Electroluminescence yield in pure krypton
EElectroluminescence yield in pure krypton
R.D.P. Mano, a C.A.O. Henriques, a F.D. Amaro a and C.M.B. Monteiro a, a LIBPhys-UC, Physics Department, University of Coimbra,Rua Larga, 3004-516 Coimbra, Portgal
E-mail: [email protected]
Abstract:
The krypton electroluminescence yield was studied, at room temperature, asa function of electric field in the gas scintillation gap. A large area avalanche photodiodehas been used to allow the simultaneous detection of the electroluminescence pulses as wellas the direct interaction of x-rays, the latter being used as a reference for the calculationof the number of charge carriers produced by the electroluminescence pulses and, thus,the determination of the number of photons impinging the photodiode. An amplificationparameter of 113 photons per kV per drifting electron and a scintillation threshold of 2.7 Td( 0.7 kV cm − bar − at 293 K ) was obtained, in good agreement with the simulation datareported in the literature. On the other hand, the ionisation threshold in krypton was foundto be around 13.5 Td (3.4 kV cm − bar − ), less than what had been obtained by the mostrecent simulation work-package. The krypton amplification parameter is about 80% and140% of those measured for xenon and argon, respectively. The electroluminescence yield inkrypton is of great importance for modeling krypton-based double-phase or high-pressuregas detectors, which may be used in future rare event detection experiments. Corresponding author. a r X i v : . [ phy s i c s . i n s - d e t ] J a n ontents The electroluminescence yield of gaseous xenon and argon has been studied in detail, bothexperimentally (e.g. see [1–7] and references therein) and through simulation tools [8–12].At present, the main drive for those studies is the ongoing development of dual-phase [13–19] and high-pressure gaseous [20–23] optical Time Projection Chambers (TPC), whichmake use of the secondary scintillation, - electroluminescence (EL) - processes in the gasfor the amplification of the primary ionisation signals produced by radiation interactioninside the TPC active volume. The R&D of such TPCs aims at application to Dark Mattersearch [13–17] and to neutrino physics, such as neutrino oscillation [18, 19], double betadecay [20–22] and double electron capture [24] detection. The physics behind these rareevent detection experiments is of paramount importance in contemporary particle physics,nuclear physics and cosmology, justifying the enormous R&D efforts carried out by thescientific community.The radioactivity of Kr has been a drawback for the use of krypton in rare eventdetection experiments, being this gas the less studied one among the noble gases. To the bestof our knowledge, the electroluminescence in krypton has only been studied by simulation[8, 10] and there haven’t been published any experimental results, up to now, to benchmarkthe simulation tools. Nevertheless, there are two experiments that make use of krypton,namely the measurement of the double electron capture in Kr [25–28] and the searchfor solar hadronic axions emitted in the M Kr nuclei [29–32]. Moreover, Kr has also been proposed for inelastic dark matter search [33]. The enrichment of agiven isotope of a noble gas is, nowadays, a matured technique, not significantly expensive,allowing for the reduction of the radioactive isotope to tolerable levels for a particularexperiment.The double-electron capture half-life is an important benchmark for nuclear structuremodels [34–38], providing vital experimental constraints. In addition, it presents a signifi-cant step in the search for neutrinoless double electron capture. The latter can complement– 1 –he search for neutrinoless double beta decay. Both would unveil the Majorana nature ofthe neutrino, access the absolute neutrino mass and contribute to understand the domi-nance of matter over antimatter by means of leptogenesis. On the other hand, axions andaxion-like particles are potential candidates for the constituent particles of dark matter,being the main reason for extensive axion searches, e.g. see [39] and references therein fordetailed theoretical and experimental reviews.The referred to above rare event search experiments, having krypton as the target, havebeen carried out with gas proportional counters using enriched krypton [25–32]. Neverthe-less, optical TPCs deliver higher gains with reduced electronic noise and overly improvedenergy resolution when compared to proportional counters [40–43]. In addition, the use ofa 2D-readout for the EL signal allows the reconstruction of the topology of the ionisationevent [42, 43] in a more effective way than the complex analysis of the wave form associatedto the ionisation events in the above-mentioned proportional counters. Therefore, the useof optical TPCs will allow larger sensitive volumes, better event discrimination and a moreeffective background reduction than the present proportional counters, hence having thepotential for improved sensitivity and accuracy.Having this in mind, in this work we present an experimental study of the electrolu-minescence yield of pure Kr and compare the obtained results with those attained fromsimulation studies [8–10]. The setup is described in section 2, and in section 3 we discussthe methodology that was followed in order to obtain the absolute EL yield, while in sec-tion 4 we present the obtained results and corresponding discussion, summarising the mainconclusions in section 5.
In this work, we used a gas proportional scintillation counter (GPSC) [40], Fig.1,
Figure 1 . Schematic of the GPSC with a large-area APD as the photosensor. – 2 –rradiated with a 1.5-mm collimated beam of 5.9-keV x-rays emitted from a Fe radioac-tive source, having a Cr filter to absorb the most part of the 6.4-keV Mn K β line. Theprimary electron cloud resulting from the x-ray interactions in the absorption region aredriven towards the scintillation region by a weak electric field, with intensity below thegas excitation threshold. The intensity of the electric field in the scintillation region is setabove the gas excitation threshold, but below the ionisation threshold, to prevent electronmultiplication. Upon crossing the scintillation region, the electrons are accelerated by theelectric field, gaining enough energy to excite the gas media by electron impact, leadingto an electroluminescence pulse with a large number of VUV photons as a result of thegas de-excitation processes. The number of VUV photons is proportional to the number ofprimary electrons crossing the scintillation region and, thus, to the incident x-ray energy.The detector response to x-rays has been studied in detail for Xe [46], Ar [47] and Kr [48]gas fillings.The GPSC depicted in Fig.1 has a 2.5-cm deep absorption region, a 0.9-cm deep scin-tillation region and is filled with Kr at a pressure of 1.1 bar, continuously purified throughSt707 SAES getters [44]. G1 and G2 are meshes, made out of stainless steel wires, 80- µ m indiameter with 900- µ m spacing. The radiation window holder and its focusing electrode aremade of stainless steel and are kept at negative voltage, while the stainless steel G2-holderand the detector body are maintained at ground potential. The voltage difference betweenthe radiation window and G1 determines the electric field in the absorption region, whilethe voltage of G1 determines the electric field in the scintillation region. A Macor pieceisolates the radiation window holder, the G1 holder and its feedthrough, being vacuum-sealed onto the stainless steel using a low-vapour pressure epoxy. The electroluminescencepulses are readout by a VUV-sensitive silicon large-area avalanche photodiode (LAAPD)[45], having a 16-mm diameter active area. The LAAPD is vacuum-sealed by compressingits enclosure against the detector bottom plate, using an indium ring. The LAAPD signalsare fed through a low-noise, 1.5 V/pC, charge pre-amplifier followed by an amplifier with 2 µ s shaping time and are pulse-height analysed with a multi-channel analyser (MCA). Most of the 5.9-kev x-rays interact in the absorption region producing, in the LAAPD,signals of large amplitude as a result of the electroluminescence. Nevertheless, a smallfraction of the 5.9 keV x-rays are transmitted through the gas and interact directly in theLAAPD producing signals with lower amplitude when compared to those resulting fromthe x-ray interactions in the gas. Fig.2 depicts a typical pulse-height distribution of thesignals at the LAAPD output, obtained when irradiating the detector with 5.9-keV x-rays.It includes the Kr electroluminescence peak, more intense and in the high-amplitude re-gion, the peak resulting from the direct interactions of the x-rays in the LAAPD, much lessintense and in the low-amplitude region, and the electronic noise tail in the low-amplitudelimit. While the amplitude of the electroluminescence peak depends on both scintillationregion- and LAAPD-biasing, the amplitude of the events resulting from direct x-ray inter-action in the LAAPD just depends on the LAAPD-biasing. In addition, the latter peak– 3 –s present even for a null electric field in the scintillation region and/or when the detectoris under vacuum. For pulse amplitude measurements, the pulse-height distributions werefit to Gaussian functions superimposed on a linear background, from which the Gaussiancentroids were determined. C oun t s x α Mn K5.9 keV β Mn K6.4 keV
LAAPD direct x-rays
Kr VUV pulses
Figure 2 . Typical pulse-height distribution obtained for 5.9-keV x-rays and electric field intensitiesof 0.34 and 3.4 kV cm − bar − in the absorption and scintillation region, respectively. The LAAPDbias voltage was 1840 V, corresponding to a gain of ∼ The presence of the peak of direct x-ray interactions in the LAAPD is of utmost impor-tance, as the average number of charge carriers produced in the silicon wafer by the x-rayinteractions, N RX is well known and it is used as a reference for the electroluminescencepeak. Comparing both pulse-heights, a ratio can be found between the pulse amplitudesresulting from the electroluminescence and from the direct x-ray absorption in the LAAPD.This ratio allows a direct quantification of the number of charge carriers produced in theLAAPD by the electroluminescence pulse and, thus, the number of VUV-photons imping-ing the LAAPD, given its quantum efficiency. The concurrent detection of the light pulsesand the x-rays in the photosensor under the same conditions in the same setup allows astraight forward measurement of the number of photons impinging on it. This method hasbeen used for measuring Xe and Ar electroluminescence yield in uniform electric fields [1, 2]and in electron avalanches of GEMs, THGEMs and Micromegas micropatterned structures[49, 50]. The number of charge carriers produced in the LAAPD by the electroluminescencepulse is: N V UV = (cid:18) A EL A RX (cid:19) N RX (3.1)being A RX and A EL the amplitude of the peaks resulting from the direct x-ray interactionsin the LAAPD and from the scintillation produced by the x-ray interactions in the gas,read in the MCA. The non-linear response of the LAAPD to 5.9-keV x-rays, e.g. see Fig.20– 4 –f [51], was considered as the ratio A EL / A RX corrected for this effect. A factor of 0.94 wasused in this correction, for a LAAPD biasing voltage of 1840 V used throughout this work,which corresponds to a LAAPD gain about 150. Knowing the quantum efficiency, QE, ofthe LAAPD and the optical transparency of G2, T , and the fraction of the average solidangle, Ω f = Ω / π subtended by the active area of the photosensor relative to the primaryelectron path, the total number of photons produced in the electroluminescence pulse isobtained by: N total,V UV = (cid:18) A EL A RX (cid:19) N RX QE × T × Ω f (3.2)The electroluminescence yield is defined as the number of photons produced per driftingprimary electron per unit path length: Y = (cid:18) A EL A RX (cid:19) N RX QE × T × Ω f × N e × d (3.3)where N e is the number of primary electrons produced in Kr by a 5.9 kev x-ray interactionand d is the scintillation region depth.As the w-value in silicon is 3.62 eV, e.g. [52] and references therein, the average numberof free electrons produced in the LAAPD by the full absorption of the 5.9-keV x-rays is N RX = 1.63 × electrons. The w-value in Kr is 24.2 eV [53], thus being N e = 244 electrons.The optical transmission of G2 mesh is T=83 % and the fraction of the average solid anglehas been computed by Monte Carlo simulation [54] to be Ω f =0.215.At atmospheric pressures, the electroluminescence of Kr consists of a narrow line peak-ing at 148 nm with 5 nm FWHM [55], called second continuum, being the emissions in thevisible and in the IR regions below few percent in comparison with that in the VUV range[55, 56], thus considering its contribution negligible. The processes leading to emission inthe second continuum can be schematized as e − + Kr → e − + Kr ∗ , Kr ∗ + 2 Kr → Kr ∗ + Kr , Kr ∗ → Kr + hν .The electron impact with Kr atoms induces excited atoms, which through three-bodycollisions creates excited excimers, Kr ∗ , that decay emitting one VUV photon, hν . It cor-responds to transitions of the singlet and triplet bound molecular states, from vibrationallyrelaxed levels, to the repulsive ground state.A LAAPD QE was measured to be 0.90 for Kr EL [57]. According to the manufac-turer, the LAAPD fabrication technology is well established, and quite good reproducibilityis obtained and it is expected that the behaviour observed for individual LAAPDs is rep-resentative for any of these devices. In fact, [58] have measured the relative QE of ∼ ∼ ± ∼ σ ) for the LAAPD QE, beingthis the major source of uncertainty in our measurements.– 5 – Experimental results and discussion
In Fig.3 we depict the reduced electroluminescence yield, Y/N, i.e. the electroluminescenceyield divided by the number density of the gas, as a function of reduced electric field,E/N, in the scintillation region. The data was taken using a constant electric field of 0.36kV/cm in the absorption region, while varying the electric field in the scintillation region.Three independent runs have been performed with several days of interval and with a roomtemperature between 25 and 26 ◦ C, showing a good reproducibility of the experimentalresults. The reduced EL yield exhibits an approximately linear trend with the reducedelectric field, a behavior similar to that of Ar and Xe and mixtures of Xe with He or withmolecular additives [1–12]. For each run, Fig.3 also depicts a linear fit superimposed to theexperimental data, excluding the two data points taken at the highest reduced electric fieldwhere secondary ionisation is already non-negligible. Simulation results from [8, 10] are alsodepicted for comparison. To the best of our knowledge, there are no other experimental orsimulation results in the literature. -1 atom V cm -17
E/N [ x1000.20.40.60.811.21.41.61.82 ] - a t o m c m - pho t on s e l e c t r on - Y / N [ x run Figure 3 . Krypton reduced electroluminescence yield as a function of reduced electric field forthree different runs (this work) and the respective linear fit to the data below 14 Td, as well as fordata obtained from Monte Carlo simulation [8, 10].
The reduced electroluminescence yield can be approximately represented as Y / N (10 − photons electron − cm atom − ) = 0 .
113 E / N − . (4.1)where E/N is given in Td (10 − V cm atom − ). This equation can also be represented asa function of pressure, at a given temperature used to convert the gas density into pressure.At room temperature Eq.4.1 can be expressed as– 6 – / p (photons electron − cm − bar − ) = 113 E / p − (4.2)The slope of the linear dependence denotes the scintillation amplification parameter,i.e., the number of photons produced per drifting electron and per volt, and is in goodagreement with what has been obtained by Monte Carlo simulations for room temperature[8, 10]. In addition, the excitation threshold for Kr, defined as the extrapolation of the lineartrend to zero scintillation, 2.7 Td (0.7 kV cm − bar − at 293 K), is also in good agreementwith the values presented in the literature [8, 10, 59]. On the other hand, at a reducedelectric field of 14 Td (3.5 kV cm − bar − ) the experimental data depart from the lineartrend, denoting already the presence of a non-negligible amount of charge multiplication inthe scintillation region. Therefore, from the experimental data we can conclude that theKr ionisation threshold should be around 13.5 Td (3.4 kV cm − bar − ). This value is abovethat obtained from the MC simulation of [8] and is lower than foreseen by the most recentsimulation toolkit [10], demonstrating the importance of the data obtained in this work, forthe development of future optical-TPCs based on Kr filling.For comparison, Fig.4 depicts the experimental results obtained with the present methodfor the electroluminescence yield in Xe [1], Kr (this work) and Ar [3] along with the re-spective simulation results obtained by the most recent simulation work-package for ELproduction in noble gases [10]. The scintillation amplification parameter in Kr and Ar isabout 80 % and 60 % , respectively, of that for Xe. -1 atom V cm -17
E/N [ x1000.511.522.533.5 ] - a t o m c m - pho t on s e l e c t r on - Y / N [ x Krypton - This workXenon [1]Argon [3]Simulation Krypton [10]Simulation Xenon [10]Simulation Argon [10]
Figure 4 . Reduced electroluminescence yield in Xe, Kr and Ar as a function of reduced electricfield. Data points correspond to experimental data from [1], this work and [3], while the curvescorrespond to simulation data [10]. – 7 –
Conclusions
We have performed experimental studies on the reduced electroluminescence yield of pureKr at room temperature and compared it with those obtained by Monte Carlo simula-tion. For the experimental measurements we used a gas proportional scintillation counter(GPSC), having a VUV-sensitive large area avalanche photodiode (LAAPD) for the scin-tillation readout. We used 5.9-keV x-rays to induce electroluminescence in the GPSC orto interact directly in the LAAPD. The concurrent detection of the electroluminescencepulses and the x-rays in the photosensor under the same conditions and in the same setup,allows to use the number of charge carriers produced by the x-rays interacting directly inthe LAAPD as a reference for determining the number of charge carriers produced by theelectroluminescence in the LAAPD, allowing a straightforward measurement of the num-ber of photons impinging on it. The reduced electroluminescence yield exhibits a lineardependence with reduced electric field, with an amplification parameter of 113 photons perkV, per electron, and a scintillation threshold of 2.7 Td (0.7 kV cm − bar − at 293 K), ingood agreement with the simulation data present in the literature. Above 14 Td (3.5 kVcm − bar − ) the reduced electroluminescence yield departs from the linear trend, showinga faster increase due to the additional scintillation produced by extra secondary electronsresulting from the charge multiplication onset. The Kr amplification parameter is about80 % and 140 % of that measured for Xe and Ar, respectively. Acknowledgments
This work is funded by FEDER, through the Programa Operacional Factores de Compet-itividade - COMPETE and by National funds through FCT - Fundação para a Ciência eTecnologia in the frame of project PTDC/FIS/NUC/1534/2014 and UID/FIS/04559/2020(LIBPhys).
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