Suitability of superheated droplet detectors for dark matter search
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 kgday−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 cms−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 bywhere 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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2012, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCharacteristics of acoustic emissions during nucleation of superheated droplets
2011, Physics Letters, Section A: General, Atomic and Solid State PhysicsCitation Excerpt :For spontaneous nucleations the high frequency events also increase with increase in temperature. It is to be noted that the radioactive contaminations [21] present in the detector and the ambient background radiations also contribute to the spontaneous nucleation events since the experiments are performed at higher degree of superheat where the liquid is sensitive to energetic photons and cosmic muons [22]. The higher frequency events in spontaneous nucleation data may actually be induced by these ambient background radiations.
Neutron-gamma discrimination by pulse analysis with superheated drop detector
2010, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :Gamma-ray sensitivity for detectors with sensitive liquids CCl2F2, C3F8, C4F10 and C2ClF5 were also studied by exposing to 57Co, 22Na, 137Cs and 60Co gamma sources. It was observed that the detectors became sensitive at around 34 °C and reached an efficiency plateau between 40 and 45 °C [11]. In the above studies of gamma-ray sensitivities of various active liquids, the experiments were done by measuring the count rate as a function of temperature.
Dark matter spin-dependent limits for WIMP interactions on <sup>19</sup>F by PICASSO
2009, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy PhysicsCitation Excerpt :Other dark matter experiments based on similar techniques are SIMPLE, using droplets of C2ClF5 and CF3I [15,16] and COUPP, which operates a bubble chamber filled with CF3I [17]. Previous physics results of our studies have been published in [18,19]. In the meantime progress has been made in several areas and data are presently being taken with 32 detector modules with an increased active mass of 19F of about 65 g per module, with lower intrinsic background and an increased droplet size (Section 3).
Two-state modeling of the superheated emulsion detector
2009, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment