2013 JINST 8 C01021

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED: September 30, 2012ACCEPTED: November 29, 2012

PUBLISHED: January 15, 2013

14th INTERNATIONAL WORKSHOP ON RADIATION IMAGING DETECTORS,1–5 JULY 2012,FIGUEIRA DA FOZ, PORTUGAL

Directional detection of fast neutrons by the Timepix

pixel detector coupled to plastic scintillator with

silicon photomultiplier array

P. Masek,

a,b,1J. Jakubek,

aJ. Uher

cand R. Preston

d,e

aInstitute of Experimental and Applied Physics, Czech Technical University in Prague,Prague, Czech Republic

bFaculty of Electrical Engineering, Czech Technical University in Prague,Prague, Czech Republic

cAmsterdam Scientific Instruments,Amsterdam, The Netherlands

dCSIRO, Process Science and Engineering & Minerals Down Under Research Flagship,Lucas Heights, Australia

eUniversity of Wollongong, Centre for Medical Radiation Physics,Wollongong, Australia

E-mail: petr.masek@utef.cvut.cz

1Corresponding author.

c� 2013 IOP Publishing Ltd and Sissa Medialab srl doi:10.1088/1748-0221/8/01/C01021

2013 JINST 8 C01021

ABSTRACT: Fast neutrons are conventionally detected by scintillators of large volume, low spatialresolution and poor, if any, directional sensitivity. In this paper we present a detection techniquebased on the tracking of protons recoiled by fast neutrons. In this approach we use the siliconpixel detector Timepix attached in contact planar geometry to a fast plastic scintillator. The protonsrecoiled by neutrons in the scintillator are detected by the pixel detector while scintillation light issensed by a 4⇥4 array of silicon photomultipliers (SiPM). Each photomultiplier is equipped withan independent amplifier and discriminator providing a fast trigger signal to the pixel detector. Vari-able threshold level allows adjustment of the trigger sensitivity. Single events in the pixel detectorcan be tagged and triggered by the scintillating detector. Position and energy sensitivity of the scin-tillator together with the position and the energy sensitivity of the pixel detector allow obtaininginformation about the position and the spectrum of the neutron source. The Timepix detector isoperated with the FITPix readout interface and the Pixelman software package providing control,DAQ and online visualization. The assembled prototype has been tested with fast neutrons from alaboratory radioactive source (AmBe) and a Van de Graaff accelerator (D-T reaction). The detec-tor architecture, comprising the Timepix device, the scintillator and the segmented SiPM, allowsstacking several such units for increased detection efficiency and enhanced directional sensitivity.

KEYWORDS: Neutron detectors (cold, thermal, fast neutrons); Trigger detectors; Particle trackingdetectors (Solid-state detectors)

2013 JINST 8 C01021

Contents

1 Introduction 11.1 Pixel detector Timepix 11.2 Directional detection of fast neutrons with pixellated semiconductor detector 2

2 Enhanced fast neutron detector 22.1 Detection architecture 22.2 Scintillating light detection 32.3 Operation and configuration of coincidence 42.4 Assembled prototype 4

3 Results 4

4 Conclusions 5

1 Introduction

A novel technique for directional fast neutron detection is presented. We combine two differenttypes of detectors — Timepix, a pixellated hybrid semiconductor detector with energy sensitivityper pixel, and a scintillating detector coupled to an array of silicon photomultipliers (SiPM).

The high granularity and per pixel energy sensitivity of the Timepix device provide high spa-tial resolution and spectral resolving power. The significant hydrogen content in the scintillatingdetector volume ensures reasonable detection efficiency for fast neutrons. The fast trigger obtainedfrom the scintillator and the application of coincidence technique serve to suppress background andunwanted events and enhance the signal to noise ratio. We solved several of the shortcomings offast neutron detection and aim to provide both direction and energy detection ability.

1.1 Pixel detector Timepix

Pixel hybrid semiconductor detector [1] consists of a radiation sensitive sensor bump bonded to apixellated readout chip (matrix of 256⇥256 pixels with pitch 55 µm). The sensor can be made upof different materials (Si, CdTe, GaAs) and thicknesses (e.g. 300, 700, 1000 µm). Here the typicalconfiguration of 300 µm thick silicon sensor was employed. The total sensitive area is 2 cm2.

Each of 65536 pixels is equipped with its own preamplifier, discriminator and 14-bit counter.Each pixel can be individually configured to operate in one of the three modes supported: countingmode which counts the number of incoming particles, time-of-arrival mode which registers the timeof interaction and time-over-threshold mode which measures the energy deposited in the pixel.

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Figure 1. Experimental setup consisting of plastic scintillator with single photomultiplier and the pixeldetector (left). The detection response is shown without neutron converter (a), with the plastic converterplaced in front of the pixel detector partially covering it (b) and with trigger signal from the plastic scintillatorused for event selection and background suppression [2].

1.2 Directional detection of fast neutrons with pixellated semiconductor detector

The principle of triggered detection of fast neutrons with the pixel detector Timepix coupled to aplastic scintillator with single photomultiplier has been demonstrated [2]. The layout of this setupand detection response with and without coincidence is shown in figure 1 [2]. The advantage of thecoincidence technique for background and unwanted event suppression is apparent.

The recoiled proton behaves as a billiard ball recoiling with identical mathematical description.The energy and direction of the elastically scattered neutron can be calculated from the energy anddirection of the recoiled hydrogen nucleus when the position of the neutron source is known. Amultilayer approach was tested to get information on both the neutron energy and direction [3]. Aneutron entering the first scintillator undergoes a collision and transfers part of its energy to therecoiling proton. The interaction point and direction is recorded by the first pixel detector. Thescattered neutron moves on with changed trajectory. A collision in the second layer generates thesecond recoiled proton whose detection in the second pixel detector determines the second point.The neutron source lies in the scattering plane defined by the two interaction points and the vectorof direction of the first proton. The intersection of these planes is in the forward direction to thesource (figure 2).

2 Enhanced fast neutron detector

2.1 Detection architecture

The detection principle based on the triggered detection and tracking of the proton recoiled fromneutron converter is used. The geometry layout is also adapted to maximize the probability thatthe recoiled proton reaches the pixel detector. When a neutron interacts in the scintillator, thesignal produced by the recoiling proton is registered. The isotropic emission of the light allows itsdetection in other than the forward direction into which the proton was recoiled. Therefore, theSiPM array is placed at the front side of the scintillator. The schematic diagram of the detectionarchitecture is depicted in figure 3.

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Figure 2. Scheme of multilayer fast neutron detection system (left) where the process of double scatter eventis illustrated. The scattering plane is determined by the two interaction points and the recoil vector of thefirst collision (right) [3].

Figure 3. Schematic diagram of the coupled detector. The pixel detector measures the protons recoiledfrom the hydrogen rich plastic scintillator. The opposite side of the plastic scintillator is connected to seg-mented silicon photomultipliers. The signal from the photomultipliers is amplified and compared. An FPGAimplements the logic providing the trigger signal.

2.2 Scintillating light detection

Information about the neutron collision provided by the plastic scintillator (polystyrene doped by2% pTP and 0.03% POPOP) can be used to further enhanced event discrimination. The detectionof the recoiled proton can be thus driven by the trigger signal from the scintillator which starts thecharge measurement in the pixel detector. The detector does not need to be polled periodically asthe data are read out only when the collision occurred. The background signal is thus suppressed.Given the size of the pixel detector the plastic scintillator with the similar area is employed — size14⇥ 14⇥ 0.7 mm3. The SiPMs are mounted on the large opposite side to the pixel detector. The4⇥4 array of silicon photomultipliers (ArraySL-4 from SensL) is optically connected to the scin-tillator. The spectral response of the scintillator has maximum about 470 nm and fits the radiation

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Figure 4. Fully assembled prototype (left). The inset shows the very close positioning (air gap estimated of200 µm) of the plastic scintillator (0.7 mm) optically connected to the SiPM (1.5 mm) and the pixel detectorTimePix (outer surface at 300 µm above the chipboard plane). Each sensitive layer consists of two easilyconnectable parts. More layers can be stacked on top.

spectrum of the used photomultipliers. Four central segments of the SiPM array are led to a bankof two-stage voltage amplifier circuits with total amplification factor 100. The amplified signal iscompared to an adjustable threshold voltage which allows discrimination against dark pulses andelectronic noise.

2.3 Operation and configuration of coincidence

With different logic schemes the setup can be triggered on recoils occurring in different regions ofthe scintillator. The binary logic values from the comparators are fed into the FPGA. Coincidenceor anticoincidence of the channels can be configured and managed remotely via USB connectionfrom a PC graphical user interface. The result of the predefined logic function generates the triggersignal for the pixel detector interface FITPix [4] which starts the data acquisition process. Eventsare further processed after the acquisition is finished.

2.4 Assembled prototype

Construction of the first prototype consists of one detection layer combining the Timepix pixeldetector, the plastic scintillator and the segmented SiPM array. The stackable structure allows forthe addition of more layers to provide a multi-layer system while the compactness of the wholesystem is preserved, see figure 4. Both the pixel detector and the SiPM array are operated andcontrolled from a single PC.

3 Results

The experiments were performed with 15 MeV neutrons using the D-T reaction of the Van deGraaff accelerator at the IEAP CTU Prague (figure 5a) [4]. The detector system was mountedon a rotating holder in order to change the incidence angle of the neutron beam onto the detectorplane. Perpendicular and tilted positions were tested. The neutron flux at the detector was about104 n/cm3/s. In the first test, one channel of the SiPM array (channel no. 1) was considered in thecoincidence unit while the others were ignored. By triggering on channel 1 only, scintillations, thusevents, which occur in the proximity of channel 1 are selected. The integral frame from Timepixshows the symmetrical region below the chosen channel while the background is suppressed in the

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2013 JINST 8 C01021

Figure 5. Experimental setup at the VdG accelerator where the neutron detector is irradiated by quasi-monochromatic fast neutrons (e.g. 15 MeV) from a tritium target using the D-T reaction (a). The angle ofincidence of the neutrons can be changed to study the perpendicular and tilted directions. Recorded eventsin coincidence with the SiPM segment no. 1 while other segments are ignored (b). Coincidence of channelno. 1 and anticoincidence of channels 2–4 make the cloud asymmetrical and shifted away (c).

rest of the detector (figure 5b). By running in anti-coincidence with channels 2–4, scintillations thatoriginate in proximity to these SiPMs are rejected giving the asymmetric distribution (figure 5c).

Another measurement focused on the directional dependency of the detection. The setup wasirradiated perpendicularly and then in the tilted direction (65�). The trigger signal was generatedfrom two adjacent SiPM channels (3 and 4) in coincidence. This selects scintillations (protonrecoils) occurring in the region of the scintillator above the border between the two SiPMs. About9000 clusters were recorded in each of the two positions. Total measurement time was 2 hours, onefor each position. The integral image is shown in figure 6. Horizontal and vertical projections weremade to investigate the position of centroids and projection profiles.

4 Conclusions

The proposed concept combining a position sensitive scintillator and a semiconductor pixel detectorallows for the directionally sensitive detection of fast neutrons in a single device. The proof-of-principle prototype of a compact fast neutron detector with reduced number of channels wasdesigned and successfully tested with fast neutrons of energy 15 MeV. Even the single layer deviceis capable to provide basic directional information without further data analysis.

Future work will focus on extension of the prototype functionality. Utilization of all the 16channels of SiPM array will increase the exploitable area of Timepix detector. Improved coin-

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2013 JINST 8 C01021

Figure 6. Integral image of the perpendicular measurement (a) and the tilted setup (b). The angle of rotationof the neutron field and the detector was 65�. The setup cross sectional view is shown on top. Verticaland horizontal projections of the recorded cloud are calculated in (c). The mean positions of the verticalprojections stay unchanged while a significant shift in the horizontal projection is observed.

cidence unit will be fully configurable providing exhaustive logic combination. More detectionlayers will be stacked and together with improved data analysis the direction detection ability willbe evaluated and further improved.

Acknowledgments

Research is carried out in frame of the Medipix Collaboration. This work was supported by GrantTA01010164 of the Technology Agency of the Czech Republic. We also thank the company Cryturfor scintillation detector support and fabrication.

References

[1] X. Llopart, R. Ballabriga, M. Campbell, L. Tlustos and W. Wong, Timepix, a 65k programmable pixelreadout chip for arrival time, energy and/or photon counting measurements,Nucl. Instrum. Meth. A 581 (2007) 485.

[2] J. Jakubek and J. Uher, Fast neutron detector based on TimePix pixel device with micrometer spatialresolution, IEEE Nucl. Sci. Symp. Conf. Rec. (2009) 1113.

[3] J. Jakubek, J. Uher and P. Soukup, Fast neutron tracker based on 3D position sensitive semiconductorvoxel detector, IEEE Nucl. Sci. Symp. Conf. Rec. (2010) 302.

[4] V. Kraus et al., FITPix — fast interface for Timepix pixel detectors, 2011 JINST 6 C01079.

[5] C. Granja et al., Neutron sources for test and calibration of neutron detectors for space research,AIP Conf. Proc. 1423 (2012) 446.

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