Cryogenic particle detectors

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Cryogenic detectors are detectors that operate usually at very low temperatures, well below 77 kelvins, the liquefaction temperature of nitrogen. These sensors absorb a particle or a photon and deliver a signal whose amplitude is proportional to the energy deposited during the absorption process.

1 Introduction
2 Reading out a classical calorimeter with a resistor
3 References
4 See also

[edit] Introduction

There are several reasons to engage into the trouble to cool down a sensor. Several properties of matter are only accessible at low temperature and electronic perturbations (noise) can often be reduced.

Originally, astronomy as well as particle physics were the fields that pushed the development of cryogenic detectors. Since the 1990s other fields of science have manifested interest in this technology.

One way to illustrate the benefit of operating a sensor at low temperature is to consider a classical calorimeter. It is a piece of matter that has a heat capacity $C$ , kept at a temperature $T$ . If now some energy $E$ is deposited into this matter (called the absorber), the temperature will rise by an amount $\frac{E}{C}$ . At low temperatures, the quantity $C$ reduces. Thus, for the same amount of deposited energy there is a larger temperature excursion. This process can be repeated, and if there is no path for heat to leak out, each additional event adds energy and the temperature of the system increases but remains not well defined. A heat link must be established between the absorber and a reservoir. The amplitude of thermal fluctuations between absorber and reservoir sets a limit to the smallest energy that can be resolved with such a system.

[edit] Reading out a classical calorimeter with a resistor

The goal is to read out the temperature excursion $\Delta T = \frac{E}{C}$ ; this requires some kind of thermometer. Several types of thermoemeter exist. At low temperature, a semiconductor exhibits an increasing resistance $R$ with decreasing temperature (see thermistor). At very low temperature, the variation of $R (T)$ is strong and a small temperature change induces a measurable resistance variation. In mathematical language: $\Delta R = \frac{d}{dT} R(T) \Delta T$ . It is now possible to readout this change in electrical resistance by measuring

a voltage change (while keeping the current $I$ through the thermometer constant): $Δ V = Δ R I$

a current change (by keeping the voltage $V$ difference at the thermometer constant): $Δ I = V / Δ R$

In principle, several types of resistance thermometers (see thermistors) can be used. The abrupt superconducting phase transition is used quite effectively, provided one takes measures to keep the whole device at a temperature within its transition range. This is a technical challenge, but mature solutions exist.

At this point the second reason to operate these devices at low temperature needs to be clarified. The limit of the voltage difference $Δ V$ measurement is given by the smallest meaningful resistance fluctuation which in turn is determined (see above) by thermal fluctuations. Since all resistors $R$ exhibit natural (Johnson noise) voltage fluctuations (even without any net current flowing through) a reduction of temperature is often the only way to achieve, for a given measurement bandwidth, the required sensitivity. Thus, a voltage measurement requires low input voltage noise instrumentation whereas a current measurement requires low input current noise instrumentation.

Bolometer and calorimeter differ in the sense that the first measures a flux of energy (power) whereas the second measures distinct energy deposition events.