Dark Matter Detection

PHY 210 project

Spring 2007

Bogdan, Doug, Jay and Ragnhild

The idea

WIMPs (Weakly Interacting Massive Particles) are hypothetical particles that offer one solution to the “dark matter problem”: that the measured mass of galaxies is much lower than the mass of matter we can seeWIMPs, by hypothesis, do not interact with electromagnetism, so cannot be seen directly. The idea is to detect them from nuclear recoils with some target nuclei in a detector.

In theory…

Build a detector to measure the recoil of the nuclei of some atoms from the dark matter. Calculate from this the relation between the event rate and the recoil energy, to infer various things about the structure of dark matter.

Possible things to measure/infer:Mass of dark matter particleIncident kinetic energy of DM particleEvent rate/flux of DM through the earth (lab).

In practice…

Build a detector filled with Argon. Scintillation from nuclear recoil is picked up by photomultiplier tubes. After processing these signals through electronics/LabView, we can get the energy spectrum of the particles.

From those results…

After we have the spectrum (calibrated with known sources) we can find the energy of nuclear recoil and infer the event rate. From here, the equationdR/dER=(R0/E0r)e-E(R)/E(0)r

will allow us to find DM mass, flux, etc., where R is the event rate per unit mass, ER is the recoil energy, E0 is the incident kinetic energy, R0 is the total event rate, and r is a kinematic factor involving the mass of the nucleus of the target particle and the mass of a DM particle.

Why Argon?

Really, any old noble gas should do. The key is that we want something which is extremely stable, and scintillates under a fairly wide range of incident kinetic energies. Argon seems to fit the bill.

In more serious experiments, Argon is most commonly used (in its liquid form) for exactly these properties. So, at least we have something to model our experiment after.

SetupHigh-voltage supply for phototubes

Oscilloscope, so we cansee what’s going on

Fancy electronics

Photomultiplier tubes

Container with Ar

The container itself is a cylindrical steel tube

Plenty of electrical tape to ensure light doesn’t leak in

Photomultiplier tubes were glued into a G10 holder, which was

screwed into the steel endpieces

One of the endpieces had an outlet used for evacuating, and

pumping in Ar

The detector itself

The electronics

The signal from the phototubes goes to the VME Crate…

From the VME crate, through a discriminator,

and then to a coincidence counter

Signal from phototube sent to computer, triggered on

coincidence.

(Thanks to prof. Cristiano Galbiati!)

High-voltage supply: The voltage across the phototubes was 2300 V

We viewed output from both tubes and

from coincidence counter on

oscilloscope

Scintillation

When we calibrated this apparatus, we used an organic scintillator with gamma rays from a few sources (137Cs, 133Ba, and 60Co). Scintillators emit photons with energies proportional to the incoming radiation, up to some scaling factors.

Some Challenges

Container needs to be absolutely tight, and able to take vacuum, pressure (Solution? O-rings and a LOT of glue.)

Must be absolutely dark - photons leaking in will both disturb signals and potentially burn out phototubes (Solution? A LOT of black tape.)

Technical problems

This experiment turned out to be very challenging. At first we had trouble finding two phototubes that worked. Of our original two phototubes, we found out that one didn’t work only after gluing it in. This caused a delay of about 5 days, as we had to remove it from the G10 holder, and then glue a new one in. Unfortunately, we then found this second phototube sparked, burning out part of the VME crate’s electronics in the process. So we had to settle for a smaller second phototube, as we didn’t have another tube the size of our first one. Overall, we ended up wasting about 3 weeks before having two working phototubes. Then, we had a bit of trouble running the Argon tests. When we tried putting the gas in at 12 psi one of the phototubes was pushed through its casing, which resulted in light coming in and the terminating of the run. We then tried a second run, with the gas at 7 psi, but about 3 hours into the experiment one of the phototubes registered a big weird-shaped pulse followed by no more activity. When we stopped the run, the other phototube was very hot and possibly burnt out. We believe it might have caused a spark which was the signal we picked up in Labview. We decided to try no more runs afterwards.

Normal pulse

Shape of the last pulse

Histogram of all the pulses

Total number of pulses

The histogram in semi-logarithmic

scale

Number in linear correspondence to the energy of

the last pulse

Kaboom pulse

Weird-shaped, very

disturbing pulse

This number is one order of magnitude higher than

what we used to be getting

Close-up

All Sources

Data (Pretty graphs)

CesiumBarium

Main Graph

Cobalt

Cobalt was the best graph that we got. The related peaks and decays should tell us the energy spectrum we get from Cobalt’s gamma rays (as well as a possible spectrum for muons).

Red: Original Data

Black: Original data minus no-source data

Blue: Muon data (no sources)

Deep conclusions?

Unfortunately, we never got a chance to actually take data with the argon. But we’re sure that if we had gotten data, it would turn the scientific world on its ear.

One conclusion we do have is that experiments never go right the first time, but making mistakes isn’t the end of the world. Also, that strange pulse shapes are very, very, very scary. Very.

Further development

In terms of accuracy, this experiment has a lot of room to grow. A more shielded chamber, well-calibrated phototubes (preferably of the same size and type), and a better understanding of the underlying physics are all things which could be improved.In terms of the experimental method, we’re fairly spot-on. The experiments which are being performed now use the same basic setup – PM tubes and scintillators, argon-filled chambers, and computerized data analysis. Such experiments also have a longer time scale, with many repeated measurements and runs being performed for hours at a time.

Credits

Lyman Page

Cristiano Galbiati

Wei Chen

Joe Horvath