1. Romualdo Santoro Università dell’Insubria 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

2. 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 γ rejection power  γ-rays spectrometry  Other requirements  Mobile system  Scalability and flexibility to match a specific monitoring scenario  Remote control, to be used in covert operations

3. 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 τ at the ns and μs 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

4. 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 γ-ray spectroscopy capability

5. 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

6. 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-Müller regime  Common bias is applied to all cells (few % over breakdown voltage)  Each cell has its own quenching resistor (from 100kΩto several MΩ)  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

7. 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-Müller regime  Common bias is applied to all cells (few % over breakdown voltage)  Each cell has its own quenching resistor (from 100kΩto several MΩ)  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

8. 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

9. 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 @ 25°C ≈ 60 ph @ 21.6°C We expect 255 photons / matrix for 100 keV deposited energy assuming the 95% of reflectivity

10. 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

11. Counting measurements Test performed measuring:  Background, n and γ 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 (ΔT)  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 γ event typical n event

12. 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 γ rejection power at the 10 6 level [ 10 counts in 1000s, for a number of γ given by acceptance*activity*time = 1/3 * 3 * 10 4 * 10 3 ~ 10 7 ] Result for the different trigger scheme @ 28°C

13. 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 4μs.  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 γ and n events in sample 3  Filter γ from n through a multivariate analysis  Measure the rejection power of interacting γ 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

14. 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

15. 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, γ 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 γ induced events  2 nd step: the procedure is reiterated to define the n/γ selection criteria ( 60 Co sample used to build the p.d.f.) : signal over noise distributions

16. 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

17. 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

18. 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 γ events The bin at P>0.995 contains ~99% of the γ +n events for Cf  Repeat the exercise for all the quantities and combine the c.d.f. as follow

19. 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 γ and n events in 252 Cf can be measured: γ- events in the 60 Co data-set n and γ composition in the 252 Cf data-set

20. 2 nd step: γ 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 γ 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 γ/n separation using TOF technique  Tube layout optimization  Improved electronics (see next slide)

21. 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

22. Customized electronics R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 22 ≈ 60 ph @ 19.6°C ≈ 30 ph @ 19.6°C 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

23. 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

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

25. Typical events with PMTs R. Santoro 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD13) 7-10 Oct. 2013, Siena, Italy 25  Neutron/γ 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)

26. 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)

27. 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

28. 2 nd step: γ 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% γ rejection power= 92% P_value=0.995 Neutron eff = 94% γ rejection power= 92% measured with a data-set with pure bkg

29. 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