1. Experimental Search for Dark Matter
Overview
Our current understanding of dark matter is lacking, despite it
making up roughly 23% of the universe. While many theories describe
what we see in astrophysical data, none have been widely accepted as
of yet, despite much experimental effort. Progress seemed to have been
made when DAMA published their data, which implied a modulating dark
matter signal coming from their NaI detectors. Despite this observation,
the results obtained by DAMA are widely contested, with no other
experimental efforts fully agreeing with their modulating data. As such,
the DM-Ice group at the University of Illinois at Urbana – Champaign
seeks to better understand NaI detectors, in an effort to construct better
detectors to be located at the South Pole in order to try and reproduce
the modulating data observed by DAMA under different conditions.
We report here on research and development being done to
accurately assess the trace contaminants of NaI scintillating crystals in
an effort to develop purer crystals to test DAMA’s claim.
The Detector
Since our detector consists of a NaI scintillating crystal coupled to a
photomultiplier tube, we had to calibrate the area readings we were
getting in order to get energies. To calibrate our detector, we used
standard radioactive sources of Cesium, Sodium, Cobalt, and
Potassium, which have known peaks at known energies. We can then
plot their energies versus their areas in order to convert our area
readings into energy values.
Willie Zuniga1, Kyle Coda2, Raanan Gluck2, Liang Yang2
University of Illinois at Urbana – Champaign2 , Urbana, Illinois 61801
Harvey Mudd College1, Claremont, CA 91711
Acknowledgements
We gratefully acknowledge the Summer Research Opportunities
Program at the University of Illinois at Urbana – Champaign, as well as
the Research Board Award received by professor Liang Yang from the
university to allow this research to continue.
Initial Analysis
In order to analyse pulses, we used the C++ library ROOT,
developed by CERN. To identify and characterize events seen by our
detector, we initially set out to discriminate between alpha and gamma
events. Since alpha particles in general have a shorter lifetime, we
expect alpha pulses to decay faster than gamma pulses, which allows us
to separate them. We used a parameter called mean time, developed by
the DM-Ice17 team, to categorize their decay times. Mean time is
defined as follows:
Where ADCn is the charge collected in the nth bin of the waveform,
and n naught is the time when the waveform fire reached 50% of it’s
maximum value. By using this metric, we were able to separate alpha
and gamma events above 2,200 KeV.
Figure 2. Example calibration curve of data from Na22 and Co60. This plot
was made by histogramming the area of pulses detected, then matching
those area peaks to known energy peaks to arrive at this conversion between
area and energy.
Figure 1. A) Fermilab, 330 Ft. underground. B) Our detector is encased
inside this lead tomb.
Fermilab
Our detector is located 330 ft. underground at Fermilab. It is housed
in a lead tomb to shield it from excess radiation. This location allows us
to see mostly events occurring from the contaminants of the test crystal.
Figure 4. This figure shows a histogram of data taken above 2,200 KeV
where we sort the pulses by mean time. For these pulses, we see a
near 100% separation of alpha and gamma events.
Future Work
This summer, the team plans to perform more analysis on data
taken from our detector in order to identify concentrations of common
contaminants in our NaI crystal. We will be analyzing concentrations of
certain decay chain members to try and identify what decay chains are
taking place in our detector, and if they are broken or not. Doing so will
allow us to determine the concentrations of trace contaminants in our
crystal, which will give us a way of rigorously measuring the purity of
future NaI crystals.
A B
Figure 3. Schematic of a standard photomultiplier tube optically
coupled to a scintillator. This is essentially what our detector is. Taken
from http://web.stanford.edu/groups/scintillators/imgfund/pmt.jpg
Mean time!Calibration Curve
The red line is a fit of
our most recent source
data
The black line is a fit of
source data that was
taken one year ago
References
Cherwinka, J., D. Grant, F. Halzen, K. M. Heeger, L. Hsu, A. J. F.
Hubbard, A. Karle et al. "First data from DM-Ice17." Physical Review
D 90, no. 9 (2014): 092005.
Cherwinka, J., R. Co, D. F. Cowen, D. Grant, F. Halzen, K. M. Heeger, L.
Hsu et al. "A search for the dark matter annual modulation in South Pole
ice."Astroparticle Physics 35, no. 11 (2012): 749-754.
Ade, P. A. R., N. Aghanim, C. Armitage-Caplan, M. Arnaud, M.
Ashdown, F. Atrio-Barandela, J. Aumont et al. "Planck 2013 results. XVI.
Cosmological parameters." Astronomy & Astrophysics 571 (2014): A16.