As a few of you might have read in my posts, I am a professional physicist and I comment often on subjects of Dark Matter, Particle Physics and
occasionally astronomy. I did my undergraduate in the UK, an MPhys in Physics and Astronomy (Masters). I followed this with a PhD in a long baseline
Neutrino oscillation experiment, T2K. The subject of my theses was technical, covering aspects of photon detectors used in the T2K near detector.
I moved on after my PhD and am currently a Postdoc working for a direct dark matter search experiment.
Since I talk about things I have been involved in during my posts here I thought some of you might like to read a little bit about the types of things
we do in the mysterious world of deep underground physics.
Direct searches for dark matter involve in all cases attempting to observe dark matter interacting with normal baryonic matter. The current
theoretical best candidate for dark matter is the WIMP, the weakly interacting massive particle. It has mass, is electrically neutral and interacts
only via the weak force, similar to that of neutrinos, only the coupling is even weaker. The reasons behind this i will not talk about in this post,
but might later if there are any follow up questions.
So we are searching for the interaction of WIMPs with normal matter, typically in the form of a billiard ball type scattering event. In this event,
the WIMP smacks into an atom and gives it some energy, causing it to 'recoil' this energy can be detected with current technology.
So how do we do this? and what are the challenges?
The most significant challenge are that of target mass and purity.
- The predicted interaction cross section is extremely low, it is expressed in units of barns, it is a likelihood of an interaction
occurring for any closely passing particles. Current limits on dark matter interaction cross sections are of the order 10^-44 cm^2 in affect it is
like having an object that small and firing it at the earth, it is theoretically possible that with a cross section as low as this, that it will pass
right through the Earth and not touch a single thing. This is in comparison to a low energy neutrino which is of the order 10^-37... Most of those
will pass straight through the Earth no problem, but those we have built detectors that can observe them at a fairly low rate (one or two a day
So in order to give you a better chance of the event happening, you need a large amount of sensitive material! This is often both a technical and a
- The biggest challenge is radiation. Every material around you is radioactive, anyone who has ever seen a radiation
monitor or simple Geiger counter will tell you "Hey that thing clicks every few seconds!" Well what I am trying to tell you here is that, in order to
search for dark matter, you have to get the background radiation level down to 1 or 2 counts over the course of YEARS... there are a few tricks to
that, but that is what we are gunning for.
That means that we have to build detectors using only the cleanest possible materials, or materials that can be cleaned to purity levels that high. We
have to control every aspect of exposure and natural content of elements such as uranium and thorium. The reasons why these elements are painful for
dark matter experiments is that they produce a chain of alpha decays that result in long lived elements. These elements 'grow' in and they represent a
background that slowly and always increases over time. Much of these elements also love to stick to surfaces. An example being Polonium and Lead 210.
Polonium is not as major an issue, though lead is terrible. It has a 22 year half life and plates out on every surface.
OK so those are the two main challenges, so what do we do? how do we reduce our backgrounds? Firstly these experiments are performed deep underground.
This is both a blessing and a curse. It is a blessing because the deeper you go, the more and more cosmic rays are shielded, but it is a curse because
deep underground labs tend to be in hard rock mines or tunnels. The rocks tend to be high in uranium and thorium and so give us a huge radon
concentration in comparison to the surface (Radon chain gives us lead 210... bad bad bad) So typically these labs operate clean facilities where there
are strict controls on moving items into the lab and in some cases moving people into the lab. Sometimes basically your day will start with a shower
underground depending on the facility.
So what about the detectors themselves?
We have to screen materials that go into critical areas, or that are part of the actual active region. Active targets have to be highly purified, this
is a given. I will talk about the types of detectors later, but typically if it is a Gas detector, you have to pass the gas through a cold trap,
usually a charcoal trap to filter out radon. A semiconductor detector has to be grown using highly purified material. Crystal detectors have to be
grown under extremely controlled conditions.
Detector materials have to be selected for purity, and any mechanical structure built wit this in mind. That means that any welding must be done as
fusion welds, either pure tungsten or ceriated welds... any thorium is a big no no. That is not hard to control, but even welding tips that have been
ground down on a wheel used to grind 1-2% thorium rods gives a weld that is far far too hot.
Typically good materials are stainless steel, copper, acrylic (depending on handling and type). There are a few others but those are the main ones.
Stainless steel forms a passive chromium oxide layer on its surface, this layer tends to be pure and traps radio purities inside the bulk material.
Copper can be sourced with intrinsic high purity, this is because elements that are typically found with copper ores are things like silver and
gold... sooooo these ores get highly purified to recover those goodies. It can also be electro-formed, this is where you basically grow pure copper
from electrolysis... amazingly pure.
Acrylic - If made with pure monomer, not slush caste using recycled material, the bulk acrylic contains what ever radioactive contaminants were in the
process systems of the plant forming it. Acrylic likes to absorb things despite it being glass plastic and you don't think of it absorbing anything at
all. So Radon will diffuse into the surface. Not all bad though because if you remove that surface, you end up with a nice clean and pure material to
make your detector from.
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edit on 27-2-2014 by ErosA433 because: (no reason given)