ElectrondriftAvalanche

Garfield++

Asphericalproportionaldetectorforthestudyofverylowradonconcentration

FCassol,H.Tedjditi,CHugon,JBustoAixMarseilleUniversité,CNRS/IN2P3,CPPMUMR7346,13288,Marseille,France

We present a new type of proportional radiation detector based on a spherical geometry for monitoring of the presence of 222Rn in the gas of the tracker of theSuperNEMO experiment, dedicated to the search of neutrinoless double beta decay and soon installed in the underground laboratory of Modane (France). Themain challenge of this projetct is to demonstrate the correct performing of this detector with He gas and/or with the gas mixture imposed by the SuperNEMOtracker (96% He + 4% Ethanol+ 1% Ar), which are both not standard gases for proportional counters. We present the preliminary performances of the detectorwith several gases and their comparison with a Monte Carlo simulation based on Geant4 and Garfield++.

TheSuperNEMO experiment:

cassol@cppm.in2p3.fr - ANIMMA2017,Liège,Belgium,19- 23June2017

Thesphericalchamber:

r1=32mm,r2=3mm

Gasin

Gasout

241Am

Testsource:241Am5.5MeValphasgothrougha3mmholecoveredbya0.01mmMylarfoil• Alphaenergy:4.5MeV• Alpharate:∼ 1Hz

Gases(1atm)1. P10:90%Ar +10%CH42. He3. SN:96%He+4%Ethanol(neglecting1%Ar)

The SN tracker, composed of 2000 Geiger tubes,demands a radon contamination level of less than150 𝜇Bq/m3 in its gas mixture in order to minimizeone of the main backgrounds of the data analysis.The idea is to introduce one spherical detector in thegas flow circuit of the tracking chambers in order todetect the alphas produced by an eventual accidentalincrease of the Rn component. This monitoring canbe performed directly before or after the filter usedto recycle the He component of the mixture. For that,after a study with standard P10 gas, we tested thedetector both with He and with the SN gas mixture.

Referencegas:P10

Hegas SNgas

HV=2500– 7000V

Pulseatanode

PulseafterpreamplifierCrematCR-Z-111(0.13V/pC,RC=150𝜇s)

Anode

The detector works in a proportional regime. The 4.5 MeV alphas from the Am source appear as avisible peak in the pulse spectrum (𝛥E/E<4%) and can be easily distinguished from the backgroundevents, which consist mainly of cosmic rays (low amplitude pulses) and of two not identified sources.The Monte Carlo simulation reproduces both the detector gain as function of the applied voltageand the rise time of the alpha pulses, which is around 0.038 ms.

MonteCarlosimulation:Geant4foralphadE/dxGarfield++forelectrondriftandavalanches

Sourceremoved

241Am241Am

Cosmicrays Unidentifiedsources

Gaincurve(DataandMC)

Riseanddecaytime(Data)

HV=5600V HV=6000V

ms

ms

HV=6600V

HV scan: Alpha pulses start at 5500 V. Increasing the HV, the pulseamplitude increases but for HV > 6800 V the cosmic ray pulsesstart to overwhelm the alpha signals. In order to keep a low rate,the amplitude threshold must be increased at higher voltages.

HV=6800V

threshold threshold

ms

Pulses are much shorter thanwith the P10 gas. They start at~2800 V, but not proportionalregime can be reached. Thesignal baseline is very unstable,so as the general behaviour ofthe detector. It is not possibleto separate the 241Am signalsfrom the background pulses.

Ethanol acts as a quenching component which permits toobtain pulses similar to those in the P10 gas. They start at~2500 V, but again not proportional regime can be reached.By using a higher rate source we could test that alpha andcosmic rays pulses are mixed together in the unique largebump of the spectrum.

We could verify that the spherical chamber worksin a proportional regime in the case of gas P10. Itsenergy resolution is below 4% for alpha particlesof 4.5 MeV and signals can be easily separatedfrom background performing selections on therise and the decay time of the pulses. Thechamber can also be employed with the SN gas,but exclusively as a counting detector. In fact,despite a stable behaviour, it does not reach aproportional regime. Moreover, all pulses presentsimilar rise and decay time, which prevents anytype of signal selection. Finally, the use of thechamber with He gas is not recommended due toan inherent instable behaviour, most probablycaused by the absence of a quenchingcomponent in the gas.

The Monte Carlo simulation reproduces the P10results but it fails to describe the He and the SNgas behaviour.

Conclusions:

Gasfilter

SNgas

He

Ethanol+Ar