(3) He Spin Filter for Neutrons

Abstract

The strongly spin-dependent absorption of neutrons in nuclear spin-polarized (3)He opens up the possibility of polarizing neutrons from reactors and spallation sources over the full kinematical range of cold, thermal and hot neutrons. This paper gives a report on the neutron spin filter (NSF) development program at Mainz. The polarization technique is based on direct optical pumping of metastable (3)He atoms combined with a polarization preserving mechanical compression of the gas up to a pressure of several bar, necessary to run a NSF. The concept of a remote type of operation using detachable NSF cells is presented which requires long nuclear spin relaxation times of order 100 hours. A short survey of their use under experimental conditions, e.g. large solid-angle polarization analysis, is given. In neutron particle physics NSFs are used in precision measurements to test fundamental symmetry concepts.

Keywords: 3He polarizer and compressor; neutron spin filter; optical pumping; parity violation; polarization analysis; polarized neutrons; relaxation.

Fig. 1

Performance of a ³He NSF: total transmission (T_n), polarizing (analyzing) efficiency (P_n) and figure of merit (Q) as a function of the filter opacity O’. Solid lines: characteristic parameters for a ³He nuclear spin polarization of P_He = 80 %; dashed-dotted lines: P_He = 50 %.

Fig. 2

Sketch of the Mainz ³He Polarizer and Compressor. The whole apparatus is located in a homogenous magnetic field of 8 G. In reality, we have five optical pumping cells with a total volume of 36 L. For further explanations, see text.

Fig. 3

Build-up of ³He nuclear polarization in the OP-volume of the polarizer. Inset: Build-up of ³He nuclear polarization in a sealed-off OP-cell. The maximum polarization of 91,2 % is higher than in the open system of the polarizer due to gas purity reasons.

Fig. 4

Performance of the Mainz ³He polarizer and compressor with the old (LNA-laser 8W, lower line) and the new (fibre-laser 30 W, upper line) laser system. The nuclear polarization is plotted versus the flux (in bar L d⁻¹).

Fig. 5

Experimental setup for NSF-tests at the Mainz TRIGA-reactor. The polarized ³He cell (lower filled cell) is placed in the thermal neutron beam in a magnetic holding field. The n-beam undergoes then a Bragg-reflexion and is monitored by a detector. From the measurement of the n-transmission T_n through the ³He NSF cell, the ³He nuclear polarization was determined: P_He = (72 ± 1) %. The upper empty cell indicates that the measurement also has to be performed with an evacuated NSF cell in order to determine the neutron transmission of the quartz glass.

Fig. 6

Large solid-angle polarization analysis: Experimental setup at the D1B-two axis diffractometer at ILL. Neutrons with a wavelength of 2.5 Å are polarized by a supermirror, pass a spin flipper and are then scattered at a ErY₆Ni₃-sample. Afterwards, they pass the ³He NSF analyzer and are monitored by a multidetector.

Fig. 7

Extracted nuclear and magnetic cross-sections as a function of the scattering vector Q. For details, see text and [4].

Fig. 8

³He NSF as neutron spin-flipper for high precision PV-experiments on light nuclei. The neutron spin can be flipped via an adiabatic fast passage (AFP)-spin flip of the ³He nuclei.