Elsevier

Astroparticle Physics

Volume 44, April 2013, Pages 9-14
Astroparticle Physics

Analysis of defect formation in semiconductor cryogenic bolometric detectors created by heavy dark matter

https://doi.org/10.1016/j.astropartphys.2013.01.005Get rights and content

Abstract

The cryogenic detectors in the form of bolometers are presently used for different applications, in particular for very rare or hypothetical events associated with new forms of matter, specifically related to searches for dark matter. In the detection of particles with a semiconductor as target and detector, usually two signals are measured: ionization and heat. The amplification of the thermal signal is obtained with the prescriptions from the Luke–Neganov effect. The energy deposited in the semiconductor lattice as stable defects in the form of Frenkel pairs at cryogenic temperatures, following the interaction of a dark matter particle, is evaluated and consequences for measured quantities are discussed. This contribution is included in the energy balance of the Luke effect. Applying the present model to germanium and silicon, we found that for the same incident weakly interacting massive particle the energy deposited in defects in germanium is about twice the value for silicon.

Highlights

► Formation of defects in cryogenic detectors due to WIMPs interactions is analysed. ► Luke–Neganov effect is extended to consider the energy stored in defects. ► The energy of WIMPs recoils is partitioned in ionization, phonons and defects. ► Consequences of defect formation for detection are analysed.

Introduction

In the last decades, great developments in low temperature detectors in the form of bolometers, in the technologies of semiconductors, superconductors or scintillator crystals were obtained. These cryogenic detectors are able to detect radiations and particles with a threshold in the range of eV.

If the pioneering idea of the bolometric detectors goes back to 1935, year when Simon suggested an “Application of Low Temperature Calorimetry to Radioactive Measurements” [1], the modern applications started after the ’70. Nowadays, there are a lot of reviews in this thematic: see for example those of Gaitskell [2] and Sarazin [3]. Bolometric detectors are used in different applications in experimental physics, e.g. in searches for neutrinoless double beta decay and neutrino mass – for example the experiments CUORE & Cuoricino, for total energy measurements of free electron lasers [4], to measure the cosmological microwave background [5] constituents of the dark matter, etc.

There is clear evidence that a large part of the dark matter in the Universe is non-baryonic, non-luminous and non-relativistic and the search for it has become a very active research area in the last decades. Hypothetical Weakly Interacting Massive Particles (WIMPs) are proposed as possible particle candidates that satisfy all of the above criteria. Thus, their direct detection using the experimental information of low-energy nuclear recoils originating from WIMPs interactions is one of the detection methods usually used in bolometric detectors.

If in the first generation of these experiments only the heat deposited in detectors as phonons was used in the detection, in more recent experiments phonons and ionization (or light from scintillation signals) are measured simultaneously, trying to discriminate both between electron – nucleon/nuclei recoils and also between different sources of the phenomena: ordinary matter or constituents of the dark matter. As detector materials, silicon and germanium or scintillator crystals (Al2O3:Ti or CaWO4, CaMoO4 etc.) are used.

One of the effects produced by the slowing down of particles in crystalline semiconductors is defect production, which is a phenomenon present at all temperatures. Defect formation after electron and gamma irradiation at temperatures around and lower than liquid He was studied in InP, Si, Ge, and SiC since 1995 [6], [7], [8].

In this paper we discuss the effects introduced in the energy balance by the formation of long time stable defects in materials for bolometers and possible consequences for the identification of the particles. In the next section, general aspects related to defect formation in the process of slowing down of selfrecoils in silicon and germanium are reviewed, with emphasis on the existing experimental data related to defect formation following cryogenic irradiation. The energy stored in Frenkel pairs is calculated, and the formulae relating it to the measured quantities in heat and ionization detectors are derived. Concrete applications related to direct WIMPs searches with these detectors are discussed, underlying the influence of the energy stored in defects.

Section snippets

Energy balance in heat and ionization cryogenic detectors

After the primary interaction of an incoming particle in the semiconductor, a selfrecoil of energy E is left. It loses energy in both electronic and nuclear collisions.

Let ν(E) be the energy deposited in the semiconductor in the form of atomic collisions, and η(E) the total energy given to the electronic system, both calculated using Lindhard’s theory [9]. Part of the energy ν(E) is stored in lattice defects (ED), the other part being given to the lattice in the form of excitations (phonons).

If

Physical processes related to WIMPs direct detection

The nature and characteristics of DM is a question of central importance in cosmology, astrophysics and astroparticles. The list of candidates and the possible signatures of DM have greatly expanded due to recent experimental results and observations [34], [35]. A summary of dark matter particle candidates, their properties, and the potential methods for their detection was recently given in Refs. [36], [37]. WIMPs are the most studied from all DM candidates, are found in many particle physics

Results and discussion

In the discussion which follows, a WIMP with mass in the range 5–100 GeV, having a velocity of 260 km/s in respect to a terrestrial detector is considered. It has a single interaction in a Ge or Si cryogenic detector.

For silicon, the first result of the energy partition between ionization and other processes using the complete Lindhard theory was obtained by Lindhard and later published in the paper of Simon [45]. Analytical approximations of the Lindhard equations both for silicon and germanium

Summary

The possibility of defect formation in bolometric semiconductor detectors at cryogenic temperatures was studied, with application to WIMPs direct searches.

The models for the partition factor between the energy transferred by the primary recoil to the atomic and electronic systems of Si and Ge were reviewed, starting from Lindhard’s theory. Part of the energy transferred to the atomic system is stored in defects. At sub-Kelvin temperatures the defects are Frenkel pairs and they do not anneal out.

Acknowledgment

MLC and SL thank the NIMP Core Programme PN09-450101 for financial support.

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