Excess backgrounds in Dark Matter detectors and physics of glasses Author Sergey Pereverzev affiliation LLNL

2025-04-27 0 0 236.26KB 7 页 10玖币

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Excess backgrounds in Dark Matter detectors and physics of glasses

Author: Sergey Pereverzev

affiliation: LLNL

Abstract

Multiple mechanisms allow energy accumulation in materials and later releases. Interactions between

excitations, defects, or other configurations carrying excess energy can lead to avalanche energy releases

and more complex phenomena like self-organization and self-replication effects in systems with energy

flow. Exact theoretical models of these phenomena are often impossible because of insufficient

knowledge of interactions inside materials. Still, comparison and analogies between excess low-energy

backgrounds in solid-state detectors, mechanisms of inelastic deformations of crystals, and relaxation

processes in glasses allow essential predictions for the background properties and insight into the physics

of glasses.

The presence of avalanche-like releases of stored energy is expected for NaI(Tl). We observed energy

accumulation and release processes in the form of delayed random luminescence. We also demonstrated

suppression of delayed luminescence by exposure to red light. We cannot yet clearly confirm or refute the

presence of small avalanches or excess coincidences in the delayed photon flux. An interplay of fast and

delayed luminescence response depends on the type of particles and temperature. One can expect that

other environmental factors could cause modulations of low-energy background. More studies of material

responses to radiation are required.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore

National Laboratory under Contract DE-AC52-07NA27344. LLNL-PROC-840806

Introduction

Practically all dark matter and CEvNS detectors demonstrate excess low-energy backgrounds [1]. In this

paper, we continue to discuss the role of energy accumulation and release processes in the production of

excess backgrounds. Properties of excess backgrounds – a sharp increase in the number of events for

lower events energies, an increase of backgrounds with the growth of mechanical stress in solid-state

detectors, and an increase of low-energy background with the ionization load in noble-liquid detectors are

pointing to the possible common mechanism or scenario for this background production[2]. Interactions

between states bearing excess energy can lead to several emerging phenomena. For systems with energy

flow, 1977 Noble Laureate Ilia Prigogine outlined possibilities of forming dissipative systems,

generation of complexity, and transitions from havoc to order [3]. The Self-Organized Criticality theory

investigates systems where internal interactions lead to avalanche-like releases of stored energy [4,5].

When relaxational avalanches are present, systems often demonstrate common properties: the spectrum of

relaxation events is polynomially decreasing with energy (i.e., catastrophic events are possible), and the

power noise spectrum is close to 1/f (pink noise). Reinforcement of relaxation on a small scale

(quenching of excitations) can suppress the number of large avalanches[4,5].

Though the properties of materials and excitations and interactions could be different, searching for

common scenarios helps understand the effects observed in various types of detectors. The excess low-

energy background was observed in practically all low-temperature solid-state experiments looking for

dark matter particles and coherent scattering of low-energy neutrinos; several EXCESS workshops were

devoted to this matter [1,6]; see also [7]. Yearly modulation of low-energy background in NaI(Tl)

scintillator detector was attributed to interaction with dark matter particles by DAMA-LIBRA

collaboration [8]. Still, other experiments do not confirm these conclusions. We discuss condensed matter

aspects of low-energy background production in solid-state cryogenic detectors and in NaI(Tl)

scintillators. The assumption that DAMA-LIBRA modulation includes energy accumulation and releases

in the material was first formulated by David Nugren [9].

Low-energy background in low-temperature solid-state detectors, microcracking and microscopic

mechanisms of inelastic deformation of single crystals

Low-temperature solid-state detectors use different single-crystal sensors: Si, Ge, SiO2, CaWO4, etc.,

cooled to 10-50 mK temperatures. Experiments look for signatures of nuclear recoils in these crystals

using a thermometer attached to the crystal and looking for the temperature spikes, or a superconducting

hot phonon sensor/ microbolometer on the surface of the crystal (or array of hot phonon sensors) and

looking for the appearance of hot phonon pulses. In parallel, one can look for the emission of photons

(scintillation) and electrons/conductivity pulses in the crystal. All types of detectors demonstrate excess

low-energy background raising sharply as the energy of the event goes below approximately 10-1

eV[5,6,7] (we will not discuss here questions of recoil energy calibration). In many experiments, the

intensity of the low-energy background increases with the increase of thermomechanical stress in the

crystal, and additional experiments verify this observation. The other statement is that excess background

decreases slowly after the initial cool-down of the detector on a time scale of weeks or months. This

relaxation can restart after warming up to several K temperatures- not all the way to room temperature.

The above properties of excess backgrounds were attributed to microcracking (see [7] for example) -a

process of relief of mechanical stress produced during cool-down by the differences in thermal expansion

of materials.

Microscopic mechanisms of small inelastic deformation (flow) of single-crystal samples (Si and Ge most

often) were studied under different load conditions, including inhomogeneous loading and micro-

indentations [10,11]. It was found that flow has a form of small steps, each consisting of transformation in

a microscopic volume of material. These transformations can be changes in crystallographic structure,

chemical transformations, the appearance of the twin boundaries, sliding plains, appearance and motion of

dislocations. These transformations are dissipative events and should result in heat or hot phonon

productions. In many cases, the formation and movement of defects, dislocations, etc., in dielectrics,

semiconductors, and metal samples can be accompanied by photon emission and electrons from the

sample's surface [12,13]. As we can see, "microcracking" likely has the same meaning as inelastic

deformation /flow of crystal material. Thus, the low-energy background we see is a natural relaxation

process of mechanical stress release in single crystals and solids in general. As a term, "microcracking"

could be misleading, as the production and motion of defects in the crystal are not immediately leading to

mechanical breaking or failure. The Inelastic deformation of metals has similar mechanisms, but the

limits of plasticity (ductility) can be much more significant for metals.

Below we will discuss other relaxation processes in solids that can generate small discrete energy release

events. We argue that the observed excess low-energy backgrounds are a cumulative effect of various

glass-like relaxation processes.

Relaxation processes in glasses and Excess low-energy backgrounds in low-temperature solid-state

detectors

At temperatures below the glass transition, amorphous (disordered) materials are out of mechanical and

thermodynamic equilibrium; relaxation processes in the glass state (response to force or other stimuli)

became long and dependent on the internal state of the material (i.e., history-dependent). When one

applies force for some time and then removes it, slow "back-relaxation" can be present. Such effects mean

energy accumulation and releases can be present when the system demonstrates glass-like relaxation

properties.

At low temperatures, many subsystems in materials demonstrate complex and history-dependent

relaxation properties. Here we provide multiple references on low-temperature physics papers: charges

localized on boundaries and interphases in SQUIDs [14], the motion of charges in dielectric substrate

probed by single-electron transistor on the surface [15], magnetic moments of impurities in

superconductors [16]. The glass-like properties of the relaxation processes become more pronounced with

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ExcessbackgroundsinDarkMatterdetectorsandphysicsofglassesAuthor:SergeyPereverzevaffiliation:LLNLAbstractMultiplemechanismsallowenergyaccumulationinmaterialsandlaterreleases.Interactionsbetweenexcitations,defects,orotherconfigurationscarryingexcessenergycanleadtoavalancheenergyreleasesandmorecomplexp...

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Excess backgrounds in Dark Matter detectors and physics of glasses Author Sergey Pereverzev affiliation LLNL

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