A neutron polarimeter with high energy resolution

NUCLEAR

INSTRUMENTS

AND METHODS

64 (I968) 7 7 - 7 9 ;

© NORTH-HOLLAND

PUBLISHING

CO.

A NEUTRON POLARIMETER WITH HIGH ENERGY RESOLUTION M. SIEMII'~ISKI, Z. WILHELMI, W. ZYCH and P. ZUPRAI
Institute of Nuclear Research, Warsaw, Poland and Institute for Experimental Physics, Warsaw University, Warsaw, Poland Received 18 April 1968 A new type of neutron polarimeter utilizing the (n,~) scattering as polarization analyser is described. The energy resolution obtained for 14 MeV neutrons amounts to about 14%.

a°(0) is the cross section for elastic scattering of unpolarized neutrons, P2(0) is the analysing power of the scatterer,

1. Introduction Information on the polarization of particles emitted in nuclear reactions is known to provide a valuable and often conclusive test of reaction mechanism. However, the body of information gathered so far is rather scanty and especially lacking is the data on the polarization of complex spectra neutrons emitted in nuclear reactions. This is obviously due to the fact that neutron polarization experiments are really very hard to perform and in many cases the energy resolution of neutron polarimeters has not been good enough to allow the resolution of neutron groups of different energies. In order to overcome this difficulty we have developed a neutron polarimeter of a new type trying to get a high energy resolution even if we should have to sacrifice the efficiency.

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2. The polarimeter A general idea of the polarimeter is outlined in fig. 1. The neutron polarization is based on the measurement of the left-right asymmetry after the scattering of the neutrons by helium nuclei. Neutrons originating in a target (T) pass through a hole in a brass collimator (C), enter a helium filled chamber (Ch) and scatter on helium nuclei. Recoiled alpha particles pass through one of two angular selectors of a special construction SI or $2 and are detected in a silicon surface barrier detector (E 1 or E2). Neglecting the energy loss of alpha particle on its path in gas, the pulses produced in Ea or E2 counter are proportional to the energy of the recoiling alpha particle and, because of the fixed angle of recoil, proportional also to the energy of the incident neutron. The cross section for elastic scattering of polarized neutrons by an angle 0 is

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a(0)'= a'°)(O) [1 + PaP2(O)nan2], where Pa is the polarization of the incident neutrons,

Fig. 1. Schematic diagram of the polarimeter.

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M. SIEMII~SKI et al.

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Fig. 2. Block diagram of the electronic circuitry.

nln 2 are unit vectors perpendicular to the reaction and the scattering planes, respectively. If n a and n2 are parallel or antiparallel

a(+ O) = a(°)(O) [1 + PIP2(O)], o ( - 0) = o-(°)(0) [1 - P,P2(O)], and from the ratio r = R/L = ~ ( + 0 ) / o ( - 0 ) , the value of P1P2 c a n be calculated:

are fed through conventional charge sensitive preamplifiers into two subgroups of memory of an amplitude analyser gated by appropriately biased pulses from the ionization chamber. The width of the gate is 1.5/tsec. A block diagram of the electronic circuitry is shown in fig. 2. With a helium pressure of 1.6 atm the chamber is run at electrical field strength of 600 V/cm. The ionization chamber preamplifier is mounted directly on the

PIP2 = ( r - - 1 ) / ( r + l ) . Taking the differential of this expression we have

dPt/Pl = {2r/(1

-

r2)}dr/r.

The expression r / ( 1 - r 2) is a decreasing function of P2 for any value of P1. Hence the choice of an analyser with a large analysing power as possible is desirable. The recoil angle was chosen at 22030 ' in the laboratory system corresponding to 135 ° neutron scattering angle in the centre of mass system. The analysing power for this angle of (n,e) scattering is close to unity over the wide range of neutron energies from about 3 to 20 MeV1). Another favouring factor for this choice is that at the recoil angle of 22°30 ' the alpha particle gets as much as about 55% of the incident neutron energy which allows measurements of polarization of low energy neutrons and the use of high helium pressures. The angular selector for permitting alpha particles with a particular recoil direction to pass to the E detectors are made from a pile of parallel steel foils 22 mm long, 18 mm wide and 2 mm apart. The recoil angle variation permitted is from 19°54 ' to 25°6 ' with the mean angle set at 22o30' in the lab system. The azimuthal angle permitted is _ 11 °. In order to reduce the background caused by charged particles being emitted in neutron induced reactions in silicon and detected in the semiconductor counter, recoiling alpha particles are made to pass between two horizontal plates of an ionisation chamber located in the scattering region. (The shaded area in fig. 1 denoted by AE.) Pulses from the two semiconductor detectors

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Fig. 3. Recoiled alpha particle spectrum for (d,T)neutrons.

A NEUTRON

POLARIMETER

WITH

chamber housing to decrease the input capacitance. A continuous gas purification system with hot calcium turnings is also applied. A careful alignment of the polarimeter axis along the neutron beam is obtained with help of wed thread crosses put on the front and on the back of the brass collimator. The alignment of the polarimeter is performed with a point light source put at the front of beam defining apertures at the beam pipe extension of the accelerator viewed by an optical telescope situated at about 5 m from the target. The polarimeter is mounted on a turn-table allowing a change of the reaction angle; it may also be rotated around its axis for the change of the scattering plane from 0 ° to 180 °. 3. Results

An example of the results obtained for one detector in a preliminary run with the (d,T) neutrons is shown in fig. 3. The energy resolution obtained amounts to about 14%. A main contribution to the width of the peak constitutes the geometry (the acceptance of the angular filter and the angular spread of neutrons from the target) as the energy loss spread of alpha particles in gas is negligible. The number of accidental coincidences does not account for the background seen in fig. 3 so the background is believed to come from charged particles emitted in neutron induced reactions in silicon which loose a part of their energy in the semiconductor detector and enter the ionization chamber.

HIGH

ENERGY

RESOLUTION

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We have also performed a preliminary measurement of the asymmetry for neutrons emitted at 29 ° in the cM system under the 420 keV deuteron b o m b a r d m e n t of tritium target. The asymmetry run has been performed in the usual procedure of alternate changes of the position of the two semiconductor detectors from left to right and yielded the value R/L = ( 1 + 4)% in agreement with previous measurements2). 4. Conclusions

The energy resolution obtained with the new polarimeter is good enough to allow the resolution of neutron groups leading to ground and first excited states of final nuclei for (d,n) reactions on most light nuclei. The disadvantage is a rather small detection efficiency amounting to about 10-7 (per incident neutron) for 14 MeV neutrons but we hope that this could be well compensated for by a high reliability owing to an extreme simplicity of the system. A promising way for increasing the efficiency seems to be either a simultaneous use of another pair of semiconductor detectors placed at different recoil angle or putting the same additional system of ionization chamber, angular selectors and semiconductor detectors downstream of the neutron beam in the helium chamber. References 1) B. Hoop, Jr. and H. H. Barschall, Nuclear Physics 83 (1966) 65° 2) R. B. Perkins and J. E. Simmons, Phys. Rev. 124 (1961) 1153.