RU2532859C1 - Method of detecting ionisation signal in charge emission detectors - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to low-background experiments to search for rare events, e.g., interactions of dark matter with ordinary matter, and can be used in experiments to investigate interaction of neutrinos (antineutrinos) with energy of 1-100 MeV with matter. The method of detecting an ionisation signal in charge emission detectors includes generating an electric field in liquid xenon, pulling ionisation electrons towards the liquid-saturated vapour boundary surface, drawing (emitting) ionisation electrons into the gas phase and subsequent detection thereof in the gas phase, wherein an electronegative substance is dissolved in the liquid xenon, said substance having a high capture coefficient for ionisation electrons, thermalised under the boundary surface of liquid and gaseous xenon, and simultaneously having a low capture coefficient for ionisation electrons drifting in the liquid xenon.

EFFECT: significant reduction of the background of captured subsurface electrons, low energy threshold of detection and high sensitivity.

1 dwg, 1 tbl

Description

The invention relates to the field of low-background experiments to search for rare events, for example, interactions of dark matter with ordinary matter, and can be used for experiments to study the interaction of neutrinos (antineutrinos) with an energy of 1-100 MeV with matter.

A known method of recording [1] rare events in an emission detector is when ionization (electrons + ions) is created as a result of the interaction of the investigated radiation with liquid xenon, and under the influence of an external electric field, the electrons drift in liquid xenon to the interface, are pulled into the gas phase and recorded using photoelectronic multipliers for intense luminescence (electroluminescence) in an electric field.

In the method of registration closest to the proposed method in terms of technical nature and the achieved result, implemented in [2] of FIG. 1, ionization electrons drift in liquid xenon with an energy of about 1 eV, approach the interface, emit into the gas phase and then register at help photomultiplier tubes for intense luminescence (electroluminescence) in an electric field. In this case, in the known method, a part of the ionization electrons is not able to immediately overcome the liquid – gas phase interface, therefore they remain below the surface, where in elastic collisions with xenon atoms they drop their energy to about 0.02 eV and then randomly emit over time into the gas phase. In close to the proposed registration method [2], the frequency of emission of subsurface electrons into a gas is about 100 Hz, which is associated with the ionization of liquid xenon by background gamma rays from radioactive contamination of the detector body material and space muons.

The known method does not allow to register rare events during ionization with low energy transfer, when the result is the formation in liquid xenon and emission of one ionization electron into a gas. Such events are indistinguishable from the emission of single thermalized electrons from the background, delayed below the surface of liquid xenon.

The technical result of the proposed method is a significant decrease in the background from the detained subsurface electrons randomly emitting into the gas phase, and, as a result, a decrease in the energy threshold of the registration of the method and an increase in the sensitivity of the method, which makes it possible to study such rare processes with low energy transfer as coherent scattering neutrinos on nuclei - a fundamental physical process predicted by the standard model of electroweak interactions, but so far not observed in practice.

To obtain a technical result, a substance having a high capture cross section for electrons with an energy of about 0.02 eV is dissolved in liquid xenon in order to quickly capture them and turn them into ions. A high capture cross section should ensure a short lifetime of subsurface thermalized electrons. Moreover, to ensure low electron losses during drift to the surface of liquid xenon, the capture cross section for electrons with energies of about 1 eV should be low for the selected substance.

It is proposed to use SF6 gas (SF6) and chlorine (Cl) as such a substance. Table 1 presents the literature data on the capture cross sections for two values of electron energy - 0.02 and 1 eV, estimates of the concentration of these substances necessary to ensure the electron drift length before capture in liquid xenon of 10 meters and obtained estimates of the characteristic lifetimes of subsurface thermalized electrons.

The calculations were carried out using the formulas known from the theory of low-ionized plasma:

, где λ - длина пробега электронов до захвата, см; $$\lambda =\frac{\mathrm{one}}{\sigma N^{\text{'}\text{'}}}$$

where λ is the mean free path of the electrons to capture, cm;

σ is the cross section for electron capture by an impurity, cm ² ;

N is the impurity concentration, cm ^-3 ;

где τ - время жизни электрона до захвата, с; $$\tau =\frac{\mathrm{one}}{N\sigma {\upsilon }^{\text{'}\text{'}}}$$

where τ is the electron lifetime before capture, s;

σ is the cross section for electron capture by an impurity, cm ² ;

N is the impurity concentration, cm ^-3 ;

υ is the thermal velocity of the electron, at an energy of 1 eV it is 6 × 10 ⁷ cm / s, at an energy of 0.02 eV it is 9 × 10 ⁵ cm / s

Figure 1 shows the operation of the emission detector.

The method is as follows.

In liquid xenon dissolve SF6 (Cl) with a concentration of 10 ¹⁵ 1 / cm ³ (2.5 × 10 ¹⁵ 1 / cm ³ ), create a drift electric field with a strength of about 1 kV / cm. As a result of the studied interaction, for example, elastic scattering of neutrinos on the xenon nucleus, ionization of liquid xenon occurs with the formation of pairs (electron + ion). In an applied electric field, the electrons are “heated” to an energy of about 1 eV and drift to the interface. The length of the electron drift before capture, amounting to 10 m (see Table 1) at the indicated concentrations of the selected gases, does not practically impair the amplitude characteristics of the registration method for the typical used drift gap of the detector ~ 10 m. In the applied electric field, the electrons pass (emit) through the phase interface and exit into the gas phase, where they are recorded using photomultiplier tubes by luminescence (electroluminescence). Part of the electrons, which are not able to emit into the gas phase, remains below the surface of liquid xenon and, in elastic collisions, quickly discharges its energy to about 0.02 eV, coming into thermal equilibrium with the surrounding xenon atoms. At the indicated concentrations of dissolved gases, the characteristic lifetime before the capture of such electrons by dissolved SF6 (Cl) molecules is 5 × 10 ^-7 (1.3 × 10 ^-6 ) s.

The technical efficiency of the proposed method compared to the prototype consists in the strong suppression of the background from the emission of subsurface electrons due to their rapid capture in liquid xenon on the molecules of introduced impurities SF6 (Cl). The technical efficiency of the proposed method compared with the prototype is to reduce the energy threshold of registration of the method and increase the sensitivity of the method. It becomes possible to investigate rare events during ionization with such a transfer of energy, when the result is the formation in liquid xenon and the emission of a single ionization electron into a gas. Such events in the prototype are not distinguishable from the emission of single thermalized electrons from the background delayed below the surface of liquid xenon. This makes it possible to study such rare processes with low energy transfer as coherent neutrino scattering by heavy nuclei - a fundamental physical process that has not been explored so far, predicted in the framework of the standard model.

Information sources

1.2. M. Hornetal. Nuclear recoil scintillation and ionization yields in liquid xenon from ZEPLIN-III data. Phys. Lett. B 705 (2011) 471-476; arXiv: 106.0694 [physics.ins-det].

2. D. Yu. Akimov, I.S. Aleksandrov, V.A. Belov, A.I. Bolozdynya et al. Measurement of Single-Electron Noise in a Liquid Xenon Emission Detector. Instruments and Experimental Techniques 55 (2012) 423-28.

3. Hayes T.R., Wetzel R.C. and Freund R.S. "Absolute Electron - impact-ionization Cross sectionmeasuremensotf the Halogen Atoms", Phys. Rev. 35 (2), 578-584, 1987.

4. Kouichi Ono, Mutumi Tuda, Hiroki Ootera, and Tatsuo Oomori, "Electron cyclotron resonance plasma etching of Siwith C12: plasma chemistry and mechanisms", Pure & Appl. Chem., Vol. 66, No. 6, pp. 1327-1334, 1994.

Claims (1)

A method for recording an ionization signal in emission radiation detectors, including generating an electric field in liquid xenon, pulling ionization electrons to a liquid-saturated vapor interface, pulling (emission) ionization electrons into the gas phase, and then recording them in the gas phase, characterized in that liquid xenon dissolves an electronegative substance with a high capture coefficient for ionization electrons thermalized under the interface between liquid and gaseous senon and simultaneously possessing a low capture coefficient for ionization electrons drifting in liquid xenon.

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