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Finding an Alpha Rejection Factor in 40 torr
CS2
Nate Villaume
Faculty Advisor: D. Snowden-Ifft
For a
variety of reasons, it is presumed that Dark
Matter constitutes up to 90% of the mass in
the universe. We believe that Weakly
Interacting Massive Particles (WIMPs) are a
promising candidate for Dark Matter.
Positive identification of WIMPs has yet to
occur; the Dark Matter project at Occidental
is hoping to use a DRIFT (Directional Recoil
Identification From Tracks) chamber to
observe WIMPs.
Because the expected imbalance between
background and WIMP event rates in the DRIFT
chamber will be in favor of the background,
it is crucial to the success of this
detector for it to be able to discriminate
between the two types of events. The DRIFT
chamber has the capability to measure the
total amount of ionization per event, and
the x- and z-components of the ionization
track left by the ionizing particle. The x-
and z-components are combined into a
variable, R2, which represents the length of
the projection of the ionization track onto
the x-z plane. Mathematically, R2 is :
In this
experiment, we sought to use R2 and total
ionization to distinguish alpha particles,
(one type of unavoidable background) from
neutrons (a WIMP mimic-particle). 210Po
and 252Cf were the respective
sources of these particles.
The
figure of merit in this experiment is the
rejection factor. It is defined to be the
percentage of alphas in an ionization window
which lie below a given range cut.
Typically, the range cut is determined by
the range for which ~90% of the neutrons in
the same ionization window have smaller
ranges. If alphas fall below this range cut,
then you can not tell them apart from the
neutrons. For high ionization windows
(2000-5000 Nips) we found the rejection
factor to be less than 0.6 -1.8%. However,
of particular interest is the 500 -1000 Nips
window; here the rejection factor is 4-8%,
depending on the background subtraction
augmentation factor.
Support provided by:
Ford/Anderson Grant, Occidental College SRP |
