- Email: [email protected]
Characterization of a GEM-based fast neutron detector
Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Characterization of a GEM-based fast neutron detector B. Esposito a,n, D. Marocco a, R. Villari a, F. Murtas b, R. Rodionov c a b c
Associazione Euratom-ENEA sulla Fusione, Via E. Fermi, 45, I-00044 Frascati, Roma, Italy Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, Via E. Fermi, 40, I-00044 Frascati, Roma, Italy SRC RF TRINITI Troitsk, Moscow, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 10 November 2013 Accepted 26 November 2013 Available online 10 December 2013
The neutron efficiency of a Gas Electron Multiplier (GEM)-based detector designed for fast neutron measurements in fusion devices was determined through the combined use of Monte Carlo (MCNPX) calculations and analysis of deuterium–deuterium and deuterium–tritium neutron irradiation experiments. The detector, characterized by a triple GEM structure flushed with a Ar/CO2/CF4 – 45/15/40 gas mixture, features a digital read-out system and has two sub-units for the detection of 2.5þ 14 MeV neutrons and 14 MeV neutrons (UDD and UDT, respectively). The pulse height spectra (PHS) determined from the curves of experimental efficiency as a function of the detector's high voltage (HV) and the MCNPX-simulated PHS were compared using a fitting routine that finds the best match between the experimental and simulated PHS by assuming a parametric model for the relation between HV (that determines the detector's gain) and the energy deposited in the gas. This led to express the experimental neutron efficiency as a function of the discrimination level set on the deposited energy (energy threshold). The detector sensitivity to γ-rays was also analyzed and the operational range in which the γ-ray contribution to the signal is not negligible was determined. It is found that this detector can reach a maximum neutron efficiency of 1 10 3 counts/n at 2.5 MeV (UDD sub-unit) and of 4 10 3 counts/n at 14 MeV (UDT and UDD sub-units). & 2014 EURATOM ENEA. Published by Elsevier B.V. All rights reserved.
Keywords: Neutron detector GEM Fusion
1. Introduction This paper describes the work of efficiency characterization of a GEM-based [1] prototype detector designed for fast neutron measurements in thermonuclear fusion devices [2,3]. Neutron flux monitors for fusion applications must be able to cover a wide dynamic range and sustain high count rates: both features can in principle be provided by a GEM-based neutron detector characterized by a large number of pixels with high counting rate capability (up to 10 MHz/cm2 [4]). In fusion plasmas 2.5 MeV neutrons are produced through deuterium–deuterium (DD) reactions and 14 MeV neutrons through deuterium–tritium reactions (DT); a few percent of the total neutron yield in pure DD plasmas is due to 14 MeV neutrons produced by triton burn-up. The detector consists of two sub-units respectively for total (2.5 MeV (DD)þ14 MeV (DT)) and DT neutron measurements (UDD and UDT respectively) and has a multilayer structure. The first layer is a proton recoil neutron converter made of polyethylene. The thickness in each sub-unit is optimized respectively to maximize the neutron efficiency at 2.5 and 14 MeV (0.7 mm in the UDD and 2 mm in the UDT). The second layer is a thin aluminum foil (0.2 mm) only present in the UDT sub-unit that provides a suitable energy cut-off for the protons produced by 2.5 MeV neutrons [3]. The third layer is n
Corresponding author. Tel.: þ 39 06 94005152. E-mail address: [email protected] (B. Esposito).
constituted by an amplifying structure in which the recoil protons produce a detectable electron signal through ionization of gas atoms: it is a triple GEM based on a Ar/CO2/CF4 gas mixture (45/15/40 percentages in volume), consisting of a drift gap (3 mm wide) where the primary ionization is produced, two transfer gaps (1 mm and 2 mm wide) and an induction gap (1 mm wide) for the charge collection. The three GEM foils are stacked between a cathode (a Mylar layer 11.5 μm thick with an Al deposit 0.5 μm thick on the bottom side for electrical contact) and a segmented anode (128 pads, 8 16 matrix, size of single pad¼ 12.3 6 mm2) connected to the electronic readout system. The layout of the detector is shown in Fig. 1. Further details on the composition of the detector are given in Section 3 (see Table 1). Each GEM consists of a thin kapton foil (50 μm thick), copper clad on each side (copper thickness¼5 μm) perforated with holes (biconical, with external and internal diameters of 70 μm and 50 μm, respectively, and a pitch of 140 μm). An electric potential difference is applied between the copper sides of each GEM allowing for electron multiplication in the holes. The sum of the three voltage differences is referred to as high voltage (HV). Moreover, external electric fields are necessary to transfer the electrons in the drift (3 kV/cm), transfer (3 kV/cm) and induction gaps (5 kV/cm): the values of these fields are the standard ones used for triple GEM detectors in the LHCb experiment at CERN [5]. An earlier paper [2] reported on the results of DT and DD neutron irradiations of the detector prototype carried out at the
0168-9002/$ - see front matter & 2014 EURATOM ENEA. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.11.103
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Frascati Neutron Generator (FNG) [6] up to 107 n/cm2/s as well as on preliminary estimates of the neutron efficiency based on a simplified MCNPX model of the detector. It was found out that the detector is also sensitive to γ-rays, although a proper selection of the working HV may lead to a suppression of the γ-ray contribution. Moreover, an excessively large signal was observed during the DD irradiation in the UDT sub-unit: this was unexpected as the protons from to DD interaction in the converter should be efficiently blocked by the Al layer and, therefore, produce a negligible signal under DD irradiation in the UDT sub-unit. In the present work a detailed analysis of the detector neutron efficiency is performed through the comparison of the differential proton spectra deduced (see Section 4 for details) from the curves of experimental efficiency as a function of HV (εexp(HV), counting curves) measured in [2] and those simulated using a refined MCNPX model. The issues of the detector sensitivity to γ-rays and of the UDT sub-unit response to DD neutrons are also discussed.
Table 1 Materials used in the MCNPX model of the GEM detector.
Fiberglass (wall)
Gold (pad)
Mass density (g/cm3) 1.864
19.6
2. Experiments The irradiation experiments [2] were carried out by placing the GEM detector perpendicularly to the FNG beam at a distance d¼ 38 cm from the FNG target (with the neutron source roughly aligned with the center of the detector) for which the point source approximation holds within a good degree of accuracy. The neutron flux incident on the detector was kept constant and the number of pulses recorded was measured as a function of the detector gain obtained by selecting different HV values (the voltage difference applied to each of the three GEMs being the same). The electronic threshold, setting the minimum charge that can be detected by the readout electronics (CARIOCA-GEM chips [7]), was kept fixed throughout the FNG experiments (Qmin ¼ 32 fC, see Ref. [2]). The digital read-out system based on the CARIOCA-GEM chips does not enable to obtain pulse height spectra (PHS), but only to measure the counting rate. Two irradiations were made: 1) DD irradiation: using 2.5 MeV neutrons with a measured total DD neutron yield (emitted over 4π) YDD ¼1.85 108 n/s (uncertainty 8%) and a small contribution due to 14 MeV neutrons ( 1/200 YDD) due to triton burn-up in the deuterium target. The DD yield was measured by means of a NE213 scintillator calibrated using the neutron activation method (indium foil). MCNP calculations were used to evaluate the indium activity per source neutron; the quoted uncertainty includes the terms due to the indium activity measurement and indium cross section added in quadrature (the MCNP error is negligible) [8]. 2) DT irradiation: using 14 MeV neutrons with a measured YDT ¼4.6 109 n/s (uncertainty 3%). The DT yield was measured by means of the associated α-particle method with a silicon surface barrier (SSD) detector; the quoted uncertainty includes the terms due to the anisotropy factor, the collimator solid angle and the target-SSD detector solid-angle determination added in quadrature [9].
Fig. 1. Scheme of the triple-GEM neutron detector.
Material
197
Composition
Weight fraction
Si O Al Ca Mg B Na K C H N O
1.66E–01 3.09E–01 5.02E–02 8.56E–02 1.27E–02 1.45E–02 1.19E–03 1.13E–03 2.50E–01 1.52E–02 3.65E–02 5.76E–02
Au
1
Gas (Ar 45%–CO2 15%–CF4 40%)
0.00251
C O F Ar
1.10E–01 7.99E–02 5.13E–01 2.97E–01
Kapton (GEM foils)
0.71
H C N O
2.64E–02 6.91E–01 7.33E–02 2.09E–01
Copper (GEM foils)
4.48
Cu
1
Mylar
1.4
H C O
4.20E–02 6.25E–01 3.33E–01
Polyethylene
0.94
H C
1.44E–01 8.56E–01
Aluminum
2.7
Al
1
In both cases a γ-ray field was also present during the irradiation. No γ-ray flux or spectral measurements were available. The counting curves (experimental efficiency, εexp(HV), as a function HV) are shown in Fig. 2 for the case of the UDD sub-unit under DD and DT irradiations. The incident neutrons have been calculated using a point source/isotropic emission approximation and the neutron yield values (YDD and YDT) given above. It has to be noted that, as each particle event can induce a signal in more than one pad, the cluster size correction factor (fCS) must be applied to the measured counts (Cmeas) shown in Fig. 2 in order obtain the experimental counts (Cexp ¼Cmeas/fCS): the cluster size is the average number of pads in which a signal is induced by a particle event [10]. fCS was measured for this detector [11] and found to increase linearly with HV (from 1.0 at 800 V up to 1.6 at 1200 V).
3. MCNPX model and simulations Calculations with MCNPX [12] were performed to predict the detector response to the FNG mixed neutron/γ-ray field. A detailed MCNPX model was developed in order to take into account all details of the GEM detector. In particular, the walls of the detector, the three amplification stages and the Mylar layer used as electric contact in both sub-units were included (see the model in Fig. 3 and the list of all materials used in the MCNPX analysis in Table 1). This detector model, together with a detailed model of the FNG DD and DT neutron sources (see [8] and references therein), was used to simulate the irradiations described in the previous section.
198
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 2. Experimental counting curves at constant neutron flux (FNG irradiations [2]).
Fig. 3. MCNPX model of the GEM detector (dimensions are not in scale): (left) vertical section; (right) front view: all pads have the same size; note that the black lines enclose the 28 central pads (for each sub-unit) that are less prone to edge effects and whose data have therefore been used in the analysis (both simulation and experiment).
The calculated spectra of neutrons and γ-rays at the detector position during DT and DD irradiations are shown in Fig. 4. The calculated total neutron fluence on the detector is 6 10 5 n/ cm2/source neutron. The low-energy neutrons and prompt γ-rays are produced by the interaction of primary neutrons with the target materials. The contribution of low-energy neutrons (those below the 14 MeV DT and 2.5 MeV DD neutron peaks) is respectively 15% (DT) and 6% (DD) of the total neutron fluence. The γ spectrum peaks at 1 MeV and the calculated total γ fluence amounts to 11% (DT) and 7% (DD) of the total neutron fluence. The incident spectra for the FNG DD and DT irradiation experiments were used by MCNPX to calculate the energy deposited in the gas (Edep) by protons (neutron signal) and γ-rays entering the detector and the respective PHS dΝMCNPX/dEdep (in particles/(source neutron MeV)) were calculated for the FNG irradiation DD and DT experiments (Fig. 5). For the PHS calculation we considered, in first approximation, only the energy deposited in the drift gap since events produced in the remaining gaps will have much lower amplitude due to the lower number of amplification stages. Note that the small contribution due to 14 MeV neutrons was included in the DD MCNPX calculations. The electromagnetic component interacts weakly with the GEM and deposits only a small fraction of its energy in the gas. The neutron signal due to proton energy deposition events mostly corresponds to the range 0.01–0.1 MeV, whereas γ-ray energy deposition occurs at lower energy.
The level of the neutron signal determined by the MCNPX simulation might be somehow underestimated because of a missing contribution due to the fact that the charged particle reactions for some of the elements are not accurately modeled in MCNPX.
4. Response to neutrons The final objective is to determine the neutron efficiency values at 2.5 MeV and 14 MeV. In order to do this we will first analyze the detector response to the non-monochromatic neutrons in the mixed γ-ray/neutron field of the DD and DT irradiation experiments by determining the experimental (εexp) and simulated (εMCNPX) neutron efficiency values. 4.1. Simulated efficiency Assuming a point-like neutron source, the calculated efficiency in protons/incident neutron (pMCNPX/N) for a given threshold on the deposited energy (Ethr) is given by: Z 1 2Z 1 p 4πd dN MCNPX dεMCNPX εMCNPX ðEthr Þ ¼ MCNPX ¼ dEdep ¼ dEdep ð1Þ dE dEdep dep N AGEM Ethr Ethr where N ¼AGEM/4πd2 is the number of neutrons incident on a detector of area AGEM from a source (located at a distance d from the detector) emitting one neutron.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
199
Fig. 4. Calculated FNG neutron and γ-ray spectra at the detector position. Note that the units are different in the two figures: in the figure on the right the incident neutrons are those that impinge on the detector, while in that on the left the source neutrons are those emitted by the source.
Fig. 5. MCNPX calculations (statistical errors shown): PHS in the GEM due to γ-rays and protons (neutrons): UDD sub-unit under FNG DD (left) and DT (right) irradiation.
4.2. Experimental efficiency
with:
The counting curves can be used to obtain the experimental efficiency as a function of Ethr by means of the following formula: 2 Z 1 r exp ðHVÞ 4πd dNexp ¼ εexp ðHVÞ ¼ dEdep Ni Y N AGEM Ethr ðHVÞ dEdep Z 1 dεexp ¼ ð2Þ dE dEdep
A¼
Ethr ðHVÞ
dep
where rexp, Ni and YN are, respectively, the experimental count rate (counts/s) at HV, the neutrons/s incident on the detector and the FNG neutron yield (neutrons/s). The relation between Ethr and HV can be determined by the formula Q min ¼ nel eGðHVÞ ¼
Ethr ðHVÞeGðHVÞ W ion
ð3Þ
where nel ¼number of electrons produced in the drift gap, G(HV)¼ effective gain (ratio between electrons collected at the anode and nel), Wion ¼energy dissipation in the gas for the production of an ionelectron pair, e¼electron charge; the Wion in the chosen gas mixture can be assumed to be 27.6 eV [5]. This formula sets for each HV the threshold energy Ethr to be deposited in the drift gap by an ionizing particle in order to produce at the anode the minimum charge detectable by the readout electronics (Qmin). Since, experimentally, the gain can be expressed as a function of HV GðHVÞ ¼ α expðB HVÞ
ð6Þ
The parameters A and B can be determined through the comparison between the PHS simulated using MCNPX and the PHS determined from the experimental counting curves as described below. 4.3. Determination of the energy scale In order to compare εexp(HV) and εMCNPX (Ethr) a mapping from HV to Ethr is needed. This was obtained through a fitting process determining the optimal energy scale and gain such that the difference between the calculated (dεMCNPX/dEdep) and experimental (dεexp/dEdep) pulse height spectra is minimized. A χ2 minimization procedure was implemented consisting of the following steps: 1- choice of input counting curve εexp(HV); 2- mapping of εexp(HV) on Ethr, after a test energy scale (Ethr (HV)) is set using A and B as free parameters (using Eq. (5)); 3- calculation of the derivative dεexp/dEdep; 4- dεexp/dEdep is divided by a third free parameter K and compared energy bin by energy bin with dεMCNPX/dEdep (i.e.: dεexp = dEdep ¼ K ðdεMCNPX =dEdep Þ) by evaluating the χ2. K is a scale factor (substantially an efficiency multiplier) accounting for any deviation between the levels of the PHS due to systematic and statistical errors.
ð4Þ
where α and B are two constants. By substituting formula (4) in Eq. (3), we obtain Ethr ðHVÞ ¼ A expð B HVÞ
Q min W ion eα
ð5Þ
The minimization procedure (summarized in the scheme of Fig. 6) was applied to the DD irradiation and the UDD sub-unit and restricted to the energy range [0.06–0.5] MeV in which the γ-ray contribution predicted by MCNPX is negligible (see Fig. 5). The
200
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 6. Scheme of minimization procedure.
Fig. 7. (left) Gain for triple GEM with Ar/CO2/CF4–45/15/40 gas mixture: present analysis (solid curve with statistical errors only (2s) on fit (dashed curves)) and literature measurements (error on point at HV ¼ 970 not given in [13] arbitrarily set to 20%); (right) Ethr as a function of HV.
following values were found for the three parameters: A ¼ (3.27 0.4) MeV, B ¼(0.0157 70.0007) V 1 and KDD ¼ 2.47 70.09. The constants A and B inserted in Eqs. (4) and (5) yield G(HV) and Ethr(HV) as shown, respectively, in Fig. 7(left) and Fig. 7(right). The gain obtained with this procedure agrees within uncertainties with the gain measured experimentally for the same type of triple GEM with Ar/CO2/CF4 – 45/40/15 gas mixture [5,13]. The results of the minimization procedure for the DD irradiation and the UDD sub-unit are shown in Fig. 8(left): note that in the plot experimental data refer to all events (neutrons and γ-rays), while MCNPX data refer just to neutron events. The energy scale so determined (A and B) must hold for all measurements (irradiation type, sub-units), while the K factor may depend on the irradiation type. By applying the values obtained to the DD irradiation and UDT sub-unit case the results shown in Fig. 8(right) are found. The minimization procedure was repeated (with the same low and high energy limits) for the DT irradiation and the UDD sub-unit (Fig. 9(left)) using the previously obtained A and B and leaving KDT as the only free parameter. The following value was obtained: KDT ¼3.4 71.2. The application of these values to the DT irradiation and UDT sub-unit case led to the results shown in Fig. 9(right). It must be stressed that only parameters A and B appear in the energy scale, while the parameters KDD and KDT, that account for
differences between experimental and simulated PHS due to systematic and statistical errors, are just multipliers of the simulated PHS.
4.4. Comparison between experimental and calculated efficiency and error analysis The energy scale (i.e. the relation Ethr(HV) from Eq. (5)) with the two parameters A and B obtained through the minimization procedure using the DD irradiation and UDD sub-unit counting curve as input was employed to determine the experimental neutron efficiency curves as defined in (2) εexp ðEthr Þ ¼
r exp ðEthr Þ C meas ðEthr Þ=ðf CS ðEthr ÞΔtÞ ¼ 2 Ni Y N ðAGEM =4πd Þ
ð7Þ
where is the rexp(Ethr)¼ experimental count rate (counts/s) in the considered active part of each sub-unit; the Ni ¼neutrons/s incident on the detector; the Cmeas(Ethr) ¼counts measured in time bin Δt in the considered active part of each sub-unit; the fCS ¼ cluster size;the Δt¼time bin ¼10.4 s; the YN ¼neutrons/s emitted by the source (4.6 109 n/s in DT and 1.85 108 n/s in DD); the d¼ distance between source and GEM detector ¼38 cm; and AGEM ¼ active area of each sub-unit ¼20.16 cm2.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
201
Fig. 8. DD irradiation: comparison between pulse height spectra in UDD sub-unit (left) and UDT sub-unit (right). MCNPX spectra refer to protons only.
Fig. 9. DT irradiation: comparison between pulse height spectra in UDD sub-unit (left) and UDT sub-unit (right). MCNPX spectra refer to protons only. The fitting procedure was used for determination of KDT only (the energy scale is the same as in Fig.8).
Fig. 10. DD irradiation on UDD (left) and UDT (right): neutron efficiency from experiment (points) and MCNPX (solid curve).
In Figs. 10 and 11 we compare, respectively for the DD and DT irradiations (UDD and UDT sub-units), the MCNPX-calculated neutron efficiency (εMCNPX, obtained by integration of the dεMCNPX/ dEdep spectrum, as in formula (1)) with the experimental efficiency εexp from formula (7). In all plots there is agreement between the curves (within maximum error bars) in the energy range characterized by negligible sensitivity of the detector to γ-rays (such energy range is wider in the DT irradiation case, as expected theoretically (see Fig. 5)). The MCNPX-simulated efficiencies are systematically lower than the experimental counterparts and this may be related to the inaccurate MCNPX modeling of some charged particle reactions (see Section 3). The dashed lines around εMCNP represent the boundaries of the maximum error (calculated using the 2s statistical errors of the MCNPX simulations), while the maximum relative error on εexp is given by δεexp ðEthr Þ ¼ δðΔtÞ þ δY N þ δAGEM þ 2δd þ δf CS ðEthr Þ þ δC exp ðEthr Þ
ð8Þ
An estimate of the various constant terms of δεexp provides: δðΔtÞ ¼ 0:1%, δY DD ¼ 8%, δY DT ¼ 3%, δAGEM ¼ 1%, δd ¼ 5% (this also includes geometrical effects such as, for example, the possibility of a misalignment of the detector plane with respect to a plane perpendicular to the beam, resulting in a different effective distance of the detector from the source). fCS was determined through a linear fit to the available experimental cluster size
data [11] and δf CS was calculated using the 2s error on the fitted parameters and the formula of maximum error propagation: δf CS is in the range 16–25% depending on HV. Concerning δC exp three terms were considered and only the first one was found to be non-negligible: 1) 2s counting statistical error (δC exp_stat ) varying with Ethr (up to 7 20% in DD and 75% in DT); 2) systematic errors due to the contribution to the neutron signal due to energy deposited by protons in the first transfer gap. A contribution to the neutron signal can derive from energy deposition events occurring in the first transfer gap (therefore having only 2 stages of amplification). As the voltages of the three amplification stages were set to be the same, it can be assumed that the gain for events occurring in the first transfer gap (Gtransfer1) is equal to G2/3 where G is the total effective gain. The energy scale for the events occurring in the first transfer gap is as follows Ethr_transf er1 ðHVÞ ¼
Gtransf er1 ðHVÞ Ethr ðHVÞ ¼ ½GðHVÞ 1=3 Ethr ðHVÞ GðHVÞ ð9Þ
The resulting PHS due to protons events occurring in the first transfer gap is plotted in Fig. 12 together with the
202
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 11. DT irradiation on UDD (left) and UDT (right): neutron efficiency from experiment (points) and MCNPX (solid curve).
that was not considered in the MCNPX simulations at the time. The difference between the proton PHS with and without the Mylar layer is very large in the UDT sub-unit, as shown by Fig. 13 (left). The Mylar layer produces additional protons due to interactions of 2.5 MeV neutrons: such protons are not blocked by any subsequent Al layer and therefore lead to a significant increase of the total neutron signal recorded by the UDT in DD irradiation experiments. The calculated neutron efficiency values that would have been obtained in the DD irradiation for the UDT sub-unit without Mylar layer are presented in Fig. 13(right). Therefore, in order to have a UDT sub-unit with zero neutron efficiency at 2.5 MeV, the Mylar foil needs to be removed (this is foreseen in the new generation of detector prototypes). Fig. 12. DD irradiation, UDD sub-unit: contribution to the neutron signal of proton events occurring in the first transfer gap (dashed).
5. Response to γ-rays experimental and calculated PHS due to proton events occurring in the drift gap. It is clear that this contribution is negligible in the energy range considered in the minimization procedure. 3) systematic errors due to the attribution to the neutron signal of events in reality due to γ-rays. For the energy range in which the minimization procedure was carried out (Ethr 40.06 MeV), the calculated PHS due to γ-rays is orders of magnitude lower than the PHS due to neutrons (see Fig. 5) and, consequently, the contribution to the neutron signal of events due to γ-rays was considered to be negligible. This is confirmed a posteriori by the fact that the experimental and calculated neutron efficiency curves overlap within experimental errors (see Figs. 10 and 11) for Ethr 40.06 MeV. In the case of experimental data, we must also take into account the absolute error ΔEthr on the energy scale (Ethr) as the original count rate data were taken on the HV scale and we transformed the HV scale into the Ethr scale through a fitting procedure. ΔEthr was determined from Eq. (5) using the 2s error on the fitted parameters and the formula of maximum error propagation. 4.5. Effect of the Mylar layer The 2.5 MeV neutron efficiency of the UDT sub-unit (i.e.: efficiency in a pure 2.5 MeV neutron field) is expected to be zero due to the presence of the Al layer whose thickness completely blocks 2.5 MeV neutrons [3]. However, as the FNG 2.5 MeV field is associated to a small 14 MeV contribution, the UDT sub-unit anyway detected some signal in our DD irradiation experiment. In Ref. [2] it was reported that such signal was higher than what MCNPX simulations suggested. Fig. 10(right) shows that, within the error bars, such discrepancy is now absent. The cause of the discrepancy reported in Ref. [2] was the presence of the Mylar layer used as electric contact in the GEM prototype (located below the Al layer used to block 2.5 MeV neutrons in the UDT sub-unit)
The γ-ray efficiency of the detector depends on the chosen HV (and therefore Ethr). MCNPX calculations in a simplified geometry (monochromatic beam source) indicate that the efficiency for 1 MeV γ-rays (at Ethr ¼ 0.001 MeV) is of the order of 3 10 3 counts/γ-ray. The efficiency decreases rapidly with increasing Ethr and already drops by 3 orders of magnitude by setting Ethr ¼0.06 MeV (Fig. 14). The situation is different with neutrons; in this case, a long efficiency plateau exists followed by a fast decrease at higher Ethr (Figs. 10 and 11). The neutron efficiency at 2.5 MeV of the UDD sub-unit is flat up to Ethr ¼ 0.1 MeV, while the neutron efficiency at 14 MeV of both sub-units is flat up to Ethr ¼0.01 MeV. The possibility of using the present GEM-based detector for fast neutron measurements in a mixed neutron/γ-ray field therefore depends on the chosen HV setting and on the relative number of the incident neutrons and γ-rays. In the mixed neutron/γ-ray field of the FNG irradiation experiments (in which the γ-ray field is not monoenergetic and ranges from 0.01 MeV to 10 MeV, see Fig. 4), the γ-ray sensitivity is negligible for Ethr 4Ethr_min, i.e. the Ethr value above which there is agreement between the calculated neutron efficiency and experimental efficiency curves. For Ethr o Ethr_min the calculated neutron efficiency is lower than the experimental efficiency because the γrays events are not included in the calculated curve. The Ethr_min values that can be deduced from Figs. 10 and 11 are 0.06 MeV and 0.01 MeV respectively for DD and DT irradiations, and they roughly agree with the MCNPX predictions (compare with the Ethr values where the dashed γ-ray curves cross the solid neutron curves in Fig. 5). Given the steep decrease with Ethr of the γ-ray PHS shown in Fig. 5 (that can be considered roughly representative of PHS due to γ-ray fields in fusion devices) it can be expected that even in presence of large γ-ray fields at the detector (e.g. ratio γ/n 10) it should still be possible to reject the γ-ray component by suitably setting the HV, without need to use operational HV corresponding to the region of decreasing of neutron efficiency.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
203
Fig. 13. MCNPX-calculated differential proton spectra (left) and neutron efficiency (right) for the UDT sub-unit (DD irradiation): comparison between case with no Mylar (dashed) and with Mylar 11.5 μm thick (solid). The actual GEM prototype used in the experiments includes a Mylar layer (11.5 μm thick) below the Al layer.
Table 2 Maximum neutron efficiency values in DD and DT irradiation (maximum errors in excess of 50%, as from Figs. 10 and 11).
Fig. 14. Simulated detector γ-ray efficiency (isotropic gamma sources: 0.66 MeV (137Cs) and 1.25 MeV (average of the two gamma lines of 60Co: 1.173 and 1.333 MeV). The HV scale is also shown on the top of the figure.
Sub-unit Efficiency (counts/n)
UDD
UDT
εexp_DD εexp_DT
1 10 3 4 10 3
3 10 4 4 10 3
values are taken in the HV plateau before any γ-ray sensitivity sets in (see Fig. 15). As explained in the sections above, the neutrons incident on the detectors in the DD and DT experiments were not monochromatic. However, the DD and DT efficiency values shown in Table 2 can be considered to be representative of the neutron efficiency at 2.5 MeV and 14 MeV taking into account that: a) the small fraction of low-energy neutrons incident on the detector (respectively 15% and 6% of the total, respectively in DT and DD irradiations); b) the large error bars on the experimental neutron efficiency; c) the negligible contribution (in the case of the DD irradiation only) of 14 MeV neutrons to εexp_DD (as these are only a fraction 1/200 of YDD) and, in addition, the fact that the proton PHS produced by 14 MeV neutrons peaks at much lower Edep than the proton PHS produced by 2.5 MeV neutrons ( 0.02 MeV against 0.1 MeV).
Fig. 15. DD irradiation: neutron efficiency of the UDD sub-unit; experiment (points), MCNPX (solid curve).
As an example in this respect, we show in Fig. 15 the efficiency curve as a function of HV for the UDD sub-unit in the DD irradiation experiment: it can be seen that the minimum HV required to maximize the neutron efficiency at 2.5 MeV and still to have negligible sensitivity to γ-rays is HV 1000 V. For lower HV the neutron efficiency decreases rapidly, while for higher HV the γ-ray sensitivity sets in (note the difference between the solid line and the points).
Note that for the UDT sub-unit the theoretical efficiency at 2.5 MeV is zero when no Mylar layer is used. Finally, note that the same neutron efficiency at 14 MeV is found for both UDD and UDT. This occurs as the higher proton conversion efficiency at 14 MeV in the UDT sub-unit, roughly a factor 1.6 with respect to the UDD case (2 mm polyethylene converter in UDT versus 0.7 mm polyethylene converter in UDD, see Fig. 2 in [3]), is compensated by the presence of the thin aluminum foil (0.2 mm) below the polyethylene converter in the UDT sub-unit, which leads to a reduction by approximately the same factor (see Fig. 3 in [3]) of the number of protons entering the gas drift gap.
7. Conclusions 6. Neutron efficiency The summary of the values of the experimental neutron efficiency in DD and DT irradiations is given in Table 2: such
A GEM-based neutron detector (a triple GEM structure based on a Ar/CO2/CF4 – 45/15/40 gas mixture) with two sub-units for the detection of 2.5 þ14 MeV neutrons and 14 MeV neutrons
204
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
respectively [2] was characterized in terms of neutron efficiency, by means of MCNPX modeling and analysis of experimental irradiations. The MCNPX model fully describes the walls of the detector, the three amplification stages and the Mylar layer used as electric contact in both sub-units and takes into account the interactions occurring both in the front and in the sides of the detector system. The irradiations were performed at a neutron generator facility (FNG) using deuterons impinging on deuterium and tritium targets (DD and DT neutrons respectively) and the counts at several high voltages were acquired (counting curves). The differential proton spectra were determined from the counting curves in the irradiation experiments by setting a parameterized energy scale Ethr(HV), differentiating the resulting εexp(Ethr) and finally determining through minimization the energy scale parameters that best match the experimental PHS with the MCNPX‐simulated one. This led to the evaluation of the experimental efficiency as a function of the discrimination level set on the deposited energy (Ethr ¼ energy threshold) and its comparison with the MCNPX-simulated neutron efficiency. The GEM-based neutron detector is also sensitive to γ-rays. However, γ-rays can be effectively rejected by selecting a sufficiently low operational HV (i.e. high Ethr). In the specific case of the FNG irradiations, where the γ-ray spectrum is in the range 0.1–10 MeV, the energy deposited in the gas by γ-rays is typically r0.06 MeV against Z0.1 MeV and Z0.01 MeV deposited respectively by 2.5 MeV and 14 MeV neutrons and the rejection can be achieved using HV 1000 V corresponding to Ethr 0.07 MeV. The detector can achieve (in the operational range where the γ-ray contamination to the signal is absent) a maximum neutron efficiency of 1 10 3 counts/n at 2.5 MeV (UDD sub-unit) and of 4 10 3 counts/n at 14 MeV (UDT and UDD sub-units); a detailed error analysis indicates maximum total uncertainties in these figures (systematic and counting statistics) in excess of 750%. Acknowledgments The authors would like to thank Maurizio Angelone and Mario Pillon for their support in the FNG experiments, Aldo Pensa and
Gerardo Claps for technical support and Antonino Pietropaolo for his very careful reading of the manuscript.
References [1] F. Sauli, Nuclear Instruments and Methods in Physics Research A 386 (1997) 531. [2] B. Esposito, F. Murtas, R. Villari, M. Angelone, D. Marocco, M. Pillon, S. Puddu, Nuclear Instruments and Methods in Physics Research A 617 (2010) 155. [3] R. Villari, M. Angelone, B. Esposito, A. Ferrari, D. Marocco, F. Murtas, M. Pillon, IEEE Transactions on Nuclear Science NS 56 (2009) 1102. [4] A. Bressan, J.C. Labbè, P. Pagano, L. Ropelewski, F. Sauli, Nuclear Instruments and Methods in Physics Research A 425 (1999) 262. [5] G. Bencivenni, P. de Simone, F. Murtas, M. Poli Lerner, W. Bonivento, A. Cardini, C. Deplano, D. Pinci, D. Raspino, Nuclear Instruments and Methods in Physics Research A 513 (2003) 264. [6] M. Martone, M. Angelone, M. Pillon, Journal of Nuclear Materials 212–215 (1994) 1661. [7] W. Bonivento, P. Jarron, D. Moraes, W. Riegler, F. dos Santos, Nuclear Instruments and Methods in Physics Research A 491 (2002) 233. [8] A. Milocco, A. Trkov, M. Pillon, Nuclear Instruments and Methods in Physics Research B 271 (2012) 6. [9] M. Angelone, M. Pillon, P. Batistoni, P. Marini, M. Martone, V. Rado, Review of Scientific Instruments 67 (1996) 2189. [10] D. Raspino, Il Rivelatore a Tripla GEM per il Sistema di Muoni dell'Esperimento LHCb, Ph.D. thesis, Universita' di Cagliari, 2004. [11] S. Puddu, Sviluppo di Sistemi Diagnostici per Plasmi Nucleari Accesi, Tesi di Laurea in Fisica, Universita’ di Cagliari, 2010. [12] G.Z. McKinney, et al., MCNPX 2.6.X Features, LANL Report LA-UR-07-2053, Presented at the M&C/SNA 2007 Conference Workshop, Monterey, CA, April 15–19, 2007. Available from: 〈http://mcnpx.lanl.gov/〉. [13] G. Croci, M. Rebai, G. Claps, M. Cavenago, M. Dalla Palma, G. Gervasini, G. Grosso, F. Murtas, R. Pasqualotto, E. Perelli Cippo, M. Tardocchi, M. Tollin, G. Gorini, Journal of Instrumentation 7 (2012) C03010.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Characterization of a GEM-based fast neutron detector B. Esposito a,n, D. Marocco a, R. Villari a, F. Murtas b, R. Rodionov c a b c
Associazione Euratom-ENEA sulla Fusione, Via E. Fermi, 45, I-00044 Frascati, Roma, Italy Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, Via E. Fermi, 40, I-00044 Frascati, Roma, Italy SRC RF TRINITI Troitsk, Moscow, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 10 November 2013 Accepted 26 November 2013 Available online 10 December 2013
The neutron efficiency of a Gas Electron Multiplier (GEM)-based detector designed for fast neutron measurements in fusion devices was determined through the combined use of Monte Carlo (MCNPX) calculations and analysis of deuterium–deuterium and deuterium–tritium neutron irradiation experiments. The detector, characterized by a triple GEM structure flushed with a Ar/CO2/CF4 – 45/15/40 gas mixture, features a digital read-out system and has two sub-units for the detection of 2.5þ 14 MeV neutrons and 14 MeV neutrons (UDD and UDT, respectively). The pulse height spectra (PHS) determined from the curves of experimental efficiency as a function of the detector's high voltage (HV) and the MCNPX-simulated PHS were compared using a fitting routine that finds the best match between the experimental and simulated PHS by assuming a parametric model for the relation between HV (that determines the detector's gain) and the energy deposited in the gas. This led to express the experimental neutron efficiency as a function of the discrimination level set on the deposited energy (energy threshold). The detector sensitivity to γ-rays was also analyzed and the operational range in which the γ-ray contribution to the signal is not negligible was determined. It is found that this detector can reach a maximum neutron efficiency of 1 10 3 counts/n at 2.5 MeV (UDD sub-unit) and of 4 10 3 counts/n at 14 MeV (UDT and UDD sub-units). & 2014 EURATOM ENEA. Published by Elsevier B.V. All rights reserved.
Keywords: Neutron detector GEM Fusion
1. Introduction This paper describes the work of efficiency characterization of a GEM-based [1] prototype detector designed for fast neutron measurements in thermonuclear fusion devices [2,3]. Neutron flux monitors for fusion applications must be able to cover a wide dynamic range and sustain high count rates: both features can in principle be provided by a GEM-based neutron detector characterized by a large number of pixels with high counting rate capability (up to 10 MHz/cm2 [4]). In fusion plasmas 2.5 MeV neutrons are produced through deuterium–deuterium (DD) reactions and 14 MeV neutrons through deuterium–tritium reactions (DT); a few percent of the total neutron yield in pure DD plasmas is due to 14 MeV neutrons produced by triton burn-up. The detector consists of two sub-units respectively for total (2.5 MeV (DD)þ14 MeV (DT)) and DT neutron measurements (UDD and UDT respectively) and has a multilayer structure. The first layer is a proton recoil neutron converter made of polyethylene. The thickness in each sub-unit is optimized respectively to maximize the neutron efficiency at 2.5 and 14 MeV (0.7 mm in the UDD and 2 mm in the UDT). The second layer is a thin aluminum foil (0.2 mm) only present in the UDT sub-unit that provides a suitable energy cut-off for the protons produced by 2.5 MeV neutrons [3]. The third layer is n
Corresponding author. Tel.: þ 39 06 94005152. E-mail address: [email protected] (B. Esposito).
constituted by an amplifying structure in which the recoil protons produce a detectable electron signal through ionization of gas atoms: it is a triple GEM based on a Ar/CO2/CF4 gas mixture (45/15/40 percentages in volume), consisting of a drift gap (3 mm wide) where the primary ionization is produced, two transfer gaps (1 mm and 2 mm wide) and an induction gap (1 mm wide) for the charge collection. The three GEM foils are stacked between a cathode (a Mylar layer 11.5 μm thick with an Al deposit 0.5 μm thick on the bottom side for electrical contact) and a segmented anode (128 pads, 8 16 matrix, size of single pad¼ 12.3 6 mm2) connected to the electronic readout system. The layout of the detector is shown in Fig. 1. Further details on the composition of the detector are given in Section 3 (see Table 1). Each GEM consists of a thin kapton foil (50 μm thick), copper clad on each side (copper thickness¼5 μm) perforated with holes (biconical, with external and internal diameters of 70 μm and 50 μm, respectively, and a pitch of 140 μm). An electric potential difference is applied between the copper sides of each GEM allowing for electron multiplication in the holes. The sum of the three voltage differences is referred to as high voltage (HV). Moreover, external electric fields are necessary to transfer the electrons in the drift (3 kV/cm), transfer (3 kV/cm) and induction gaps (5 kV/cm): the values of these fields are the standard ones used for triple GEM detectors in the LHCb experiment at CERN [5]. An earlier paper [2] reported on the results of DT and DD neutron irradiations of the detector prototype carried out at the
0168-9002/$ - see front matter & 2014 EURATOM ENEA. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.11.103
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Frascati Neutron Generator (FNG) [6] up to 107 n/cm2/s as well as on preliminary estimates of the neutron efficiency based on a simplified MCNPX model of the detector. It was found out that the detector is also sensitive to γ-rays, although a proper selection of the working HV may lead to a suppression of the γ-ray contribution. Moreover, an excessively large signal was observed during the DD irradiation in the UDT sub-unit: this was unexpected as the protons from to DD interaction in the converter should be efficiently blocked by the Al layer and, therefore, produce a negligible signal under DD irradiation in the UDT sub-unit. In the present work a detailed analysis of the detector neutron efficiency is performed through the comparison of the differential proton spectra deduced (see Section 4 for details) from the curves of experimental efficiency as a function of HV (εexp(HV), counting curves) measured in [2] and those simulated using a refined MCNPX model. The issues of the detector sensitivity to γ-rays and of the UDT sub-unit response to DD neutrons are also discussed.
Table 1 Materials used in the MCNPX model of the GEM detector.
Fiberglass (wall)
Gold (pad)
Mass density (g/cm3) 1.864
19.6
2. Experiments The irradiation experiments [2] were carried out by placing the GEM detector perpendicularly to the FNG beam at a distance d¼ 38 cm from the FNG target (with the neutron source roughly aligned with the center of the detector) for which the point source approximation holds within a good degree of accuracy. The neutron flux incident on the detector was kept constant and the number of pulses recorded was measured as a function of the detector gain obtained by selecting different HV values (the voltage difference applied to each of the three GEMs being the same). The electronic threshold, setting the minimum charge that can be detected by the readout electronics (CARIOCA-GEM chips [7]), was kept fixed throughout the FNG experiments (Qmin ¼ 32 fC, see Ref. [2]). The digital read-out system based on the CARIOCA-GEM chips does not enable to obtain pulse height spectra (PHS), but only to measure the counting rate. Two irradiations were made: 1) DD irradiation: using 2.5 MeV neutrons with a measured total DD neutron yield (emitted over 4π) YDD ¼1.85 108 n/s (uncertainty 8%) and a small contribution due to 14 MeV neutrons ( 1/200 YDD) due to triton burn-up in the deuterium target. The DD yield was measured by means of a NE213 scintillator calibrated using the neutron activation method (indium foil). MCNP calculations were used to evaluate the indium activity per source neutron; the quoted uncertainty includes the terms due to the indium activity measurement and indium cross section added in quadrature (the MCNP error is negligible) [8]. 2) DT irradiation: using 14 MeV neutrons with a measured YDT ¼4.6 109 n/s (uncertainty 3%). The DT yield was measured by means of the associated α-particle method with a silicon surface barrier (SSD) detector; the quoted uncertainty includes the terms due to the anisotropy factor, the collimator solid angle and the target-SSD detector solid-angle determination added in quadrature [9].
Fig. 1. Scheme of the triple-GEM neutron detector.
Material
197
Composition
Weight fraction
Si O Al Ca Mg B Na K C H N O
1.66E–01 3.09E–01 5.02E–02 8.56E–02 1.27E–02 1.45E–02 1.19E–03 1.13E–03 2.50E–01 1.52E–02 3.65E–02 5.76E–02
Au
1
Gas (Ar 45%–CO2 15%–CF4 40%)
0.00251
C O F Ar
1.10E–01 7.99E–02 5.13E–01 2.97E–01
Kapton (GEM foils)
0.71
H C N O
2.64E–02 6.91E–01 7.33E–02 2.09E–01
Copper (GEM foils)
4.48
Cu
1
Mylar
1.4
H C O
4.20E–02 6.25E–01 3.33E–01
Polyethylene
0.94
H C
1.44E–01 8.56E–01
Aluminum
2.7
Al
1
In both cases a γ-ray field was also present during the irradiation. No γ-ray flux or spectral measurements were available. The counting curves (experimental efficiency, εexp(HV), as a function HV) are shown in Fig. 2 for the case of the UDD sub-unit under DD and DT irradiations. The incident neutrons have been calculated using a point source/isotropic emission approximation and the neutron yield values (YDD and YDT) given above. It has to be noted that, as each particle event can induce a signal in more than one pad, the cluster size correction factor (fCS) must be applied to the measured counts (Cmeas) shown in Fig. 2 in order obtain the experimental counts (Cexp ¼Cmeas/fCS): the cluster size is the average number of pads in which a signal is induced by a particle event [10]. fCS was measured for this detector [11] and found to increase linearly with HV (from 1.0 at 800 V up to 1.6 at 1200 V).
3. MCNPX model and simulations Calculations with MCNPX [12] were performed to predict the detector response to the FNG mixed neutron/γ-ray field. A detailed MCNPX model was developed in order to take into account all details of the GEM detector. In particular, the walls of the detector, the three amplification stages and the Mylar layer used as electric contact in both sub-units were included (see the model in Fig. 3 and the list of all materials used in the MCNPX analysis in Table 1). This detector model, together with a detailed model of the FNG DD and DT neutron sources (see [8] and references therein), was used to simulate the irradiations described in the previous section.
198
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 2. Experimental counting curves at constant neutron flux (FNG irradiations [2]).
Fig. 3. MCNPX model of the GEM detector (dimensions are not in scale): (left) vertical section; (right) front view: all pads have the same size; note that the black lines enclose the 28 central pads (for each sub-unit) that are less prone to edge effects and whose data have therefore been used in the analysis (both simulation and experiment).
The calculated spectra of neutrons and γ-rays at the detector position during DT and DD irradiations are shown in Fig. 4. The calculated total neutron fluence on the detector is 6 10 5 n/ cm2/source neutron. The low-energy neutrons and prompt γ-rays are produced by the interaction of primary neutrons with the target materials. The contribution of low-energy neutrons (those below the 14 MeV DT and 2.5 MeV DD neutron peaks) is respectively 15% (DT) and 6% (DD) of the total neutron fluence. The γ spectrum peaks at 1 MeV and the calculated total γ fluence amounts to 11% (DT) and 7% (DD) of the total neutron fluence. The incident spectra for the FNG DD and DT irradiation experiments were used by MCNPX to calculate the energy deposited in the gas (Edep) by protons (neutron signal) and γ-rays entering the detector and the respective PHS dΝMCNPX/dEdep (in particles/(source neutron MeV)) were calculated for the FNG irradiation DD and DT experiments (Fig. 5). For the PHS calculation we considered, in first approximation, only the energy deposited in the drift gap since events produced in the remaining gaps will have much lower amplitude due to the lower number of amplification stages. Note that the small contribution due to 14 MeV neutrons was included in the DD MCNPX calculations. The electromagnetic component interacts weakly with the GEM and deposits only a small fraction of its energy in the gas. The neutron signal due to proton energy deposition events mostly corresponds to the range 0.01–0.1 MeV, whereas γ-ray energy deposition occurs at lower energy.
The level of the neutron signal determined by the MCNPX simulation might be somehow underestimated because of a missing contribution due to the fact that the charged particle reactions for some of the elements are not accurately modeled in MCNPX.
4. Response to neutrons The final objective is to determine the neutron efficiency values at 2.5 MeV and 14 MeV. In order to do this we will first analyze the detector response to the non-monochromatic neutrons in the mixed γ-ray/neutron field of the DD and DT irradiation experiments by determining the experimental (εexp) and simulated (εMCNPX) neutron efficiency values. 4.1. Simulated efficiency Assuming a point-like neutron source, the calculated efficiency in protons/incident neutron (pMCNPX/N) for a given threshold on the deposited energy (Ethr) is given by: Z 1 2Z 1 p 4πd dN MCNPX dεMCNPX εMCNPX ðEthr Þ ¼ MCNPX ¼ dEdep ¼ dEdep ð1Þ dE dEdep dep N AGEM Ethr Ethr where N ¼AGEM/4πd2 is the number of neutrons incident on a detector of area AGEM from a source (located at a distance d from the detector) emitting one neutron.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
199
Fig. 4. Calculated FNG neutron and γ-ray spectra at the detector position. Note that the units are different in the two figures: in the figure on the right the incident neutrons are those that impinge on the detector, while in that on the left the source neutrons are those emitted by the source.
Fig. 5. MCNPX calculations (statistical errors shown): PHS in the GEM due to γ-rays and protons (neutrons): UDD sub-unit under FNG DD (left) and DT (right) irradiation.
4.2. Experimental efficiency
with:
The counting curves can be used to obtain the experimental efficiency as a function of Ethr by means of the following formula: 2 Z 1 r exp ðHVÞ 4πd dNexp ¼ εexp ðHVÞ ¼ dEdep Ni Y N AGEM Ethr ðHVÞ dEdep Z 1 dεexp ¼ ð2Þ dE dEdep
A¼
Ethr ðHVÞ
dep
where rexp, Ni and YN are, respectively, the experimental count rate (counts/s) at HV, the neutrons/s incident on the detector and the FNG neutron yield (neutrons/s). The relation between Ethr and HV can be determined by the formula Q min ¼ nel eGðHVÞ ¼
Ethr ðHVÞeGðHVÞ W ion
ð3Þ
where nel ¼number of electrons produced in the drift gap, G(HV)¼ effective gain (ratio between electrons collected at the anode and nel), Wion ¼energy dissipation in the gas for the production of an ionelectron pair, e¼electron charge; the Wion in the chosen gas mixture can be assumed to be 27.6 eV [5]. This formula sets for each HV the threshold energy Ethr to be deposited in the drift gap by an ionizing particle in order to produce at the anode the minimum charge detectable by the readout electronics (Qmin). Since, experimentally, the gain can be expressed as a function of HV GðHVÞ ¼ α expðB HVÞ
ð6Þ
The parameters A and B can be determined through the comparison between the PHS simulated using MCNPX and the PHS determined from the experimental counting curves as described below. 4.3. Determination of the energy scale In order to compare εexp(HV) and εMCNPX (Ethr) a mapping from HV to Ethr is needed. This was obtained through a fitting process determining the optimal energy scale and gain such that the difference between the calculated (dεMCNPX/dEdep) and experimental (dεexp/dEdep) pulse height spectra is minimized. A χ2 minimization procedure was implemented consisting of the following steps: 1- choice of input counting curve εexp(HV); 2- mapping of εexp(HV) on Ethr, after a test energy scale (Ethr (HV)) is set using A and B as free parameters (using Eq. (5)); 3- calculation of the derivative dεexp/dEdep; 4- dεexp/dEdep is divided by a third free parameter K and compared energy bin by energy bin with dεMCNPX/dEdep (i.e.: dεexp = dEdep ¼ K ðdεMCNPX =dEdep Þ) by evaluating the χ2. K is a scale factor (substantially an efficiency multiplier) accounting for any deviation between the levels of the PHS due to systematic and statistical errors.
ð4Þ
where α and B are two constants. By substituting formula (4) in Eq. (3), we obtain Ethr ðHVÞ ¼ A expð B HVÞ
Q min W ion eα
ð5Þ
The minimization procedure (summarized in the scheme of Fig. 6) was applied to the DD irradiation and the UDD sub-unit and restricted to the energy range [0.06–0.5] MeV in which the γ-ray contribution predicted by MCNPX is negligible (see Fig. 5). The
200
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 6. Scheme of minimization procedure.
Fig. 7. (left) Gain for triple GEM with Ar/CO2/CF4–45/15/40 gas mixture: present analysis (solid curve with statistical errors only (2s) on fit (dashed curves)) and literature measurements (error on point at HV ¼ 970 not given in [13] arbitrarily set to 20%); (right) Ethr as a function of HV.
following values were found for the three parameters: A ¼ (3.27 0.4) MeV, B ¼(0.0157 70.0007) V 1 and KDD ¼ 2.47 70.09. The constants A and B inserted in Eqs. (4) and (5) yield G(HV) and Ethr(HV) as shown, respectively, in Fig. 7(left) and Fig. 7(right). The gain obtained with this procedure agrees within uncertainties with the gain measured experimentally for the same type of triple GEM with Ar/CO2/CF4 – 45/40/15 gas mixture [5,13]. The results of the minimization procedure for the DD irradiation and the UDD sub-unit are shown in Fig. 8(left): note that in the plot experimental data refer to all events (neutrons and γ-rays), while MCNPX data refer just to neutron events. The energy scale so determined (A and B) must hold for all measurements (irradiation type, sub-units), while the K factor may depend on the irradiation type. By applying the values obtained to the DD irradiation and UDT sub-unit case the results shown in Fig. 8(right) are found. The minimization procedure was repeated (with the same low and high energy limits) for the DT irradiation and the UDD sub-unit (Fig. 9(left)) using the previously obtained A and B and leaving KDT as the only free parameter. The following value was obtained: KDT ¼3.4 71.2. The application of these values to the DT irradiation and UDT sub-unit case led to the results shown in Fig. 9(right). It must be stressed that only parameters A and B appear in the energy scale, while the parameters KDD and KDT, that account for
differences between experimental and simulated PHS due to systematic and statistical errors, are just multipliers of the simulated PHS.
4.4. Comparison between experimental and calculated efficiency and error analysis The energy scale (i.e. the relation Ethr(HV) from Eq. (5)) with the two parameters A and B obtained through the minimization procedure using the DD irradiation and UDD sub-unit counting curve as input was employed to determine the experimental neutron efficiency curves as defined in (2) εexp ðEthr Þ ¼
r exp ðEthr Þ C meas ðEthr Þ=ðf CS ðEthr ÞΔtÞ ¼ 2 Ni Y N ðAGEM =4πd Þ
ð7Þ
where is the rexp(Ethr)¼ experimental count rate (counts/s) in the considered active part of each sub-unit; the Ni ¼neutrons/s incident on the detector; the Cmeas(Ethr) ¼counts measured in time bin Δt in the considered active part of each sub-unit; the fCS ¼ cluster size;the Δt¼time bin ¼10.4 s; the YN ¼neutrons/s emitted by the source (4.6 109 n/s in DT and 1.85 108 n/s in DD); the d¼ distance between source and GEM detector ¼38 cm; and AGEM ¼ active area of each sub-unit ¼20.16 cm2.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
201
Fig. 8. DD irradiation: comparison between pulse height spectra in UDD sub-unit (left) and UDT sub-unit (right). MCNPX spectra refer to protons only.
Fig. 9. DT irradiation: comparison between pulse height spectra in UDD sub-unit (left) and UDT sub-unit (right). MCNPX spectra refer to protons only. The fitting procedure was used for determination of KDT only (the energy scale is the same as in Fig.8).
Fig. 10. DD irradiation on UDD (left) and UDT (right): neutron efficiency from experiment (points) and MCNPX (solid curve).
In Figs. 10 and 11 we compare, respectively for the DD and DT irradiations (UDD and UDT sub-units), the MCNPX-calculated neutron efficiency (εMCNPX, obtained by integration of the dεMCNPX/ dEdep spectrum, as in formula (1)) with the experimental efficiency εexp from formula (7). In all plots there is agreement between the curves (within maximum error bars) in the energy range characterized by negligible sensitivity of the detector to γ-rays (such energy range is wider in the DT irradiation case, as expected theoretically (see Fig. 5)). The MCNPX-simulated efficiencies are systematically lower than the experimental counterparts and this may be related to the inaccurate MCNPX modeling of some charged particle reactions (see Section 3). The dashed lines around εMCNP represent the boundaries of the maximum error (calculated using the 2s statistical errors of the MCNPX simulations), while the maximum relative error on εexp is given by δεexp ðEthr Þ ¼ δðΔtÞ þ δY N þ δAGEM þ 2δd þ δf CS ðEthr Þ þ δC exp ðEthr Þ
ð8Þ
An estimate of the various constant terms of δεexp provides: δðΔtÞ ¼ 0:1%, δY DD ¼ 8%, δY DT ¼ 3%, δAGEM ¼ 1%, δd ¼ 5% (this also includes geometrical effects such as, for example, the possibility of a misalignment of the detector plane with respect to a plane perpendicular to the beam, resulting in a different effective distance of the detector from the source). fCS was determined through a linear fit to the available experimental cluster size
data [11] and δf CS was calculated using the 2s error on the fitted parameters and the formula of maximum error propagation: δf CS is in the range 16–25% depending on HV. Concerning δC exp three terms were considered and only the first one was found to be non-negligible: 1) 2s counting statistical error (δC exp_stat ) varying with Ethr (up to 7 20% in DD and 75% in DT); 2) systematic errors due to the contribution to the neutron signal due to energy deposited by protons in the first transfer gap. A contribution to the neutron signal can derive from energy deposition events occurring in the first transfer gap (therefore having only 2 stages of amplification). As the voltages of the three amplification stages were set to be the same, it can be assumed that the gain for events occurring in the first transfer gap (Gtransfer1) is equal to G2/3 where G is the total effective gain. The energy scale for the events occurring in the first transfer gap is as follows Ethr_transf er1 ðHVÞ ¼
Gtransf er1 ðHVÞ Ethr ðHVÞ ¼ ½GðHVÞ 1=3 Ethr ðHVÞ GðHVÞ ð9Þ
The resulting PHS due to protons events occurring in the first transfer gap is plotted in Fig. 12 together with the
202
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
Fig. 11. DT irradiation on UDD (left) and UDT (right): neutron efficiency from experiment (points) and MCNPX (solid curve).
that was not considered in the MCNPX simulations at the time. The difference between the proton PHS with and without the Mylar layer is very large in the UDT sub-unit, as shown by Fig. 13 (left). The Mylar layer produces additional protons due to interactions of 2.5 MeV neutrons: such protons are not blocked by any subsequent Al layer and therefore lead to a significant increase of the total neutron signal recorded by the UDT in DD irradiation experiments. The calculated neutron efficiency values that would have been obtained in the DD irradiation for the UDT sub-unit without Mylar layer are presented in Fig. 13(right). Therefore, in order to have a UDT sub-unit with zero neutron efficiency at 2.5 MeV, the Mylar foil needs to be removed (this is foreseen in the new generation of detector prototypes). Fig. 12. DD irradiation, UDD sub-unit: contribution to the neutron signal of proton events occurring in the first transfer gap (dashed).
5. Response to γ-rays experimental and calculated PHS due to proton events occurring in the drift gap. It is clear that this contribution is negligible in the energy range considered in the minimization procedure. 3) systematic errors due to the attribution to the neutron signal of events in reality due to γ-rays. For the energy range in which the minimization procedure was carried out (Ethr 40.06 MeV), the calculated PHS due to γ-rays is orders of magnitude lower than the PHS due to neutrons (see Fig. 5) and, consequently, the contribution to the neutron signal of events due to γ-rays was considered to be negligible. This is confirmed a posteriori by the fact that the experimental and calculated neutron efficiency curves overlap within experimental errors (see Figs. 10 and 11) for Ethr 40.06 MeV. In the case of experimental data, we must also take into account the absolute error ΔEthr on the energy scale (Ethr) as the original count rate data were taken on the HV scale and we transformed the HV scale into the Ethr scale through a fitting procedure. ΔEthr was determined from Eq. (5) using the 2s error on the fitted parameters and the formula of maximum error propagation. 4.5. Effect of the Mylar layer The 2.5 MeV neutron efficiency of the UDT sub-unit (i.e.: efficiency in a pure 2.5 MeV neutron field) is expected to be zero due to the presence of the Al layer whose thickness completely blocks 2.5 MeV neutrons [3]. However, as the FNG 2.5 MeV field is associated to a small 14 MeV contribution, the UDT sub-unit anyway detected some signal in our DD irradiation experiment. In Ref. [2] it was reported that such signal was higher than what MCNPX simulations suggested. Fig. 10(right) shows that, within the error bars, such discrepancy is now absent. The cause of the discrepancy reported in Ref. [2] was the presence of the Mylar layer used as electric contact in the GEM prototype (located below the Al layer used to block 2.5 MeV neutrons in the UDT sub-unit)
The γ-ray efficiency of the detector depends on the chosen HV (and therefore Ethr). MCNPX calculations in a simplified geometry (monochromatic beam source) indicate that the efficiency for 1 MeV γ-rays (at Ethr ¼ 0.001 MeV) is of the order of 3 10 3 counts/γ-ray. The efficiency decreases rapidly with increasing Ethr and already drops by 3 orders of magnitude by setting Ethr ¼0.06 MeV (Fig. 14). The situation is different with neutrons; in this case, a long efficiency plateau exists followed by a fast decrease at higher Ethr (Figs. 10 and 11). The neutron efficiency at 2.5 MeV of the UDD sub-unit is flat up to Ethr ¼ 0.1 MeV, while the neutron efficiency at 14 MeV of both sub-units is flat up to Ethr ¼0.01 MeV. The possibility of using the present GEM-based detector for fast neutron measurements in a mixed neutron/γ-ray field therefore depends on the chosen HV setting and on the relative number of the incident neutrons and γ-rays. In the mixed neutron/γ-ray field of the FNG irradiation experiments (in which the γ-ray field is not monoenergetic and ranges from 0.01 MeV to 10 MeV, see Fig. 4), the γ-ray sensitivity is negligible for Ethr 4Ethr_min, i.e. the Ethr value above which there is agreement between the calculated neutron efficiency and experimental efficiency curves. For Ethr o Ethr_min the calculated neutron efficiency is lower than the experimental efficiency because the γrays events are not included in the calculated curve. The Ethr_min values that can be deduced from Figs. 10 and 11 are 0.06 MeV and 0.01 MeV respectively for DD and DT irradiations, and they roughly agree with the MCNPX predictions (compare with the Ethr values where the dashed γ-ray curves cross the solid neutron curves in Fig. 5). Given the steep decrease with Ethr of the γ-ray PHS shown in Fig. 5 (that can be considered roughly representative of PHS due to γ-ray fields in fusion devices) it can be expected that even in presence of large γ-ray fields at the detector (e.g. ratio γ/n 10) it should still be possible to reject the γ-ray component by suitably setting the HV, without need to use operational HV corresponding to the region of decreasing of neutron efficiency.
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
203
Fig. 13. MCNPX-calculated differential proton spectra (left) and neutron efficiency (right) for the UDT sub-unit (DD irradiation): comparison between case with no Mylar (dashed) and with Mylar 11.5 μm thick (solid). The actual GEM prototype used in the experiments includes a Mylar layer (11.5 μm thick) below the Al layer.
Table 2 Maximum neutron efficiency values in DD and DT irradiation (maximum errors in excess of 50%, as from Figs. 10 and 11).
Fig. 14. Simulated detector γ-ray efficiency (isotropic gamma sources: 0.66 MeV (137Cs) and 1.25 MeV (average of the two gamma lines of 60Co: 1.173 and 1.333 MeV). The HV scale is also shown on the top of the figure.
Sub-unit Efficiency (counts/n)
UDD
UDT
εexp_DD εexp_DT
1 10 3 4 10 3
3 10 4 4 10 3
values are taken in the HV plateau before any γ-ray sensitivity sets in (see Fig. 15). As explained in the sections above, the neutrons incident on the detectors in the DD and DT experiments were not monochromatic. However, the DD and DT efficiency values shown in Table 2 can be considered to be representative of the neutron efficiency at 2.5 MeV and 14 MeV taking into account that: a) the small fraction of low-energy neutrons incident on the detector (respectively 15% and 6% of the total, respectively in DT and DD irradiations); b) the large error bars on the experimental neutron efficiency; c) the negligible contribution (in the case of the DD irradiation only) of 14 MeV neutrons to εexp_DD (as these are only a fraction 1/200 of YDD) and, in addition, the fact that the proton PHS produced by 14 MeV neutrons peaks at much lower Edep than the proton PHS produced by 2.5 MeV neutrons ( 0.02 MeV against 0.1 MeV).
Fig. 15. DD irradiation: neutron efficiency of the UDD sub-unit; experiment (points), MCNPX (solid curve).
As an example in this respect, we show in Fig. 15 the efficiency curve as a function of HV for the UDD sub-unit in the DD irradiation experiment: it can be seen that the minimum HV required to maximize the neutron efficiency at 2.5 MeV and still to have negligible sensitivity to γ-rays is HV 1000 V. For lower HV the neutron efficiency decreases rapidly, while for higher HV the γ-ray sensitivity sets in (note the difference between the solid line and the points).
Note that for the UDT sub-unit the theoretical efficiency at 2.5 MeV is zero when no Mylar layer is used. Finally, note that the same neutron efficiency at 14 MeV is found for both UDD and UDT. This occurs as the higher proton conversion efficiency at 14 MeV in the UDT sub-unit, roughly a factor 1.6 with respect to the UDD case (2 mm polyethylene converter in UDT versus 0.7 mm polyethylene converter in UDD, see Fig. 2 in [3]), is compensated by the presence of the thin aluminum foil (0.2 mm) below the polyethylene converter in the UDT sub-unit, which leads to a reduction by approximately the same factor (see Fig. 3 in [3]) of the number of protons entering the gas drift gap.
7. Conclusions 6. Neutron efficiency The summary of the values of the experimental neutron efficiency in DD and DT irradiations is given in Table 2: such
A GEM-based neutron detector (a triple GEM structure based on a Ar/CO2/CF4 – 45/15/40 gas mixture) with two sub-units for the detection of 2.5 þ14 MeV neutrons and 14 MeV neutrons
204
B. Esposito et al. / Nuclear Instruments and Methods in Physics Research A 741 (2014) 196–204
respectively [2] was characterized in terms of neutron efficiency, by means of MCNPX modeling and analysis of experimental irradiations. The MCNPX model fully describes the walls of the detector, the three amplification stages and the Mylar layer used as electric contact in both sub-units and takes into account the interactions occurring both in the front and in the sides of the detector system. The irradiations were performed at a neutron generator facility (FNG) using deuterons impinging on deuterium and tritium targets (DD and DT neutrons respectively) and the counts at several high voltages were acquired (counting curves). The differential proton spectra were determined from the counting curves in the irradiation experiments by setting a parameterized energy scale Ethr(HV), differentiating the resulting εexp(Ethr) and finally determining through minimization the energy scale parameters that best match the experimental PHS with the MCNPX‐simulated one. This led to the evaluation of the experimental efficiency as a function of the discrimination level set on the deposited energy (Ethr ¼ energy threshold) and its comparison with the MCNPX-simulated neutron efficiency. The GEM-based neutron detector is also sensitive to γ-rays. However, γ-rays can be effectively rejected by selecting a sufficiently low operational HV (i.e. high Ethr). In the specific case of the FNG irradiations, where the γ-ray spectrum is in the range 0.1–10 MeV, the energy deposited in the gas by γ-rays is typically r0.06 MeV against Z0.1 MeV and Z0.01 MeV deposited respectively by 2.5 MeV and 14 MeV neutrons and the rejection can be achieved using HV 1000 V corresponding to Ethr 0.07 MeV. The detector can achieve (in the operational range where the γ-ray contamination to the signal is absent) a maximum neutron efficiency of 1 10 3 counts/n at 2.5 MeV (UDD sub-unit) and of 4 10 3 counts/n at 14 MeV (UDT and UDD sub-units); a detailed error analysis indicates maximum total uncertainties in these figures (systematic and counting statistics) in excess of 750%. Acknowledgments The authors would like to thank Maurizio Angelone and Mario Pillon for their support in the FNG experiments, Aldo Pensa and
Gerardo Claps for technical support and Antonino Pietropaolo for his very careful reading of the manuscript.
References [1] F. Sauli, Nuclear Instruments and Methods in Physics Research A 386 (1997) 531. [2] B. Esposito, F. Murtas, R. Villari, M. Angelone, D. Marocco, M. Pillon, S. Puddu, Nuclear Instruments and Methods in Physics Research A 617 (2010) 155. [3] R. Villari, M. Angelone, B. Esposito, A. Ferrari, D. Marocco, F. Murtas, M. Pillon, IEEE Transactions on Nuclear Science NS 56 (2009) 1102. [4] A. Bressan, J.C. Labbè, P. Pagano, L. Ropelewski, F. Sauli, Nuclear Instruments and Methods in Physics Research A 425 (1999) 262. [5] G. Bencivenni, P. de Simone, F. Murtas, M. Poli Lerner, W. Bonivento, A. Cardini, C. Deplano, D. Pinci, D. Raspino, Nuclear Instruments and Methods in Physics Research A 513 (2003) 264. [6] M. Martone, M. Angelone, M. Pillon, Journal of Nuclear Materials 212–215 (1994) 1661. [7] W. Bonivento, P. Jarron, D. Moraes, W. Riegler, F. dos Santos, Nuclear Instruments and Methods in Physics Research A 491 (2002) 233. [8] A. Milocco, A. Trkov, M. Pillon, Nuclear Instruments and Methods in Physics Research B 271 (2012) 6. [9] M. Angelone, M. Pillon, P. Batistoni, P. Marini, M. Martone, V. Rado, Review of Scientific Instruments 67 (1996) 2189. [10] D. Raspino, Il Rivelatore a Tripla GEM per il Sistema di Muoni dell'Esperimento LHCb, Ph.D. thesis, Universita' di Cagliari, 2004. [11] S. Puddu, Sviluppo di Sistemi Diagnostici per Plasmi Nucleari Accesi, Tesi di Laurea in Fisica, Universita’ di Cagliari, 2010. [12] G.Z. McKinney, et al., MCNPX 2.6.X Features, LANL Report LA-UR-07-2053, Presented at the M&C/SNA 2007 Conference Workshop, Monterey, CA, April 15–19, 2007. Available from: 〈http://mcnpx.lanl.gov/〉. [13] G. Croci, M. Rebai, G. Claps, M. Cavenago, M. Dalla Palma, G. Gervasini, G. Grosso, F. Murtas, R. Pasqualotto, E. Perelli Cippo, M. Tardocchi, M. Tollin, G. Gorini, Journal of Instrumentation 7 (2012) C03010.