Elsevier

Astroparticle Physics

Volume 14, Issue 3, November 2000, Pages 227-237
Astroparticle Physics

Suitability of superheated droplet detectors for dark matter search

https://doi.org/10.1016/S0927-6505(00)00123-7Get rights and content

Abstract

We have measured the sensitivity of superheated droplet detectors in different radiation environments and we discuss the suitability of this detection technique for the search of weakly interacting cold dark matter particles. In particular, we show that our new proposed detector based on superheated carbo-fluorates can be operated at ambient pressure and room temperature in a mode where it is almost exclusively sensitive to the nuclear recoils following neutralino interaction, which allows a powerful background discrimination. The potential of the detection technique is shown by deriving a limit on the spin-dependent neutralino cross-section, making use of the very favourable neutralino–19F interaction and background suppression.

Introduction

PICASSO [1] is a new approach for the detection of cold dark matter candidates based on the metastability of moderately superheated liquids [2], [3], [4], [5]. The proposed detection method relies on the fact that for suitable operating parameters, a liquid to vapour phase transition can be triggered exclusively by heavily ionising nuclear recoils and not by minimally ionising particles. This method has the potential of a substantial increase of sensitivity and would allow to investigate the neutralino hypothesis for cold dark matter, as predicted by minimal supersymmetric models. A feasibility study was performed in our Montréal laboratory with the aim to characterise efficiencies, thresholds, and intrinsic backgrounds. We have made progress in understanding these issues and have shown that the acoustic detection of events with piezoelectric sensors is possible with an energy threshold as low as a few keV. In these studies, we used detector modules which were fabricated according to our specifications by BTI technology industries at Chalk River, Ontario.1

Section snippets

Detection technique

The detection reaction of neutralinos is an elastic scattering off a detector nucleus, and the energy deposition of the recoiling nucleus has to be detected. Under the usual assumptions for neutralino flux and masses, we expect for all detector materials recoil energies to be lesser than 100 keV, and depending on more detailed assumptions on supersymmetric cross-sections, we expect event rates below 1 counts kg−1day−1 in the energy range between 4 and 20 keV. Therefore, in order to ensure a

Systematic studies

In order to understand the response of our detectors to neutralino-induced recoils and to possible background sources, we studied the sensitivity of our detectors to various types of radiations over a large range of temperatures.

It is interesting to note that a unified description of the macroscopic properties of droplet detectors, such as neutron detection threshold, onset of γ-ray sensitivity and the limit of thermodynamic stability can be obtained by introducing a dimensionless parameter,

γ-ray and cosmic-ray muon background

The background tests were performed in a temperature-controlled room in our underground laboratory, which is covered by 7 m of earth and is shielded on one side by solid granite rock. The ambient γ-ray background in this laboratory was measured with a 490 cm3 Ge detector. A total flux of 0.12 counts cm−2s−1 was found in the range from 20 keV to 3 MeV. The spectrum showed a prominent 40K peak, as well as the characteristic γ-rays of the 238U/232Th decay chains. The cosmic muon flux in this part

Preliminary limits on spin-dependent neutralino interactions

Due to its high 19F content, PICASSO is especially suitable to search for spin-dependent neutralino interactions. 19F is a spin-1/2+ isotope and has a very favourable spin dependent cross-section for neutralinos [15], [16]. If we assume that neutralinos are distributed in our galaxy in a self-gravitating halo with isotropic Maxwellian velocity distribution, the recoil spectrum is well approximated bydRdE=R0〈ERF2(ER)expE〈ER,where ER is the mean recoil energy and F2(ER) is a nuclear form

Conclusions

The results obtained in this systematic study do show that superheated liquid droplet detectors have the potential to achieve the high sensitivity required to detect nuclear recoils with energies as low as a few keV only, as needed for the observation of neutralinos as cold dark matter candidates. At the same time, the sensitivity to minimum ionising radiation is found to be negligibly small. The detector fabrication is well understood and since the cost of the active droplet material is low

Acknowledgements

We are indebted to B. Sur, AECL Chalk River, for contributions in the initial phase of the project and for lending us a 3He counter. We thank our colleagues at TRIUMF and McGill University for providing scintillator panels for our cosmic muon veto system. Further thanks go to the SNO collaboration for their hospitality during our background measurements. Finally, the gain in sensitivity obtained with the modified BTI detectors has been essential to this project and the expertise provided by BTI

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