N. P. Hawkes

Sample-interpolation timing: an optimized technique for the digital measurement of time of flight for γ rays and neutrons at relatively low sampling rates

A unique, digital time pick-off method, known as sample-interpolation timing (SIT) is described. This method demonstrates the possibility of improved timing resolution for the digital measurement of time of flight compared with digital replica-analogue time pick-off methods for signals sampled at relatively low rates. Three analogue timing methods have been replicated in the digital domain (leading-edge, crossover and constant-fraction timing) for pulse data sampled at 8 GSa s−1. Events arising from the 7Li(p, n)7Be reaction have been detected with an EJ-301 organic liquid scintillator and recorded with a fast digital sampling oscilloscope. Sample-interpolation timing was developed solely for the digital domain and thus performs more efficiently on digital signals compared with analogue time pick-off methods replicated digitally, especially for fast signals that are sampled at rates that current affordable and portable devices can achieve. Sample interpolation can be applied to any analogue timing method replicated digitally and thus also has the potential to exploit the generic capabilities of analogue techniques with the benefits of operating in the digital domain. A threshold in sampling rate with respect to the signal pulse width is observed beyond which further improvements in timing resolution are not attained. This advance is relevant to many applications in which time-of-flight measurement is essential.

Digital dual-parameter data acquisition for SP2 hydrogen-filled proportional counters

Hydrogen-filled proportional counters perform well as neutron spectrometers in the energy region from a few tens of keV up to ∼1.5 MeV. Unfortunately, gamma rays also generate signals in these detectors. It is possible in principle to distinguish the two types of event via the rise time of their respective signal pulses, but the data acquisition system needed for this is complex to assemble and adjust if one uses conventional modular analogue electronics. In this work a digital sampling system, in conjunction with custom software, was used to measure and acquire amplitude and rise time data from type SP2 counters. The interpretation of the data was supported by a Monte Carlo calculation. The performance of the system is compared with that of a conventional 1-parameter analogue system, and the potential of the digital technique to supplant conventional methods is discussed.

CORRECTIONS ASSOCIATED WITH ON-PHANTOM CALIBRATIONS OF NEUTRON PERSONAL DOSEMETERS

The response of neutron personal dosemeters as a function of neutron energy and angle of incidence is typically measured by mounting the dosemeters on a slab phantom and exposing them to neutrons from an accelerator-based or radionuclide source. The phantom is placed close to the source (75 cm) so that the effect of scattered neutrons is negligible. It is usual to mount several dosemeters on the phantom together. Because the source is close, the source distance and the neutron incidence angle vary significantly over the phantom face, and each dosemeter may receive a different dose equivalent. This is particularly important when the phantom is angled away from normal incidence. With accelerator-produced neutrons, the neutron energy and fluence vary with emission angle relative to the charged particle beam that produces the neutrons, contributing further to differences in dose equivalent, particularly when the phantom is located at other than the straight-ahead position (0° to the beam). Corrections for these effects are quantified and discussed in this article.

ADDITIONAL CHARACTERISATION OF THE THERMAL NEUTRON PILE AT THE NATIONAL PHYSICAL LABORATORY, UK

As part of its wide-ranging neutron metrology capabilities, the National Physical Laboratory in the UK has a thermal neutron facility in which accelerator-produced neutrons are moderated within a large assembly or pile of graphite bricks. The neutron field has previously been well characterised in terms of the fluence rate and energy spectrum at various irradiation positions. However, recent changes to the structure (e.g. enlarging the central irradiation cavity) have prompted a renewal and extension of this work. We have also used Monte Carlo modelling to improve our understanding of the piles performance.