Triple GEM gas detectors as real time fast neutron beam monitors for spallation neutron sources

Transcription

1 Journal of Instrumentation OPEN ACCESS Triple GEM gas detectors as real time fast neutron beam monitors for spallation neutron sources To cite this article: F Murtas et al View the article online for updates and enhancements. Related content - The triple GEM detector as stray neutron monitor E. Aza, M. Magistris, F. Murtas et al. - Diamond detectors for fast neutron measurements at pulsed spallation sources M Rebai, L Giacomelli, C Andreani et al. - Development of large-area THGEM detectors for investigation of thermalhydraulic phenomena using neutron imaging M Cortesi, R Zboray, R Adams et al. Recent citations - Performance of the high-efficiency thermal neutron BAND-GEM detector Andrea Muraro et al - Electron-volt neutron spectroscopy: beyond fundamental systems Carla Andreani et al - Evolution in boron-based GEM detectors for diffraction measurements: from planar to 3D converters Giorgia Albani et al This content was downloaded from IP address on 27/06/2018 at 17:28

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: May 15, 2012 ACCEPTED: June 21, 2012 PUBLISHED: July 25, 2012 Triple GEM gas detectors as real time fast neutron beam monitors for spallation neutron sources F. Murtas, a G. Croci, b,1 A. Pietropaolo, c G. Claps, a C.D. Frost, d E. Perelli Cippo, c D. Raspino, d M. Rebai, c N.J. Rhodes, d E.M. Schooneveld, d M. Tardocchi b and G. Gorini c a Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, Via E. Fermi 40, Frascati, Italy b Istituto di Fisica del Plasma P. Caldirola, Associazione EURATOM-ENEA/CNR, Via R. Cozzi 53, Milano, Italy c CNISM and Dip. di Fisica G. Occhialini, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, Milano, Italy d ISIS, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom gabriele.croci@ifp.cnr.it ABSTRACT: A fast neutron beam monitor based on a triple Gas Electron Multiplier (GEM) detector was developed and tested for the ISIS spallation neutron source in U.K. The test on beam was performed at the VESUVIO beam line operating at ISIS. The 2D fast neutron beam footprint was recorded in real time with a spatial resolution of a few millimeters thanks to the patterned detector readout. KEYWORDS: Instrumentation for neutron sources; Micropattern gaseous detectors (MSGC, GEM, THGEM, RETHGEM, MHSP, MICROPIC, MICROMEGAS, InGrid, etc); Neutron detectors (cold, thermal, fast neutrons); Beam-line instrumentation (beam position and profile monitors; beam-intensity monitors; bunch length monitors) 1 Corresponding author. c 2012 IOP Publishing Ltd and Sissa Medialab srl doi: / /7/07/p07021

3 Contents 1 Introduction 1 2 Experimental results and comparison with simulations 1 3 Conclusions 6 1 Introduction Fast neutron beams available at large scale facilities are becoming strategic for industrial applications, especially in relation to the assessment of radiation hardness of silicon-based nano-sized electronic chips. Spallation facilities at ISIS [1] in U.K., LANSCE [2] in U.S., TRIUMF [3] in Canada and ANITA [4] in Sweden are well suited for chip irradiation studies as these provide neutron beams covering the energy range from thermal neutron up to multi MeV energies with an intensity distribution resembling the atmospheric one [5 7]. The intensities provided are five or six orders of magnitude higher than the natural neutron intensity, thus allowing for assessing the robustness of chips and/or complex hardware architectures to neutron irradiation in rapid times, which match the new technology production rate. A specific requirement of neutron beam lines dedicated to chip irradiation is the possibility to monitor and characterize the neutron beam above 1 MeV (the most concerning energy region of the spectrum) with a spatial resolution of millimetres. The construction of the ChipIr [8] beam line at the ISIS second target station [9] calls for the development of suitable fast neutron monitors. Several detectors were recently tested: Bonner sphere spectrometers [10], Thin Film Breakdown Counters (TFBC) [11] and the Single-crystal Diamond Detectors (SDD) [12, 13]. Within the R&D activity for fast neutron counters, the investigation of real-time, large-area, high-rate capability, sub-millimetre spatial resolution and costeffective devices is strategic for both industrial and basic research applications of neutron beams produced at spallation neutron sources. 2 Experimental results and comparison with simulations This paper presents test results of Gas Electron Multipliers (GEM) detectors [14, 15, 29] that in principle fulfil the above stated requirements. These devices are typically used in high-energy physics for tracking and triggering thanks to their good spatial resolution and timing properties, excellent rate capability and radiation hardness. Although GEM-based detectors are mostly used to detect charged particles (GEM-based charged particle beam monitors have already developed [15]), these can be adapted to reveal neutral particles, such as neutrons and photons [16, 17]. This device was used at ISIS as neutron monitor, exploiting elastic scattering off hydrogen in a plastic layer and the detection of the recoiling protons. 1

4 Figure 1. Schematic of a triple GEM detector showing the three GEM foils between cathode and readout. The cathode to readout distance is 7 mm. The tests were made at the VESUVIO beam line [18] at ISIS. Neutrons are produced by a double-bunched proton beam impinging onto a W/Ta target, each bunch being about 70 ns wide (Full width half maximum) and 322 ns apart. The detector used in the tests was a Triple GEM device [19] (figure 1), placed along the primary flight path at a distance L 0 = 12.5 m from a water moderator at T = 293 K. Each GEM in the detector is a thin (50 µm) kapton foil, copper clad on each side, perforated with high surface density of holes, each one acting as an electron multiplication channel. Each hole has a bi-conical shape with external and internal diameters of 70 and 50 µm, respectively, and a pitch of 140 µm. A typical voltage difference of V is applied between the two copper sides, giving fields as high as 100 kv/cm into the holes, resulting in an electron multiplication up to a few thousands. A Triple GEM detector consists of three GEM foils sandwiched between two conductive planes with the anode segmented in pads and connected to the readout electronics (see figure 1). In this configuration this detector can be effectively used as tracking detector with good time and position resolution performances. The ionization electrons, produced by the charged particles crossing the gas in the gap between the cathode and the first GEM foil (drift gap), cross the three GEM foils where they are multiplied. Once they are extracted from the last GEM foil, they drift towards the anode in the so-called induction gap, inducing a purely electron current signal on the anode pads. A triple GEM detector requires a High-Voltage system to create the electric fields inside the chamber and to supply the GEM foils. A dedicated High-Voltage system was designed and realized, the so called HVGEM [20], an active HV divider with seven floating power supplies similar to a set of seven batteries stacked in a row. The 12 V power supply and the low power consumption are two of the most remarkable characteristics, as reported in a previous experimental work [21]. The triple GEM specifically used in these tests features an Al cathode of 40 µm covered on the outer side by a 60 µm polyethylene sheet, which makes it sensitive to fast neutrons by means of elastic (n, p) reactions on hydrogen. The efficiency for (n, p) conversion is increased by adding a 400 µm sheet made from polypropylene. The drift region of the detector is 3 mm, while the gaps between the GEM are 1 mm (Transfer 1) and 2 mm (Transfer 2) and the induction gap is 1 mm wide. The detector volume was filled with the Ar/CO 2 gas mixture (70/30 relative concentration). The voltage difference between the GEM electrodes was 300 V and it was the same on the three GEMs, 2

5 the drift field as well as the two transfer fields were set to 3.5 kv/cm, and the in the induction field was put to 5.0 kv/cm: this electric configuration results in a total gain of 180. This low gain value and operational voltage were used to reduce the contribution of smaller signals coming from the background, in particular γ-rays, especially those travelling with the incident neutron beam and generated by the spallation reactions and/or by the moderator/reflector/poisoning system as well as by the biological shielding [22, 23]. In the configuration described above, neutron counting is performed by registering the recoiling protons that ionize the gas in the drift gap. The front-end electronics is made by a 128 channels readout, the single pad dimensions being 12 6 mm2. These boards are based on the Carioca-GEM Chip [24], already used at LHCb at CERN for the muon detectors. Each board houses 16 channels (2 chips) that produce LVDS time over threshold signals. A FPGA based mother board is finally plugged just on top of the eight boards for low voltage power supply, threshold distribution and data acquisition (TDC and rate counter with a clock speed of 200 MHz). The detector was placed in the incident neutron beam line at a distance of about 12.5 m from the water moderator of VESUVIO. The detector was placed slightly off-center with respect to the neutron beam. This was done in order to avoid a direct irradiation of the whole FPGA behind the detector [15]. Figure 2 shows the intensity profiles along the horizontal and vertical directions on a plane perpendicular to the neutron beam axis, collected over an integrated proton beam current of 355 µah (with an average proton current <Ip>= 178 µa). The intensity profiles is well described by a Gaussian beam with FWHM of about 15 mm as expected from technical designs and previous measurements [25]. 3 Figure 2. (Color online): (a) 2D map showing the beam intensity distribution as number of events above threshold recorded at each GEM readout pad for an integrated proton beam current of µah; (b) pseudo 3D plot of the data shown in (a) as function of the horizontal (x) and vertical (y) coordinates of the readout pad centres, and bimodal Gaussian fit to the data.

6 Figure 3. Experimental setup simulated using the GEANT4 tool. Figure 4. Neutron detection efficiency of the GEM simulated by the GEANT4 package for neutron energies in the range MeV and for the values of deposited energy threshold (E th ) shown in the legend. The 60 kev curve refers to the actual threshold used for the measurements at ISIS. The detector efficiency was simulated using the GEANT4 code [26]. The simulation does not take into account the full detector since the geometry comprises only CH 3, CH 2, Al and Ar/CO 2 drift gap layers without introducing GEM foils and anode (see figure 3). A monochromatic neutron beam irradiates the setup and all the secondary particles generated by its interaction are recorded. The signal is generated by the energy deposited in the drift gap gas by recoil protons created either in the polypropylene or polyethylene layers: a count is recorded when this deposited energy is greater than a certain threshold and the efficiency is calculated as the number of counts divided by the total number of generated neutrons. In order to simulate the GEM efficiency as a function of neutron energy, 10 7 neutrons have been generated for discrete energies in the range MeV. Figure 4 shows the efficiency curve from 2 to 100 MeV for different choices of the energy threshold. After an initial rise with energy, the efficiency drops quickly with increasing energy which makes it difficult to use the present detector for neutrons, say, above 20 MeV. In the range 4

7 Figure 5. Correlation plot between GEM counts and the 2.5 MeV neutron flux onto the detector provided by the Frascati Neutron Generator at the ENEA research Center in Frascati measured by means of a NE213 liquid scintillator [17] MeV the average efficiency is for the energy threshold E th = 60 kev. The latter was set as a voltage threshold on the CARIOCA chips, previously calibrated using a procedure involving a known input charge. The proton energy release in the gas is related to the total amount of charge detected by the electronics through the GEM effective gain. As can be seen from the plot, the efficiency varies when different values of threshold are used. This is due to the spectrum of proton energy deposition inside the gas. The simulation has been performed for different thresholds in order to foresee the effect of a possible modification of the threshold value. One cause that may change the gain of the detector and, therefore, the threshold is the gas temperature. However in order to have a 10% change in detection efficiency, the temperature must vary by at least 5 C. As a consequence, the small temperature variations experienced during the run would not imply substantial changes on the efficiency. The experimental value of the efficiency was calculated as the ratio of the value of the mean counts per seconds measured by the GEM during the irradiation to the VESUVIO spectral fluence rate integrated from 2 to 20 MeV and over the effective beam size intercepted by the GEM (about 22 cm 2 ). This calculation provides ε exp The GEM efficiency was also measured using the almost monochromatic neutron beam from the Frascati Neutron Generator (FNG, ENEA- Frascati) [27]. Figure 5 shows the correlation between the counts in the detector and the neutron flux measured with a monochromatic 2.5 MeV. In the figure the x-axis reports the neutron flux provided by a NE213 liquid scintillator corrected for the ratio of the GEM active area to the GEM-neutron source distance. The resulting neutron detection efficiency is about It is worth to be stressed that although GEM detectors can be setup to detect photons, the chosen gain for the present measurements on fast neutrons provide an estimated photon efficiency below 10 8 [17, 28]. The difference among the three values of the efficiency can be explained considering that the one related to ISIS is measured over 5

8 Figure 6. Time spectrum recorded by the GEM detector over an acquisition window of 100 ns delayed with respect to the ISIS clock to obtain a time scan over about 3 µs. A reference proton beam signal is also shown. The GEM time spectrum was shifted to show the correlation with the double bunch proton time structure [7] MeV neutrons, while that at FNG is from monochromatic 2.5 MeV neutrons. The value of the simulated efficiency (see figure 3) for a threshold of 60 kev (the actual one used in the FNG measurements) is The slight difference between the experimental and the simulated efficiency can be due to the fact that only a simplified setup was considered in the calculation. The time structure of the VESUVIO neutron pulse was characterized by recording over a period of 3 µs the GEM counting rate within 100 ns wide time slices. The rate distribution over the whole period is shown in figure 6, where the proton beam time structure from the accelerator is also plotted. This structure was already observed during fast neutron measurements with both TFBC and diamond detectors (see refs. [11 13]). Because of the time structure of the proton beam, the arrival time onto the detector cannot be associated to a unique neutron energy value. The long tail that is present for negative times is linked to the time resolution of the measurement that was affected by two factors: first the time binning was limited to 100 ns by the detector/daq setup; second the T0 of the proton bunch was suffering from a significant jitter. In the future we will repeat this measurement changing the DAQ setup in order to acquire data using a wider binning. 3 Conclusions Results obtained by the tests described in the present paper assess that GEM detectors are fully able to detect in real time, with millimetre spatial resolution, fast neutrons in the energy range 2 20 MeV. Efficiency in detecting fast neutrons (around 10 4 ), high insensitivity (lower than 10 8 ) to photons fields, linearity with respect to irradiation rate and high counting rate stability (around 5%, [30]), together with the above mentioned characteristics, make these devices suitable for fast neutron beam monitoring at spallation neutron sources. Furthermore the device shows a good 6

9 time resolution being capable of distinguishing the fine time structure of the fast neutron beam of ISIS (see figure 6). In perspective, the performance of the device can be further extended towards spectroscopic capabilities, by characterizing its response function over a wider energy region by properly optimizing the cathode and polyethylene thicknesses. Monte Carlo simulations and new tests on beam are expected to achieve this goal in the near future. Acknowledgments This work was supported within the CNR-CCLRC Agreement No. 01/9001 concerning collaboration in scientific research at the spallation neutron source ISIS. The financial support of the Consiglio Nazionale delle Ricerche in this research is hereby acknowledged. References [1] [2] [3] [4] A.V. Prokofiev et al., Characterization of the ANITA neutron source for accelerated SEE testing at the Svedberg Laboratory, IEEE REDW (2009) 166. [5] IEC TS Standard for the accommodation of atmospheric radiation effects via single event effects within avionics electronic equipment. Part 1 (2005), JEDEC standard JESD89A. Measurement and reporting of alpha particle and terrestrial cosmic ray induced soft errors in semiconductor devices (2006), [6] C. Andreani et al., Facility for fast neutron irradiation tests of electronics at the ISIS spallation neutron source, Appl. Phys. Lett. 92 (2008) [7] M. Violante et al., A new hardware/software platform and a new 1/E neutron source for soft error studies: testing FPGAs at the ISIS facility, IEEE Trans. Nucl. Sci. 54 (2007) [8] [9] [10] R. Bedogni et al., Characterization of the neutron field at the ISIS-VESUVIO facility by means of a bonner sphere spectrometer, Nucl. Instrum. Meth. A 612 (2009) 143. [11] A.N. Smirnov et al., Application of thin-film breakdown counters for characterization of neutron field, Nucl. Instrum. Meth. A, submitted for publication (2012). [12] A. Pietropaolo et al., Single-crystal diamond detector for time-resolved measurements of a pulsed fast-neutron beam, Europhys. Lett. 92 (2010) [13] A. Pietropaolo et al., Fission diamond detectors for fast-neutron ToF spectroscopy, Europhys. Lett. 94 (2011) [14] F. Sauli, GEM: a new concept for electron amplification in gas detectors, Nucl. Instrum. Meth. A 386 (1997) 531. [15] F. Murtas et al., Applications in beam diagnostics with triple GEM detectors, Nucl. Instrum. Meth. A 617 (2010)

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