382 research outputs found
Gas Gains Over 10 and Optimisation using Fe X-rays in Low Pressure SF with a Novel Multi-Mesh ThGEM for Directional Dark Matter Searches
The Negative Ion Drift (NID) gas SF has favourable properties for track
reconstruction in directional Dark Matter (DM) searches utilising low pressure
gaseous Time Projection Chambers (TPCs). However, the electronegative nature of
the gas means that it is more difficult to achieve significant gas gains with
regular Thick Gaseous Electron Multipliers (ThGEMs). Typically, the maximum
attainable gas gain in SF and other Negative Ion (NI) gas mixtures,
previously achieved with an Fe X-ray source or electron beam, is on the
order of ; whereas electron drift gases like CF and similar mixtures
are readily capable of reaching gas gains on the order of or greater. In
this paper, a novel two stage Multi-Mesh ThGEM (MMThGEM) structure is
presented. The MMThGEM was used to amplify charge liberated by an Fe
X-ray source in 40 Torr of SF. By expanding on previously demonstrated
results, the device was pushed to its sparking limit and stable gas gains up to
50000 were observed. The device was further optimised by varying the
field strengths of both the collection and transfer regions in isolation.
Following this optimisation procedure, the device was able to produce a maximum
stable gas gain of 90000. These results demonstrate an order of magnitude
improvement in gain with the NID gas over previously reported values and
ultimately benefits the sensitivity of a NITPC to low energy recoils in the
context of a directional DM search
Demonstration of ThGEM-multiwire hybrid charge readout for directional dark matter searches
Sensitivities of current directional dark matter search detectors using gas time projection chambers are now constrained by target mass. A ton-scale gas TPC detector will require large charge readout areas. We present a first demonstration of a novel ThGEM-Multiwire hybrid charge readout technology which combines the robust nature and high gas gain of Thick Gaseous Electron Multipliers with lower capacitive noise of a one-plane multiwire charge readout in SF6 target gas. Measurements performed with this hybrid detector show an ion drift velocity of 139 Β± 12 msβ1 in a reduced drift field E/N of 93 Td (10β17 V cm2) at a gas gain of 2470Β±160 in 20 Torr of pure SF6 target gas
Charge amplification in sub-atmospheric CF4:He mixtures for directional dark matter searches
Low pressure gaseous Time Projection Chambers (TPCs) are a viable technology for directional Dark Matter (DM) searches and have the potential for exploring the parameter space below the neutrino fog [1,2]. Gases like CF4 are advantageous because they contain flourine which is predicted to have heightened elastic scattering rates with a possible Weakly Interacting Massive Particle (WIMP) DM candidate [3,4,5]. The low pressure of CF4 must be maintained, ideally lower than 100 Torr, in order to elongate potential Nuclear Recoil (NR) tracks which allows for improved directional sensitivity and NR/Electron Recoil (ER) discrimination [6]. Recent evidence suggests that He can be added to heavier gases, like CF4, without significantly affecting the length of 12C and 19F recoils due to its lower mass. Such addition of He has the advantage of improving sensitivity to lower mass WIMPs [1]. Simulations can not reliably predict operational stability in these low pressure gas mixtures and thus must be demonstrated experimentally. In this paper we investigate how the addition of He to low pressure CF4 affects the gas gain and energy resolution achieved with a single Thick Gaseous Electron Multiplier (ThGEM)
Demonstration of radon removal from SF6 using molecular sieves
The gas SF6 has become of interest as a negative ion drift gas for use in directional
dark matter searches. However, as for other targets in such searches, it is important that radon
contamination can be removed as this provides a source of unwanted background events. In this
work we demonstrate for the first time filtration of radon from SF6 gas by using a molecular
sieve. Four types of sieves from Sigma-Aldrich were investigated, namely 3Γ
, 4Γ
, 5Γ
and 13X.
A manufactured radon source was used for the tests. This was attached to a closed loop system in
which gas was flowed through the filters and a specially adapted Durridge RAD7 radon detector.
In these measurements, it was found that only the 5Γ
type was able to significantly reduce the
radon concentration without absorbing the SF6 gas. The sieve was able to reduce the initial radon
concentration of 3875 Β± 13 Bqmβ3
in SF6 gas by 87% when cooled with dry ice. The ability of
the cooled 5Γ
molecular sieve filter to significantly reduce radon concentration from SF6 provides
a promising foundation for the construction of a radon filtration setup for future ultra-sensitive SF6
gas rare-event physics experiments
Directional dark matter readout with a novel multi-mesh ThGEM for SF6 negative ion operation
Direct searches for Weakly Interacting Massive Particle (WIMP) dark matter could greatly benefit from directional measurement of the expected induced nuclear recoils. Gas-based Time Projection Chambers (TPCs) offer potential for this, opening the possibility of measuring WIMP signals below the so-called neutrino floor but also of directional measurement of recoils induced by neutrinos from the Sun, for instance as proposed by the CYGNUS collaboration. Presented here for the first time are results from a Multi-Mesh Thick Gas Electron Multiplier (MM-ThGEM) using negative ion gases for operation with such a directional dark matter TPC. Negative ion drift gases are favoured for directionality due to their low diffusion characteristics. The multiple internal mesh structure is designed to provide a high gain amplification stage when coupled to future large area Micromegas, strip or pixel charge readout planes. Experimental results and simulations are presented of MM-ThGEM gain and functionality using low pressure pure CF4, SF6 and SF6:CF4 mixtures irradiated with alpha particles and 55Fe x-rays. The concept is found to work well, providing stable operation with gains over 103 in pure SF6
Lowering the energy threshold in COSINE-100 dark matter searches
COSINE-100 is a dark matter detection experiment that uses NaI(Tl) crystal
detectors operating at the Yangyang underground laboratory in Korea since
September 2016. Its main goal is to test the annual modulation observed by the
DAMA/LIBRA experiment with the same target medium. Recently DAMA/LIBRA has
released data with an energy threshold lowered to 1 keV, and the persistent
annual modulation behavior is still observed at 9.5. By lowering the
energy threshold for electron recoils to 1 keV, COSINE-100 annual modulation
results can be compared to those of DAMA/LIBRA in a model-independent way.
Additionally, the event selection methods provide an access to a few to sub-GeV
dark matter particles using constant rate studies. In this article, we discuss
the COSINE-100 event selection algorithm, its validation, and efficiencies near
the threshold
Microsurgical third ventriculocisternostomy as an alternative to ETV: report of two cases
OBJECTIVE: To describe a microsurgical alternative to endoscopic third ventriculocisternostomy. METHODS: Two children with shunt-dependent hydrocephalus and multiple shunt revisions were considered candidates for third ventriculocisternostomy (TVS). Because of slit ventricles, an endoscopic approach was not possible and, therefore, both patients received a microsurgical TVS by a supraorbital approach. RESULTS: In both cases, microsurgical TVS was successful and the patients became shunt free. CONCLUSION: Microsurgical TVS by a supraorbital craniotomy is a viable alternative to endoscopic TVS in selected cases
Characterisation of large area THGEMs and experimental measurement of the Townsend coefficients for CF4
Whilst the performance of small THGEMs is well known, here we consider the challenges in scaling these up to large area charge readouts. We first verify the expected gain of larger THGEMs by reporting experimental Townsend coefficients for a 10 cm diameter THGEM in low-pressure CF. Large area 50 cm by 50 cm THGEMs were sourced from a commercial PCB supplier and geometrical imperfections were observed which we quantified using an optical camera setup. The large area THGEMs were experimentally characterised at Boulby Underground Laboratory through a series of gain calibrations and alpha spectrum measurements. ANSYS, Magboltz and Garfield++ simulations of the design of a TPC based on the large area THGEMs are presented. We also consider their implications for directional dark matter research and potential applications within nuclear security
First search for a dark matter annual modulation signal with NaI(Tl) in the Southern Hemisphere by DM-Ice17
We present the first search for a dark matter annual modulation signal in the Southern Hemisphere conducted with NaI(Tl) detectors, performed by the DM-Ice17 experiment. Nuclear recoils from dark matter interactions are expected to yield an annually modulated signal independent of location within the Earthβs hemispheres. DM-Ice17, the first step in the DM-Ice experimental program, consists of 17 kg of NaI(Tl) located at the South Pole under 2200 m.w.e. overburden of Antarctic glacial ice. Taken over 3.6 years for a total exposure of 60.8 kg yr, DM-Ice17 data are consistent with no modulation in the energy range of 4β20 keV, providing the strongest limits on weakly interacting massive particle dark matter from a direct detection experiment located in the Southern Hemisphere. The successful deployment and stable long-term operation of DM-Ice17 establishes the South Pole ice as a viable location for future dark matter searches and in particular for a high-sensitivity NaI(Tl) dark matter experiment to directly test the DAMA/LIBRA claim of the observation of dark matter
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