International audienceThe BiPo pro ject is dedicated to the measurement of extremely low radioactive contaminations of SuperNEMO ββ source foils ( 208 Tl < 2 µBq/kg and 214 Bi < 10 µBq/kg). A modular BiPo1 prototype with its 20 modules and its shielding test facility is running in the Modane Underground Laboratory since February, 2008. The goal of this prototype is to study the back-grounds and particularly the surface contamination of plastic scintillators. After 2 months, a preliminary upper limit on the sensitivity of a 10 m 2 BiPo detector in 208 Tl contamination of selenium source foils can be extrapolated to: A( 208 Tl) < 7.5 µBq/kg (90 % C.L.)

Bongrand, M.

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BiPo prototype measurements for SuperNEMOM. BongrandTo cite this version:M. Bongrand. BiPo prototype measurements for SuperNEMO. J.-M. Fre`re ; L. Iconomidou-Fayard ; S. Loucatos ; J. Tran Thanh Van. 43e Rencontres de Moriond Electroweak interactionsand Unified theories, Mar 2008, La Thuile, Italy. The GIOI publishers, pp.519-522, 2008.<in2p3-00287589>HAL Id: in2p3-00287589http://hal.in2p3.fr/in2p3-00287589Submitted on 12 Jun 2008HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.BiPo PROTOTYPE FOR SuperNEMO RADIOPURITY MEASUREMENTSM. BONGRAND(on behalf of the SuperNEMO collaboration)LAL, Universite´ Paris-Sud 11, CNRS/IN2P3, Orsay, FranceThe BiPo project is dedicated to the measurement of extremely low radioactive contamina-tions of SuperNEMO ββ source foils (208Tl < 2 µBq/kg and 214Bi < 10 µBq/kg). A modularBiPo1 prototype with its 20 modules and its shielding test facility is running in the ModaneUnderground Laboratory since February, 2008. The goal of this prototype is to study the back-grounds and particularly the surface contamination of plastic scintillators. After 2 months, apreliminary upper limit on the sensitivity of a 10 m2 BiPo detector in 208Tl contamination ofselenium source foils can be extrapolated to: A(208Tl) < 7.5 µBq/kg (90 % C.L.).1 PrincipleThe BiPo detector is dedicated to the measurement of the high radiopurity levels in 214Bi and208Tl of very thin materials and especially the double beta source foils of the SuperNEMOdetector1. The expected sensitivity is A(208Tl) < 2 µBq/kg and A(214Bi) < 10 µBq/kg.In order to measure 208Tl and 214Bi contaminations, the original idea of the BiPo detector isto detect the so-called BiPo process, which corresponds to the detection by organic scintillatorsof an electron followed by a delayed alpha particle. The 214Bi isotope is nearly a pure β emitter(Qβ = 3.27 MeV) decaying into 214Po, an α emitter with a half-life of 164 µs (Fig. 1). The 208Tlisotope is measured by detecting its parent the 212Bi isotope. 212Bi decays with a branchingratio of 64 % via a β emission towards 212Po (Qβ = 2.25 MeV) which is again an α emitter witha short half-life of 300 ns. So, for these two chains a BiPo signature is an electron associated toa delayed α with a delay time depending on the isotope contamination we want to measure.210Tl(1.3 min)Pb210(22.3 y)Po214(164   s)µBi214(19.9 min)238Uβααβ0.02 %Bi212(60.5 min)Tl208(3.1 min)Pb208(stable)Po212(300 ns)232Thβααβ36 %Figure 1: BiPo processes for 214Bi and 208Tl.The particles emitted by the source foil are detected with plastic scintillators coupled to lowradioactivity photomultipliers (Fig. 2). Plastic scintillators are very radiopure and reduce thebackscattering of electrons. The detection efficiency is dominated by the capacity for an α par-ticle to escape the foil. GEANT4 simulations give a total efficiency of 6.5 % for contaminationsin selenium foils (40 mg/cm2) with 1 MeV threshold for α. Therefore the energy threshold ofthe detector must be as low as possible. Moreover the energy converted into scintillation lightis much lower for α compared to electrons. This quenching factor depends on the energy of theα and has been measured with a dedicated test bench2. For example, a 1 MeV α will producesame amount of light than a 40 keV electron.SourceβαScintillatorTime~ 164 µs212BiPo 214BiPo~ 300 nsFigure 2: BiPo detection principle with plastic scintillators and time signal seen with PMTs. Dots represent thecontamination and crosses represent energy depositions in scintillators (trigger in blue and delayed in red).2 BackgroundsThe BiPo measurement consists in the detection of the electron in one scintillator and thedetection of the delayed α particle in the other scintillator. This strong BiPo signature constrainsthe background of the detector to only 3 processes:- bismuth (212Bi or 214Bi) contaminations in the volume of the scintillator. In such decay theelectron deposits part of its energy in the first scintillator before crossing the foil to reach theother one. The delayed α is detectable only in the first scintillator because it can’t cross the foil.This background can be rejected because two hits in time are observed in the two scintillators:this is not a BiPo event. (Fig. 2a)- bismuth contaminations on the surface of the scintillatora. In this case the electron doesn’tdeposit enough energy in the first scintillator to be detected. The delayed α particle is stilldetectable only in this first scintillator. This contamination is not distinguishable from a BiPosignal because this signature exactly corresponds to a BiPo event coming from the foil. (Fig. 2b)- random coincidences due to external γ. To reduce this background, the BiPo detector useslow background materials, is shielded and installed in underground lab. The single countingrate of each scintillator has to be less than 40 mHz to measure 208Tl and less than 10 mHz for214Bi because of 214Po longer half-life. Pulse shape discrimination also reduces this background.(Fig. 2c)Sourceβ βαγαScintillator(a) (b) (c)γFigure 3: Surface and volume bismuth contaminations of the scintillators and random coincidences backgrounds.3 BiPo1 prototypeBiPo1 prototype is divided in 20 modules. Each module is a black box containing two polystyrenbased scintillators coupled to low background 5” photomultipliers with PMMA light guides.Scintillator dimensions are 20×20×1 or 20×20×0.3 cm3, the entrance window is covered with200 nm of ultra-pure aluminum to isolate optically each scintillator and to improve the lightcollection. The sides of scintillators and light guides are covered with 0.2 mm of Teflon foraSmall thickness where the energy deposited by the electron is below the threshold (∼ 100 µm for 150 keV).light diffusion. The prototype is installed in the Modane Underground Laboratory (LSM) under4800 m.w.e.. Surrounding the modules, a shielding of 15 cm of low activity lead reduces externalγ and 3 cm of pure iron stops bremsstrahlung γ emitted in the lead by the decay of the 210Bifrom long half-life 210Pb. Radon-free air flushes the volume of each module and the inner volumeof the shielding (Fig. 4).RadonScintillatorLow activity leadLight PMTIronairfreeguideFigure 4: BiPo1 prototype in its shielding in LSM.Photomultipliers signals are sampled with VME digitizing board3, during 2.5 µs with a highsampling rate (1 GS/s) and a 12 bit high dynamic range (1 V). The acquisition is triggered eachtime a pulse reaches the 150 keV energy threshold and the 2 photomultipliers signals from amodule are stored. The delayed hit research is performed later by the analysis of the signals(Fig. 5).4 BiPo1 calibrationThe first BiPo1 module has been dedicated to the validation of the detection principle for212Bi contaminations inside a foil. A 150 µm aluminum foil (40 mg/cm2) with a contaminationmeasured with HPGe detectors of A(212Bi→212Po) = 0.19 ± 0.03 Bq/kg, has been installedbetween the two scintillators. After 141 days of data taking, 1501 BiPo events have beendetected. Taking into account the efficiency calculated by GEANT4 simulations, it correspondsto a reconstructed activity of A(212Bi→212Po) = 0.22 ± 0.01 Bq/kg, in good agreement withinitial HPGe measurement. The delay between the two hits is also measured, and the fit ofthe decay law perfectly corresponds to the 212Po half-life (Fig. 5). These results are a strongvalidation of the measurement principle and the calculated efficiency.Figure 5: Example of a BiPo event observed in BiPo1 and delay distribution between the β and the α decays.Particle identification in the plastic scintillator could help to sign the BiPo process and rejectrandom coincidences of γ. Indeed, longer half-life states in scintillators are excited by α particlesbut not by electrons. More light is therefore observable in the tail of the pulse for α particles.A pulse shape discrimination, using tail-to-total charge ratio as discrimination factor, has beenapplied on the photomultiplier signals from the data of the calibration foil. A good separationhas been observed for prompt (e−) and delayed (α) signals (Fig. 6). Using this discrimination,it is possible to reject 80 % of the random coincidence background and to keep 90 % of the trueBiPo events.Figure 6: Distributions of the tail-to-total charge ratio and the tail charge q as a function of the total charge Qfor the prompt and delayed signal from the aluminum foil in the first module of BiPo1.5 BiPo1 surface radiopurityThe other modules of BiPo1 are dedicated to the measurement of the surface radiopurity ofscintillators. BiPo events, coming from the contact surface between the two scintillators, areobserved by a hit in one scintillator and a delayed hit in the other one. After 2 months ofdata taking, 7 BiPo events have been observed on a statistics equivalent to 0.8 m2×month. Itcorresponds to an activity of about 2 µBq/m2. Extrapolating this background to a final 10 m2BiPo detector for a one month measurement of 4 kg selenium source foil for SuperNEMO, thethallium preliminary sensitivity is: A(208Tl) < 7.5 µBq/kg (90% C.L.).6 ConclusionThe BiPo1 prototype demonstrated the validity of the experimental technique to measure 208Tlcontaminations in thin materials. The preliminary sensitivity achieved is 10 times better thanstandard HPGe measurements. Particles identification with simple plastic scintillators enhancethe performances of the prototype. New modules using “phoswich” scintillators, a compoundof a thin fast scintillator to detect α and a thick slow scintillator for electrons, will improvethe discrimination between the 2 particles. A second prototype, BiPo2, using large scintillatorplates (0.56 m2) will also be tested in LSM before this summer.References1. L. Simard, Status of the SuperNEMO Project, Workshop on calculation of double-beta decaymatrix elements, Prague, Czech Republic, 20072. M. Bongrand, The BiPo Detector for Ultralow Radioactivity Measurements, Topical Workshop onlow radioactivity Techniques, LRT 2006, Aussois, France, 2006, arXiv:physics/0702070v13. D. Breton, E. Delagnes and M. Houry, Very high dynamic range and high sampling rate VMEdigitizing boards for physics experiments, Nuclear Science, IEEE Transactions on Volume 52,Issue 6, Dec. 2005 Page(s): 2853 - 2860

BiPo prototype measurements for SuperNEMO

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