PHY326/426

The DRIFT Dark Matter Experiment

Daniel Walker

DRIFT - Directional Recoil Identification From Tracks

Introduction

● WIMP Overview

● Dark Matter Galactic Halo

● WIMP Wind

● Boulby Underground Laboratory

● Overview of the DRIFT Detector

WIMP Overview

● There are a range of potential dark matter candidates, including Weakly-Interacting Massive Particles (WIMPs).

● Masses of WIMPs thought to range from ~1 to ~1000 GeV/c2 (1 GeV/c2 ~ mass of a proton).

● Simplest models suggest that WIMPs exist as an approximately spherical halo surrounding the visible mass of the galaxy.

● A potential candidate is predicted by supersymmetry (SUSY), however supersymmetry remains undetected so far. Detection would greatly strengthen the case for WIMPs being a major dark matter component.

● There are other theoretical candidates, for example axions.

Detection of WIMPs● It is useful to consider the orientation of the solar system:

The orbit of the earth around the sun (ecliptic plane of the solar system) is inclined at an angle of ≈62.5o into the plane of the galaxy.

The Sun has a circular velocity around the galactic centre of ≈220 km/s.

● If we measure the mean direction of the WIMP flux from Earth, this is expected to be coincident with the direction of solar motion. This WIMP wind could give an important signature in looking for WIMPs.

WIMP FluxThis WIMP wind potentially gives two detection signatures, which could be useful in identifying dark matter.

1) The velocity of the Earth with respect to the WIMP Halo is expected to vary approximately sinusoisally with a period of 1 year.

•A detector may measure the WIMP flux across the entire sky, which is dependent on the WIMP density and the mean velocity of the WIMP wind. The expected annual oscillation in velocity would lead to a corresponding annual modulation in WIMP counts.

WIMP Flux2) There is thought to be a diurnal (approximately daily) variation of the Earth's position

with respect to the WIMP wind. This is caused by the Earth's rotation over the course of a sidereal day (approx 23hrs 56 mins), which is the time it takes the Earth to rotate relative to fixed stars.

A detector sensitive to the direction of incoming WIMPs could detect this variation. DRIFT is one such detector, located at a latitude of 55o as shown in the diagram below. Note the Earth is inclined by approximately 62.5o relative to the WIMP wind.

Over the course of 12 hours, the mean direction of the WIMP wind is thought to change from being directly into the top of the detector to through the side of the detector. This daily modulation could be an important signature in WIMP detection, in particular as it is thought to be distinct from any variation from terrestrial sources.

Site Location● Choosing the site of a detector is particularly important, as the WIMP rate expected is

extremely low and the detectors are also sensitive to background radiation.

● The detector location ideally has minimal natural radioactivity. For WIMP detection, neutron radiation is a concern as it can mimic WIMP events. Additional neutron shielding is erected around the detector. Alpha particles leave a different signature in the DRIFT detector so can more easily be distinguised.

● Cosmics rays produce muons in the atmosphere, then produce neutrons upon interacting with the materials of the lab. It is hence necessary to place the detector as far underground as possible to minimise the cosmic ray and hence neutron flux.

Boulby Mine● DRIFT is located at Boulby mine, which is on the North East coast in the North

Yorkshire Moors National Park. This is a working salt and potash mine and is the deepest in the country at a depth of 1100m. The underground workings extend up to 10km under the North Sea. The dark matter labs are a short walk (< 1 km) from the two main shafts.

Boulby Mine

● A schematic of the underground lab is shown

● DRIFT-II is currently operating. Note ZEPLIN II/III and DRIFT-I have already completed operations.

● There are also experiments not related to dark matter located here. More details are available at: http://www.stfc.ac.uk/boulby/default.aspx

Boulby Mine

● The underground facility is built in the layer of NaCl rocksalt, a relatively stable layer in which the main roadways are built.

● The layers of rocks shield the detector from cosmic rays. Cosmic ray muons are reduced by a factor of ~106.

● The rocksalt is low in Uranium (U) and Thorium (Th) contaminants, which are sources of neutrons through their decay chains.

DRIFT Overview● The DRIFT dark matter detector is designed to record directional information from nuclear

recoils of WIMP interactions. The aim of the experiment is to identify a WIMP signal due to the daily variation in the direction of the WIMP flux.

● DRIFT-II is the second generation of the experiment, shown below, and was installed underground in 2005.

Neutron Shielding

● The neutron background in the lab is estimated as leading to ~1 background event a day inside DRIFT.

● Thus, neutron shielding in the form of polypropylene pellets is used to shield the DRIFT detector. Neutrons readily interact with the hydrogen present in the plastic. A depth of at least 50cm is required around the detector.

DRIFT Overview

● The detector inside the vacuum vessel covers a volume of 1 m3.

● Multi-wire proportional chambers (MWPCs), which readout the signals are positioned at each end of the detector volume. Both cover an area of 1 m2.

● A uniform electric field is supplied by the central cathode.

DRIFT Overview● The DRIFT experiment is a Negative Ion Time Projection Chamber.

● The target volume is filled with carbon disulphide (CS2) at 40 Torr (0.053 atm). An incoming WIMP

may interact with a CS2 nucleus. This recoiling nucleus creates an ionisation track inside the target

volume, produced initially of electrons.

● The electrons readily attach themselves to other carbon disulphide (CS2) molecules, creating

negatively charged CS2

- anions.

● The electric field from the central cathode causes these negative ions to drift towards one of the two MWPCs, on which the signals can be read out. These signals can be used to reconstruct the extent and direction of the ionisation track.

DRIFT Overview

● Using a low pressure gas (CS2) means that the ionisation tracks are a few millimetres in

length and are measurable.

● The operating pressure is approximately 41 Torr. Factors influencing the choice of pressure include the target gas mass (more mass means a higher chance of WIMP interactions) and diffusion of the ions as they interact with other gas molecules.

● CS2 is chosen because it is moderately electronegative, so it readily captures electrons

to form CS2

- anions. Drifting ions instead of electrons results in lower diffusion of the

track.

● In practice a mixture of gases is used: In addition to CS2, carbon tetrafluoride (CF

4) and

O2 is used. (In a ratio 30-10-1 Torr). The gas is constantly circulated through the

detector volume to maintain the purity.

● To maintain the correct pressure inside the vessel, this is continually monitored and fed back to the gas control system.

The Central Cathode● The central cathode consists of a aluminium coated mylar sheet of thickness 0.9μm. It

produces a uniform drift field.

● It is desirable for the cathode to be transparent to radiation. For example, in certain situations, if alpha particles become lost in the material and not detected this can lead background events to be mischaracterised as WIMP signal events.

● This happens with radon (Rn) decay products. Radon gas is present in the atmosphere (and in the detector) and comes from the uranium and thorium decay chains. For example, 222Rn may decay to a charged polonium ion (218Po+) and an alpha particle. The polonium ion will be attracted to the central cathode where it may plate-out. It then decays to 214Pb and another alpha particle. It is possible for this alpha particle to be hidden from detection inside the cathode surface, with the 214Pb causing a WIMP-like recoil in the gas.

Multiwire Proportional Chambers (MWPCs)● Each MWPC consists of three parallel planes of wires: A single anode plane of 512

20 μm stainless steel wires sandwiched between 2 grid planes consisting of 512 100μm diameter wires.

● The wire spacing is 2 mm within the planes and 10 mm between planes.

● The grid planes are held at a potential of -3 kV, with the anode plane connected to readout electronics.

MWPC Wires

Charge Avalanche in the MWPCs● A single ionisation track will be made up of many ions, but for illustration we will focus on a

single CS2

- ion.

● This ion drifts towards the anode plane of one of the MWPCs. Inside the MWPC the field is uniform apart from in the immediate vicinity of the wires. Once the ion arrives within a certain distance of an anode wire, the field becomes strong enough to strip an electron from it.

● This freed electron causes an avalanche of ionisation as it accelerates towards the anode wire, colliding with other CS

2

molecules (on a timescale of ~1 ns)

● The remaining positive ions drift towards the grid wires. These moving charges induce a current on both the grid and anode wires, which is then read by the electronics.

Detector Readout● The signals from every eighth wire are grouped together, before being amplified and shaped.

They are saved to disk if they exceed a certain threshold.

● Example events are shown below. An alpha particle (shown on the left) creates a characteristic pattern of signals, due to the long range of the alpha particle inside the detector.

● The waveform signals are later analysed in software, with the aim of removing events that may be due to background, for example electronics noise, or alpha particles.

DRIFT - IIe

● The next generation of the experiment, currently being tested in the US.

● Similar to DRIFT-IId but improved design. Modified wire plane readouts

DRIFT - III● A future proposed scaled-up version.

● Total proposed volume is between 8 and 24 m3

Summary

● There is expected to be an annual modulation in the mean WIMP flux and a daily modulation in the mean WIMP direction.

● Boulby Mine is located deep underground, which results in reduced levels of cosmic ray induced backgrounds.

● DRIFT detector Overview:

→ Sensitive to the direction of the interacting particle.

→ Negative ions are drifted towards the MWPCs, where an avalanche process occurs. Signals are processed and saved for later analysis.