A.M. Ghose

Measurement of coherent scattering cross-sections of gamma rays at very low momentum transfer

Abstract The coherent scattering cross-sections of gamma rays were measured for C, Al, Cu, Sn, Hg, Pb, Bi and Th at very low momentum transfers (in the range of 0.015 to 0.12 mc) using 145 keV photons obtained from a radioactive 141Ce source. The results obtained for high Z elements are then compared with the results obtained from Hartree-Fock-Slater (HFS) from factor calculations and those obtained by Hauser and Mussgnug1). Hauser and Mussgnug observed large experimental departures from the form factor values for same high Z elements. However the present measurements do not confirm the findings of Hauser and Mussgnug. Further as the previously available experimental data at low momentum transfers for low Z elements are not adequate to compare with the form factor values, the present measurements give us an opportunity to check the experimental observations with HFS form factor values. The comparison reveals that the HFS form factor calculations are generally in good agreement at very low momentum transfers for all elements under investigation.

Uniform spectral sensitivity photon counter

Abstract It is shown that a photon counter, the efficiency of which is independent of the energy of the photon over a wide range can be obtained by combining a suitable filter with a conventional NaI(Tl) scintillation counter. A theory is developed which makes it possible to choose the constitution as well as the optimum thickness of the filter for a particular dimension of the crystal. Performance of the system for paraxial as well as wide beam geometries have been considered. Experimental investigations for verifying the theory have been given.

SPHERE TRANSMISSION METHOD FOR THE MEASUREMENT OF ATOMIC PHOTO-ELECTRIC CROSS-SECTIONS

Abstract A new method for the measurement of atomic photo-electric cross-sections of high Z atoms has been developed. The method is based on the measurement of photon transmission through spherical shells of absorbers. Theoretical calculations have been presented to consider the effects of multiple scattering in the absorber shell. It has been shown that the absorption in the shell can be approximated by an effective cross-section which can be expanded in a polynomial in the thickness of the shell. The effect of truncating the polynomial by another polynomial of lower order has been discussed. Formulae for the evaluation of correction factors for finite size of the source, self-absorption in the source, non-uniformity of detector sensitivity, scattering in the absorber holders, small deviations from spherical symmetry in the experimental arrangement and for pair production in the absorber have been derived.

A linear response photon counter

Abstract A photon counter whose efficiency is linear with the energy of the interacting photon has been developed by combining a suitable filter with a conventional NaI(Tl) crystal. The theoretical concept used by one of the authors 1 ) in developing a counter whose efficiency is independent of photon energy has been utilised in the present investigation. The efficiencies of the counter obtained experimentally for different photon energies have been compared with the theoretically predicted values and are in reasonably good agreement.

Coherent scattering of gamma rays from calcium oxide

Abstract The ratio of coherent to incoherent scattering cross sections of 136, 140.5, 264.7, 279.5 keV gamma rays have been determined for calcium oxide for several values of momentum transfers. An improved experimental arrangement incorporating multiple section filters have been employed in our measurements and the results compared with theoretical values evaluated from recent compilations.

MEASUREMENT OF ELASTIC NEUTRONS BY ORGANIC SCINTILLATORS.

Abstract A method for the measurement of elastically scattered neutrons in the presence of inelastic ones has been developed. The method consists in measuring the neutron intensity at two different biases of the recoil proton scintillation neutron counter. It has been shown that these experimental results are sufficient to determine the elastic neutron intensity unambigously. No additional data like differential elastic neutron cross sections etc. are required in this method. The experimental test of this new procedure has been carried out by measuring nonelastic cross sections of nuclei for 14.8 MeV neutrons. The experimental results indicate the consistency and the validity of the method.

On the non-elastic cross section of 14.2 MeV neutrons

Abstract Non-elastic scattering cross sections for 14.2 MeV neutrons have been measured for the elements O, Al, Cu, Cd, Sn, W and Pb employing the modified sphere transmission technique. Essentially it involves the simultaneous measurement of neutron transmission at different bias levels of the detector providing scope for better experimental accuracy. An automatic search routine was used to calculate an optical model fit to the data. An average set of parameters was obtained by analysing non-elastic as well as differential elastic cross-section data together.

Investigations on the improvement of the effective resolution in scintillation spectrometers

Abstract A method for resolving peaks in a scintillation spectrum which otherwise would overlap due to photomultiplier statistics is presented here. The arrangement is quite flexible for varying light divisions between the two photomultipliers viewing one scintillating crystal. A differential discriminator accepts pulses from a fixed portion of the spectrum from one of the tubes whereas another differential discriminator scans the spectrum from the second tube, coincidence gated by the first. Experimental results presented here show that the 1.33 MeV monoenergetic line from 60Co source was separated and resolved; the effective resolution was made better by increasing the light input to the second photomultiplier.