A gated LaBr3 (Ce) detector for border protection applications

Accepted Manuscript A gated LaBr3 (Ce) detector for border protection applications A. Etile, D. Denis-Petit, L. Gaudefroy, V. Meot, O. Roig

PII: DOI: Reference:

S0168-9002(17)30953-1 https://doi.org/10.1016/j.nima.2017.08.053 NIMA 60072

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 4 April 2017 Revised date : 27 August 2017 Accepted date : 29 August 2017 Please cite this article as: A. Etile, D. Denis-Petit, L. Gaudefroy, V. Meot, O. Roig, A gated LaBr3 (Ce) detector for border protection applications, Nuclear Inst. and Methods in Physics Research, A (2017), https://doi.org/10.1016/j.nima.2017.08.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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A gated LaBr3 (Ce) detector for border protection applications

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A. Etilea,∗, D. Denis-Petita , L. Gaudefroya , V. Meota , O. Roiga

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a CEA,DAM,DIF,

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5

F-91297 Arpajon, France

Abstract We report on the dedicated implementation of the blocking technique for a LaBr3 (Ce) detector as well as associated electronics and data acquisition system for border protection applications. The detector is meant to perform delayed γ-ray spectroscopy of fission fragments produced via photofission induced by a high intensity pulsed photon beam. The gating technique avoids saturation of the detection chain during irradiation. The resulting setup allows us to successfully perform delayed γ-ray spectroscopy starting only 30 ns after the gating operation. The measured energy resolution ranges from 5% to 6.5% at 662 keV depending on the γ-ray detection time after the gating operation.

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Keywords: Gating technique, LaBr3 (Ce) scintillator, Digital signal processing.

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1. Introduction

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C-BORD is an H2020 research and innovation program for an effective

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Container inspection at BORDer control points [1]. The statement that has

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been done is that containerized freight is a potential mean for smuggling, drug

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trafficking and illicit substances transport, being thus critical to trade and soci-

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ety. This is why it is essential to have efficient Non-Intrusive Inspection. Within

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the C-BORD project this goal will be achieved thanks to the development of

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five technologies. Each technology should provide an efficient non intrusive con-

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tainer inspection for specific targets respecting functional, practical, logistical,

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safety and financial constraints related to sea and land border control points. ∗ [email protected]

Preprint submitted to Journal of LATEX Templates

July 28, 2017

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Among targets and technologies, one pillar is the photofission technique

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based on the use of a linear electron accelerator for Special Nuclear Material

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(SNM) detection [2], [3]. Indeed, illicit traffic of materials potentially involved

20

in the fabrication of a nuclear device or a dirty bomb is one of the main threat

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European Union wants to prevent from. One specific request is the identifica-

22

tion of the SNM, i.e. discrimination between

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objective of this non destructive active system is to be deployed for a demonstra-

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tion in the ferdam harbor. The photofission detection module will be integrated

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in the current dual view X-ray imaging system: one of the two 9 MeV LINAC

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(Varian Linatron M9 [4]) currently used for imaging will be used for photofission

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measurements. The linatron produces 2.5 µs long 100 mA intensity X-ray pulses

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repeated at a frequency of 200 Hz.

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Under these conditions, it seems illusory to perform measurements during ir-

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radiations. Indeed, the high intensity of the X-ray pulse will cause saturation

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of most of standard detection systems. We therefore proposed to discrimi-

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nate between fissile and fertile actinides by measuring isomeric fission fragment

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production yield ratios through delayed gamma-rays detection. Most studies

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concerning delayed γ-rays signature in neutron-induced fission or photofission

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concern long lifetimes, whereas we focus on very short lifetimes (less than a few

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microseconds). The first works proving the feasibility of discrimination between

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239

235

U,239 Pu vs.

238

U. The final

Pu and 235 U using fission delayed γ-rays are presented in references [5] [6]. In

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addition recent calculations tend to confirm the legitamecy of the method [7].

39

The isomers of interest are

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lives of 164 ns, 511 ns and 2.95 µs and excitation energies of 1691 keV, 1555 keV

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and 1892 keV. Detecting the deexcitation of these isomers present several ad-

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vantages: i ) the delayed nature of the signal of interest with respect to the

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irradiation makes it possible to avoid saturation of the detection system as long

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as one can block the latter during irradiation pulses; ii ) the lower limit of the

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half-lives of interest (T1/2 > 150 ns) limits the background due to the beam;

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iii ) the higher limit of the half-lives of interest (T1/2 < 3 µs) implies a better

47

signal-to-background ratio due to a more intense activity as compared to longer

134m

Te,

135m

2

Te and

136m

Xe with respective half-

48

lifetimes; and iv ) the energies of the γ-rays involved in the deexcitation of the

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isomers limits the absorption of the photons by different materials and more

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importantly enables a better signal-to-background ratio as compared to lower

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γ-ray energies.

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Thanks to its high light output and fast decay time output pulse, leading to a

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fast time response and a good energy resolution, the LaBr3 (Ce) crystal is an

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excellent candidate for such measurements. Nevertheless, a method for blocking

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excessive light output due to the γ-flash is mandatory in order not to saturate

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the associated PMT for several µs after the beam pulse. The observation of a

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physical phenomenon following intense radiations is common to several areas

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such as luminescence [8], fluorescence [9] or phosphorescence spectroscopy, or

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experiments with laser applications to molecular and solid states physics. Most

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of the time the physical signal is amplified using a PMT, as in the present case,

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and the gating technique is used to block the PMT functioning during the irradi-

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ation process [10], [11]. The gating technique is a mean to turn on/off the PMT

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output by controlling the electrical potential of selected dynodes. Several solu-

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tions are commercially available in order to implement the blocking technique.

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However, they do not fulfill the constraints of our present purpose. Either the

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switching times are not compatible with the detection of the γ-rays deexciting

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the isomers of interest or the available sizes of the proposed PMTs do not match

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that of the envisioned LaBr3 (Ce) detector. We therefore implemented a dedi-

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cated blocking function for a LaBr3 (Ce) detector. First application of the gating

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method to a LaBr3 (Ce) scintillator combined to a PMT has been presented by

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K.Y. Hara in [12]. They present the effective suppression of the detector (and

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assembled circuits) overloading due to the beam pulse, allowing time-of-flight

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measurements in neutron capture reactions 1.4 µs after the γ-flash. In our case,

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measurements should start only ∼100 ns after the γ-flash where the switching

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noise is still present and the PMT gain has not yet fully recovered. These issues

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inherent to the gating technique and the solutions adopted are detailed in the

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next sections.

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In this article we first report on the technical implementation of the block3

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ing technique. The resulting detection system, including the digital acquisition

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and the corresponding data processing is then characterized using standard

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γ-sources. Finally, the detection chain is shown to successfully provide spectro-

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scopic data after an intense γ-flash produced by a commercial LINAC Varian

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Linatron similar to that to be used in the C-BORD project.

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2. Electronic gating: principle, implementation and induced gate

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noise

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Figure 1 presents the principle of the present approach. During the γ-flash

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(Tirr in Fig. 1) photoelectrons multiplication in the PMT is inhibited using a

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gate signal synchronized with the beam. The PMT is activated by switching

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the electrical potential of interest back to the working value during Tcount . Between these two steps the switching time, Tswitch , results from a propagation

Figure 1: Principle of the gating technique. Irradiation takes place during Tirr . The detector is ready to count at the end of Tswitch for a period Tcount (see text for details).

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delay of the gate signal in the gating module. In terms of measurement this

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switching time corresponds to the cooling time, Tcool , during which most of the

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beam activity decreases. Consequently over the Tcount period the PMT is not

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saturated, that is ready to detect γ-rays of interest, and the γ background is

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highly reduced.

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For the present work the gating technique has been applied to a Scintibloc

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38S38/2/B380 detector (1.5 × 1.5 inches LaBr3 (Ce) crystal) from the Saint-

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Gobain company [13] mounted on a 2 inches window R2083 Hamamatsu PMT [14].

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The initial energy resolution of the detector was 4.1 % at 662 keV.

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Figure 2 shows a schematic view of the setup of the gating technique applied to

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the LaBr3 (Ce) for interpulses measurements. A Photek Gating Module GM50-10B [15] is used to control the potential re-

Figure 2: Block diagram of the experimental setup for the use of the gating technique on a LaBr3 (Ce) for interpulses measurements. See text for explanation. 101 102

quired for the gating function. This unit provides two output voltages +50 V

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and -200 V depending on the logic signal of a TTL input drive. The switching

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time between both voltages is of about 110 ns and the switching operation can

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be repeated at a maximum rate of 10 kHz. This module has been used in order

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to control the focusing electrode voltage of the PMT. Such an approach allows

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us to inhibit the signal due to the γ-flash at the earliest possible stage of the

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PMT. Initially working with a negative polarity high voltage (HV), the base of

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the PMT tube has been modified in order to work with a positive polarity HV 5

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(+1315 V) and therefore be compatible with the use of the gating module (GM)

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for controlling the focusing electrode. The potential of the photocathode has

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been empirically adjusted so as to maximize the gain of the modified PMT-base

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ensemble. The optimal value is -50 V. With such modifications the -200 V out-

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put of the gating module applied to the focusing electrode of the PMT leads

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to a reverse electric field for electrons created at the photocathode and induces

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the desired inhibition of the PMT. On the contrary optimal working conditions

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are obtained with the +50 V output of the gating module.

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The impact of the aforementioned modifications on the energy resolution of

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the detector is relatively limited. Indeed, working in ungated mode (i.e. with a

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+50 V voltage continuously applied to the focusing electrode) and using stan-

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dard analogical electronic, the energy resolution is 4.5% for the 662 keV γ-ray

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from a 40 kBq 137 Cs source, a value in line with that of the unmodified detector.

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However, the effective use of the gating function induces a gate noise on the

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output of the PMT originating from an induction current being transferred from

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photocathode to anode [10]. Figure 3 shows the typical form of the gate noise

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induced on the PMT output measured for the present setup. For the particular

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event reported in Fig. 3 the PMT was blocked during 1.4 µs between t = 3600 ns

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and t = 5000 ns after the data acquisition trigger. As can be seen from the

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figure oscillations on the PMT output occur at the beginning and the end of

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the gating operation and last for about 1.5µs. A low-pass filter (RC constant

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of about 50 ns) allows to integrate and to reduce by a factor of 4 the amplitude

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of the gate noise. However, if not treated properly, this noise precludes us from

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detecting delayed γ-rays of interest in the present work. In the next section the

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analysis procedure adopted in order to overcome this difficulty is presented as

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well as the corresponding results.

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Figure 3: Typical waveform of the gate noise induced on the output of the PMT. Oscillations appear each time the voltage of the gating module is switched, i.e. at the beginning and the end of the gating function.

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3. Data analysis and results

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The induced gate noise on the PMT output is not compatible with the use of

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standard analogical electronics in order to extract the energy of detected γ-rays.

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Therefore, the output signal of the detector has been sent to a CAEN Desktop

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Waveform Digitizer DT5730 (500 MHz sampling rate, 14 bits resolution) [16].

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For each event the total signal coming out from the detector is saved for further

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offline analysis. This trace is composed of the superimposition of the gate noise

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and the signals resulting from γ interactions in the detector crystal. The shape of

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the induced gate noise, i.e. amplitude and frequency of the oscillations, does not

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evolve from one trace to the other. It gives the opportunity to subtract this noise

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from the total trace saved for each event therefore revealing the superimposed

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physical signal of interest. Figure 4 presents an example of this part of the offline

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149

analysis. The magenta curve in Fig. 4 (a) shows the mean induced gate noise

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starting at time t = 4970 ns when the voltage applied to the focusing electrode of

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the PMT is switched from -200 V to +50 V. This average noise is superimposed

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to a physical signal (black curve) obtained with 22 Na and 137 Cs gamma sources.

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The result of gate noise subtraction is shown as the black curve in Fig. 4 (b).

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One now clearly distinguishes two peaks around t = 5060 ns and t = 5180 ns.The

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stability of the switching noise allows one to record it before the measurement

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and subtract it from traces on an event by event basis. The next step in the

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offline algorithm is to extract the pulse height and time for each peak in the

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subtracted trace. Peak finding is performed using standard numerical method

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involving the derivative of the trace. The amplitude of each peak found in the

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trace is extracted via a χ2 minimization using the average normalized peak shape

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obtained prior the measurement of interest from a source run without gating

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operation. This normalized peak shape is placed on the time axis according to

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the position of the minimum of the studied peak. The pulse height of interest is

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the only free parameter in the χ2 minimization. A good approximation of the

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pulse height, A0 , defined as the minimal value of the trace over the peak region,

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is used as a starting point for the χ2 minimization. The latter is subsequently

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performed for three points, namely A0 and A0 ± 10%. Taking advantage of

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the second order polynomial form of the χ2 function around its minimum this

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approach allows us to extract the optimum pulse height amplitude within a

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short execution time. The result of this part of the algorithm is reported as

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the red curve in Fig. 4 (b) and shows a satisfactory adjustment of the peaks

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present in the subtracted trace. As seen from this figure, the present algorithm

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allows us to reconstruct physical signals appearing only 30 ns after the gating

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operation the latter happening at t = 4970 ns as previously stated.

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For time measurement we applied a method similar to that reported in ref. [17].

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We obtained a time resolution of 155 ps which is satisfactory for the half-lives

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considered in this work.

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The pulse heights resulting from the above algorithm cannot be directly used

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in order to construct the final energy spectrum of detected γ-rays. Indeed, a 8

Figure 4: (Color online.) (a) Gate noise (magenta curve) superimposed to a physical signal acquired with 22 Na and 137 Cs sources (black curve). (b) Physical signal (black curve) resulting from the subtraction of the gate noise from the black curve displayed in panel (a). The result of the χ2 minimization discussed in the text is shown as the red curve.

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correction accounting for the restoration time of the PMT gain after the gating

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operation has to be performed. This correction has been determined with a

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3 MBq

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of the 511 keV peak as a function of the detection time following the gating

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operation. As seen from this figure, the maximal correction amounts to about

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12% of the nominal gain of the PMT just after the gate function and it takes

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about 10 µs for the PMT to recover its nominal gain after the gating operation.

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Na source. Figure 5 shows the evolution of the relative pulse height

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Figure 5: Restoration time of the PMT gain after the gating operation.

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With the final objective of studying the decay of isomeric states with lifetimes

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of the order of few µs this correction is not negligible.

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Figure 6 presents an example of reconstructed energy spectrum for 22 Na (3 MBq)

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and

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ergy resolution at 662 keV amounts to 5.1% for events occurring at least 1 µs

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after the gating operation (i.e. events detection starting from 1 µs after the

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gating operation (switching on of the detector) up to the end of the acquisition

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window). These events are represented by the green line in Figure 6). The en-

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ergy resolution amouts to 6.4% for events occurring up to 1 µs after the gating

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operation (i.e. events detection starting from the gating operation (switching

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on of the detector) up 1µs after). These events are represented by the red line

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in Figure 6.

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Cs (33 MBq) sources data acquired using the gating function. The en-

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Figure 6: (Color online.) Reconstructed and corrected energy spectra of

22 Na

and

137 Cs

sources. In red Events up to 1 µs after the gating operation (i.e. events detection starting from the gating operation (switching on of the detector) up to 1µs. In green Events occuring at least 1 µs after the gating operation (i.e. events detection starting from 1 µs after the gating operation (switching on of the detector) up to the end of the acquisition window).

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4. In-beam experiment: γ-flash suppression

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Finally, in order to test the detection system in conditions similar to that

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expected within the C-BORD project an experiment has been performed at the

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SAPHIR facility (CEA, Saclay, France) using a LINAC Varian Linatron M9 [4].

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This LINAC delivers a 6 MeV X-ray beam in a 30 degrees aperture cone. Fig-

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ure 7 shows a schematic view of the experimental set-up. The detector was

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located 184 cm away from the beam axis (represented by the red arrow on the

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Figure 7) and ∼10 cm away from the exit window of the LINAC. A 19 MBq 60 Co

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source (represented in blue on the Figure 7) was placed ∼15 cm above the de-

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tector. A 10 cm thick lead shielding was surrounding the detector except for the

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crystal entrance window that was directly facing the beam. For this experiment,

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Figure 7: Schematic description of the experimental setup at the SAPHIR facility: (a) top view, (b) front view. See text for details.

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400 ns after the beam pulse the detector was turned on for 6 µs. Within these

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experimental conditions the high photon flux on the LaBr3 (Ce) crystal due to

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the γ-flash leads to afterglow phenomenon [18], [19] that subsequently saturates

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the PMT for several microseconds if the gating function is not used. Figure 8

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shows the reconstructed γ-ray spectrum for the

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procedure described above. The red curve shows the reconstructed spectrum

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for events where the γ-ray is detected up to 1 µs after the gating operation.

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Although the statistics is rather low, the 1173 keV and 1332 keV peaks are re-

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solved and the energy resolution amounts to 4.6% for the first energy. Statistics

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is higher for events detected at least 1 µs after the gating operation, reported

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as a green curve in Fig. 8. The energy resolution for these events does not differ

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much from that just reported and amounts to 4.4%. On these presented result

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at 6 MeV, the counting rate between 400 ns to 6 µs after the beam pulse is 65

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kHz. In futur measurements at Rotterdam, the counting rate is expected to be

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around 10 kHz.

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Co source using the analysis

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Peaks were observed shifting as the counting rate on the detector was in-

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creasing. This effect has been previously studied on LaBr3 (Ce) [20], [21]. It

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has to be taken into account in case of high count rate working conditions, spe-

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Figure 8: (Color online.)

60 Co

source γ-ray spectrum reconstructed with the proposed setup

during interpulses of a 100 mA 6 MeV X-ray beam. In red Events up to 1 µs after the gating operation ((i.e. events detection starting from the gating operation (switching on of the detector) up to 1µ). In green Events occuring at least 1 µs after the gating operation (i.e. events detection starting from 1 µs after the gating operation (switching on of the detector) up to the end of the acquisition window).

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cially when measuring delayed γ-rays from photofission reactions shortly after

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the beam pulse where the counting rate on the detector changes rapidly.

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5. Conclusion

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We report on the implementation of a dedicated gating function for a LaBr3 (Ce)

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detector to be used for homeland security application. The gate noise induced

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on the PMT output by the gating operation is suppressed during offline analysis

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of the digitized response of the detection system. The PMT gain restoration

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time after the gating function has been studied. The relative gain variation just

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after the gating operation is as large as 12% and it takes about 10 µs to reach

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its nominal value. This effect, of prime importance for the study of delayed

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γ-rays, is accounted for in the offline algorithm. The proposed solution has 13

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been characterized with standard γ sources. Depending on the time elapsed

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between the gating operation and the γ-ray detection the energy resolution of

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the present setup varies from 5% to 6.5% for the 662 keV gamma transition.

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Gamma rays can be detected down to 30 ns after the gating operation. The

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proposed setup has been tested in real conditions and is shown to efficiently

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suppress the γ-flash of a pulsed 6 MeV X-ray beam of 100 mA intensity. The

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γ-ray spectrum reconstructed for the first microsecond following the beam pulse

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shows an energy resolution of about 4.5% at 1173 keV. The resolution degrada-

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tion is around 1% when working with the gating function, so if a detector with

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an initial resolution of 3% is modified one would end with the recommended 4%

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energy resolution to separate the peaks at the energy of interest, when using

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the gating operation. New experiments must be performed using a 9 MeV X-ray

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beam to detect delayed γ-rays from the photofission of SNM using an appropri-

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ate shielding to reduce the background and optimize the signal-to-background

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ratio. Furthermore, conditions for the future measurements in Rotterdam will

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be favorable since the expected count rate is 10 kHz whereas it is 65 kHz for the

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presented results at 6 MeV.

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Acknowledgments

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The authors would like to thanks CEA LIST (Saclay, France) members for

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giving us the opportunity to realize experiments in their facility and M. Tarisien

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(CENBG, Bordeaux, France) for fruitful discussions on PMT working. This

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project has received funding from the European Union’s Horizon 2020 research

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and innovation program under grant agreement No 653323. This text reflects

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only the authors views and the Commission is not liable for any use that may

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be made of the information contained therein.

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