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1. IntroductionNeutron spectrometry is important in radiation protection, because the dose equivalent from a given neutron fluence depends strongly on the energy spectrum. Gas-filled proportional counters are a good choice of spectrometer in the energy region from a few tens of keV up to about 1.5 MeV, because of their high resolution (relative to Bonner spheres) and favourable signal-to-noise ratio (relative to organic scintillators). One popular type of proportional counter is the SP2 chamber(1), consisting of a 40 mm diameter thin metal sphere containing hydrogen gas at a pressure of about 105 – 106 Pa (1 – 10 atmospheres), and with a thin wire anode across one diameter (Figure 1).

Unfortunately gamma rays also generate signals in these chambers, and interfere with low-energy signals from neutrons. It is possible in principle to distinguish the two types of event by analysing the signal rise time(2), but with conventional modular analogue electronics the system required to do this is complex to assemble and adjust.

The aim of the present work was to assess the potential of digital sampling hardware to simplify data acquisition from SP2 counters, while still using neutron / gamma discrimination and preserving the quality of the neutron spectrum.

2. Neutron / gamma discrimination in SP2 counters

Figure 2 shows typical neutron and gamma interactions in the chamber. The neutron scatters elastically from a hydrogen nucleus, and the recoiling proton generates ionisation in the gas. The gamma ray on the other hand undergoes Compton scattering with an electron in the chamber wall, and this electron recoils into the gas. Because the stopping power of the gas is much lower for the electron than the proton, the electron track is much longer, and it typically crosses the chamber and hits the far wall. This means that there is a maximum energy that a gamma ray can deposit in the chamber, irrespective of the gamma energy. It also provides a basis for distinguishing neutron and gamma events: the primary ionisation electrons from the electron track have a greater spread of distances to travel before arriving at the wire, and so arrive over a greater span of time, producing a signal with a significantly greater rise time (Figure 3).

3. Digital 2-parameter measurements: experimental arrangementMonoenergetic neutrons were produced by the NPL Van de Graaff accelerator via the 7Li(p,n) reaction. An SP2 chamber filled to 3 atmospheres was exposed to 565 keV neutrons at an angle of 0° to the beam, and data acquired as described below. A 1 atmosphere chamber was separately used with the 70 keV neutrons that are produced at 50° to the beam when the 0° energy is 144 keV. The neutron production target generates copious high-energy gamma rays along with the neutrons.

For digital data acquisition, the signal processing chain was conventional as far as the preamplifier (Ortec model 142PC), but then the preamplifier output pulse was passed directly to a 12-bit Acqiris DP310 (Agilent U1070A-001) sampling card in a PC. The card was set so that, on receipt of a trigger, it acquired 1000 samples, spanning 5 microseconds each side of the trigger (10 ns between samples). The internal trigger of the card could not be set below 10% of the upper limit of the input voltage range, and this prevented acquisition of low energy pulses. To circumvent this the second pulse output from the preamplifier was passed to an Ortec 551 TSCA, via a fast amplifier, to produce an external trigger.

The sampling card was controlled by a LabVIEW program written for this experiment. The program included analysis routines that derived an amplitude and rise time from each set of samples as it was acquired, logging this pair of numbers for later analysis. The detailed pulse shape was not logged. Approximately 85000 – 100000 events were acquired in each run.

For comparison with the digital system, data were additionally acquired using conventional analogue electronics (main amplifier, ADC / MCA) in a 1-parameter configuration without rise time information.

4. Results and discussionThe digitally-acquired 2-parameter results are shown in Figures 4(a) and 5(a) for the 565 and 70 keV data respectively. The neutron and gamma branches are clearly visible. Unfortunately it is not obvious how the two should be separated at low energies where they overlap.

For illustrative purposes, a straight-line boundary has been assumed in each case, and used to derive the “neutron” pulse height spectra in Figures 4(b) and 5(b). The results show a highly plausible extrapolation of the spectrum to low energies. However, the boundary used is somewhat arbitrary.

Knauf and Wittstock(2) encountered a similar problem with their 2-parameter analogue data, and advocated a fitting process in rise-time space at several constant-energy slices. For the present data, the low-energy extension to be gained

by pulse shape discrimination is not large, because the maximum energy a gamma can deposit is small. A simple energy threshold, excluding all gamma events, may be the best solution (at the cost of truncating the neutron spectrum).

The 2-parameter 565 keV data show complex features (regions with a low density of counts) that are not present at 70 keV. To clarify the reason for this, a Monte Carlo simulation was written in which the energy and rise time of each neutron event are derived from the energy and geometry of the proton track. Gammas with an arbitrary high energy spectrum, interacting with electrons on the surface of the spherical volume, are included to establish an approximate scale for the rise time axis. The main differences between the 565 and 70 keV data are successfully reproduced (Figure 6), and the 565 keV features are seen to be due to the wall effect (truncation of the proton track on striking the chamber wall) because they vanish from the output of the simulation when the wall effect is turned off.

5. Conclusions1. Digital techniques simplify the acquisition of 2-parameter

data from SP2 counters very considerably compared to conventional modular analogue techniques, although the difficulties of analysing the data for low-energy neutron / gamma discrimination remain.

2. At energies where gamma pulses do not interfere, the digitally-derived neutron pulse-height spectrum is shown to be a good match to that from the conventional electronics.

3. It may be possible to eliminate the external trigger units via improved sampler hardware, or by acquiring data at two gains simultaneously and triggering on the high-gain channel. This would further simplify the digital method, to the point where it may even be preferred to 1-parameter acquisition by conventional modules.

References1. P.W. Benjamin, C.D. Kemshall and J. Redfearn, A high

resolution spherical proportional counter, Nucl. Instrum. Methods 59 (1968) 77 – 85.

2. K. Knauf and W. Wittstock, Neutron Spectrometry with Proton Recoil Proportional Counters at the Research and Measurement Reactor Braunschweig – Status of the Technique, Bericht FMRB-114, July 1987.

AcknowledgementThe authors are grateful for the financial support of the National Measurement Office, an Executive Agency of the UK government’s Department for Business, Innovation and Skills.

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

N.P. Hawkes* and N.J. RobertsNational Physical Laboratory, Hampton Road, Teddington, Middx. TW11 0LW, UK

*Presenting author. Email: nigel.hawkes@npl.co.uk

Figure 4(a). Digital two-parameter data from the 3 atmosphere chamber with 565 keV neutrons. There is clear separation into a neutron branch (near horizontal) and a gamma branch (near vertical). The straight black line shows the trial neutron-gamma boundary used to produce the neutron pulse-height spectrum in Figure 4(b).

Figure 4(b). Black trace: pulse-height spectrum as measured by conventional analogue electronics and 1-parameter data acquisition (no rise time information). Red trace: neutron pulse-height spectrum derived from the digital data using the trial neutron-gamma boundary. The vertical axis of the digital spectrum has been scaled to allow the shapes to be compared easily.

Figure 5(a). As Figure 4(a), but for the 1 atmosphere chamber and 70 keV neutrons.

Figure 5(b). As Figure 4(b), but for the 1 atmosphere chamber and 70 keV neutrons.

Figure 6. Results of a simple Monte Carlo calculation using a geometrical proxy for the rise time, and an arbitrary high energy gamma spectrum. (a): 3 atmosphere chamber and 565 keV neutrons. (b): 1 atmosphere chamber and 70 keV neutrons. The more complex shape of (a) compared with (b) is due to the wall effect (it vanishes when wall effect is turned off in the calculation), and is a good match to the measured data of Figure 4(a).

Figure 1. SP2 chamber. The electrical connector and the gas seal covers have been removed.

Figure 3. Two signals from a 3 atmosphere SP2 chamber exposed to a 241Am / Be source with lead cover. The black trace has a short rise time and high amplitude, and is identified as a neutron. The red trace has a long rise time and low amplitude, and is identified as a gamma ray.

Figure 2. Neutron and gamma interactions in SP2 chambers. Typically the proton track is short, while the electron track crosses the entire chamber, giving the gamma signal a longer rise time.