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Romualdo Santoro Universit dellInsubria Integration of SiPM in a high-pressure noble gas scintillation detector for homeland security M. Caccia, V. Chmill, S. Martemiyanov Insubria R. Chandra, G. Davatz, U. Gendotti Arktis

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MODES_SNM R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 2 Modular Detector System for Special Nuclear Material Approved by the European Commission within the Framework Program 7 The Main Goal is the development of a system with detection capabilities of difficult to detect radioactive sources and special nuclear materials Neutron detection with high rejection power -rays spectrometry Other requirements Mobile system Scalability and flexibility to match a specific monitoring scenario Remote control, to be used in covert operations

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Baseline technology R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 3 The Arktis technologies is based on the use of 4 He for the neutrons detection The main key features of 4 He Reasonably high cross section for n elastic scattering Good scintillating properties Two component decays, with at the ns and s levels Cheaper and easier to be procured wrt 3 He 44 cm diameter x 47 cm sensitive length 180 bar 4 He sealed system maintaining gas purity R. Chandra et al., 2012 JINST 7 C03035

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MODES_SNM System overview R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 4 Modular system optimized for: Fast neutron ( 4 He) Thermal neutron ( 4 He with Li converter) Gamma (Xe) All components are being integrated, we are approaching the commissioning and qualification phase With -ray spectroscopy capability

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MODES_SNM R&D R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 5 The project baseline is based on PMTs coupled with scintillating material R&D activities was planned since the beginning to investigate the possibility of using SiPM as light detector Why SiPM is so appealing? high sensitivity (single photon discrimination) compactness, robustness, low operating voltage and power consumption low cost

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SiPM R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 6 SiPM is a High density (~10 3 /mm 2 ) matrix of diodes with a common output, working in Geiger-Mller regime Common bias is applied to all cells (few % over breakdown voltage) Each cell has its own quenching resistor (from 100kto several M) When a cell is fired an avalanche starts with a multiplicative factor of about 10 5 -10 6 The output is a fast signal (T rise ~ ns; T fall ~ 50 ns) sum of signals produced by individual cells SiPM works as an analog photon detector Signals from SiPM

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SiPM R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 7 SiPM is a High density (~10 3 /mm 2 ) matrix of diodes with a common output, working in Geiger-Mller regime Common bias is applied to all cells (few % over breakdown voltage) Each cell has its own quenching resistor (from 100kto several M) When a cell is fired an avalanche starts with a multiplicative factor of about 10 5 -10 6 The output is a fast signal (T rise ~ ns; T fall ~ 50 ns) sum of signals produced by individual cells SiPM works as an analog photon detector The selected device is a large area (13.6 x 14.3 mm 2 ) monolithic array of SiPM units produced by Hamamatsu: S11829-3344M

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Lab charcterization to fullfill the simulation hints R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 8 Minimum detectable light Detector sensitivity (i.e. S/N or capability to discriminate an event against noise ) Model developed by Arktis We expect 255 photons / matrix for 100 keV deposited energy assuming the 95% of reflectivity

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Lab charcterization to fullfill the simulation hints R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 9 Firing the matrix with a calibrated photon flux, we measured the minimum detectable light a two different temperatures (different performances due to a combined effect of increased noise and gain drift) 250 ph @ 25C 60 ph @ 21.6C We expect 255 photons / matrix for 100 keV deposited energy assuming the 95% of reflectivity

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Experimental set-up for proof of principle A short tube (19 cm) used for the proof of principle Filled with 4 He at 140 bar, an integrated wavelength shifter and two SiPMs mounted along the wall (by ARKTIS) Two SIPMs read-out through the Hamamatsu electronic board (C11206-0404FB) 2-channels 3-stage amplification with leading edge discrimination (SP5600A CAEN) Digitizer with a sampling rate of 250 Ms/s 12 bit digitization (V720 CAEN) 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy R. Santoro 10

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Counting measurements Test performed measuring: Background, n and counting rate using 252 Cf and 60 Co source in contact Two triggering scheme: Trailing edge discrimination in coincidence Trailing edge and delayed gate of each single SiPM in coincidence Few parameters to be optimized: Leading and trailing threshold Delay time (T) Gate aperture 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy R. Santoro 11 1 st Trigger Scheme 2 nd Trigger Scheme typical event typical n event

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Counting measurements 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy R. Santoro 12 An amazing result, corresponding to a rejection power at the 10 6 level [ 10 counts in 1000s, for a number of given by acceptance*activity*time = 1/3 * 3 * 10 4 * 10 3 ~ 10 7 ] Result for the different trigger scheme @ 28C

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Off-line data analysis Data recorded with a minimum bias trigger Low threshold on the pulse height discrimination No coincidence between the two SiPMs For each triggered events we digitize signal of both SiPMs with sample rate of 125 MS/s and a total duration of 4s. Three data set: 1. 400 events without radioactive sources 2. 6000 events with 60 Co source in contact 3. 10000 events with 252 Cf source in contact Analysis strategies: Identify an observable allowing to measure the ratio between noise & particle induced triggers in samples 2 & 3 Filter noise from particle induced events through a multivariate analysis Identify the ratio between and n events in sample 3 Filter from n through a multivariate analysis Measure the rejection power of interacting and the selection efficiency of interacting neutrons 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy R. Santoro 13

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Sample composition: (% of Background and signal) R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 14 Each signal is baseline subtracted in the integrated time windows FAST and SLOW component is calculated as the integral of the signal to the left / right side of the peak Definition of Fast and Slow Component BKG Co60 Cf252 The areas underneath the fits are used to measure sample composition These numbers are used to estimate the selection efficiency and bkg rejection power

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A multivariate Bayesian analysis R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 15 The strategy: Select 4 no-correlated variables where bkg, and n appear to be reasonably different TOT_Diff, Charge Diff, Charge Skewness, Full_charge Bkg data-set is used to build the experimental probability density functions (p.d.f.) The corresponding cumulative distributions function (c.d.f.) I i is then constructed: The four I i s are combined to get the final distribution: Two step procedure: 1 st step: the selection criteria based on P is used to remove the background from n and induced events 2 nd step: the procedure is reiterated to define the n/ selection criteria ( 60 Co sample used to build the p.d.f.) : signal over noise distributions

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1 st Step R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 16 c.d.f. obtained for one of the selected variables (total charge), based on the bkg data-set bkg data-set As expected we have a random quantity with a flat distribution

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1 st Step R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 17 c.d.f. obtained for one of the selected variables (total charge), based on the bkg data-set Moving towards a different p.d.f ( 60 Co or 252 Cf), we have an accumulation of events on the right part of the histogram which allows as to separate the signal form bkg

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1 st step: bkg rejection R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 18 Selection for the 2 nd step These two plots show that almost all the signal is on the right part of the histograms (peak) while the bkg is a flat component on the left The bin at P>0.995 contains ~78% of the events The bin at P>0.995 contains ~99% of the +n events for Cf Repeat the exercise for all the quantities and combine the c.d.f. as follow

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2 nd step R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 19 after the Bkg has been filtered out, the ratio between and n events in 252 Cf can be measured: - events in the 60 Co data-set n and composition in the 252 Cf data-set

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2 nd step: rejection R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 20 The procedure is reiterated using the 60 Co data-set to build the cumulative distributions to identify over n in the 252 Cf data-set Results for a P cut of 0.995 Results are well beyond the expectation which are pushing us to continue with Further test to measure the neutron detection efficiency and to qualify the /n separation using TOF technique Tube layout optimization Improved electronics (see next slide)

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Customized electronics R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 21 Adjustable bias voltage for each of 16 channels SiPM temperature readout and gain compensation The thermo-chip placed onto the SiPM generates a digital pickup noise which cannot be removed The frequency of the temperature readout is settable The board include a lemo connector that can be used for veto trigger Improved minimum detectable light and dynamic range

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Customized electronics R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 22 60 ph @ 19.6C 30 ph @ 19.6C Pedestal Adjustable bias voltage for each of 16 channels SiPM temperature readout and gain compensation The thermo-chip placed onto the SiPM generates a digital pickup noise which cannot be removed The frequency of the temperature readout is settable The board include a lemo connector that can be used for veto trigger Improved minimum detectable light and dynamic range

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Conclusion R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 23 The Proof of Concept of SiPM arrays in the tube has been successfully completed The tube design optimization is certainly required A new front-end electronics (designed at Uni. Insubria) has been characterized in the lab and fulfils the requirements New test campaign is on the way to optimize the on-line and off-line analysis

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Spares 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy R. Santoro 24

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Typical events with PMTs R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 25 Neutron/ discrimination is based on the difference between the fast/slow component of the scintillation light Typical neutron and gamma events Typical plot from the 4 He detectors showing the discrimination between neutrons (Am-Be) and gamma ( 60 Co)

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Bkg_rejection R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 26 Bkg flat distribution (1 st selection) Co60 flat distribution (2 nd selection)

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2 nd step R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 27 Distributions of the four discriminant variables used in the second step of the procedure after having filtered the noise induced events

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2 nd step: rejection R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 28 P_value=0.995 Neutron eff = 94% rejection power= 92% P_value=0.995 Neutron eff = 94% rejection power= 92% measured with a data-set with pure bkg

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Few spectra used in the linearity plot R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 29