nelson, D. F. and Pidd, R. W... Dissertation, University of Hamburg, Phys. Letters, 1963, 7, 52. Nuclear Phys., 1961, 26, 280. Phys. Rev. Letter, 1959, 2, 435. Phil. Mag., 1957, 2 (8), Physica, 1951, 17, 17; Rev. Modern Phy., 1956, 28, 277. Phys. Rev., 1959, 114, 728.