Sources and Mitigation of Radon Progenies in the DRIFT Gas Detector Relevant to
WIMP Dark Matter Searches
abstract
Low energy O(100 keV) events from progeny nuclear recoils and energy-degraded alpha
particles that are a bi-product of the radon decay chain are a major source of background
for direct search dark matter experiments, difficult to distinguish from the nuclear recoils
expected from Weakly Interacting Massive Particles (WIMPs). Presented here are
studies of the sources of such events in the DRIFT II directional dark matter experiment
and of new strategies developed to mitigate their effects. DRIFT, by making use of gas
Time Projection Chamber technology with excellent low energy particle identification
and tracking capability, is ideal for such radon studies. Results are presented on
techniques to remove short-lived RPRs and on the use of nitric acid cleaning to remove
long-lived RPRs due to 210Pb plate-out. Results from a new radon emanation test facility
are also described. These reveal that high rates can be expected from many commonly
used detector materials, most notably RG58 cable, found to emanate 5.0 +/- 0.4 x 10-3 Rn
atoms s-1 m-1. The efforts presented here successfully resulted in a reduction of the
observed RPR rate in DRIFTII of ~ x150.
paper outline and context
In paper 1 (Astroparticle Physics 28 (2007) 409–421) we showed that we achieved all
normal background rejections in DRIFT II (gammas, sparks etc) but were left with an
anomalous population of nuclear recoil like events. These were identified as radon
progeny recoils (RPRs) associated with radon contamination in the detector. The
mechanism for RPRs in DRIFT was described and the likely source as 218Po suggested.
The suggestion was made that other progenies might also be present. Measurements
were presented of the rate of contained alphas in the detector, so-called Gold Plated
Cathode Crossers (GPCCs), consistent 222Rn. The rate of these alphas and the RPRs were
examined as a function of time after gas filling and found to rise together toward an
equilibrium level entirely consistent with the hypothesis that RPRs are the current
limiting background in DRIFT.
In paper 2 (Nuclear Instruments and Methods in Physics Research A 584 (2008) 114–
128) a more detailed study of the radon, radon daughters and decays in DRIFTII was
undertaken. By using 3D track reconstruction to produce range spectra it was possible to
reveal clearly separate populations of alphas from the decay of 218Po, 214Po, 216Po and
212
Po. This indicated that there are likely several possible populations of RPR events.
The short-lived component alpha decays from 222Rn, 218Po, 220Rn and 216Po were all
identified. The total GPCC rate was also measured and results were deduced for the
proportion of uncharged progenies in the gas. Calculations from these results allowed an
estimated to be made of the total radon level in DRIFT II, found to be ~1 Hz.
In this paper (paper 3) we further extend this study: firstly to identify the specific sources
of this ~1 Hz of radon in DRIFT II; secondly to determine the relationship between these
sources and the RPR events, separating in the process the populations of short-lived
RPRs and those long-lived RPRs resulting from 210Pb radon plate-out; and thirdly,
through this process, to develop first techniques for the mitigation of this recoil-like
background. The study focussed on examining and comparing the effect on the rate of
GPCC events (contained alphas in the gas) and on the RPR rate of various changes made
to the detector configuration and operation as follows:
(1) the effect of removal of internal detector items determined by emanation tests to be
sources of radon - in the process producing new results on the emanation of radon from
various common detector materials
(2) the effect of sealing the inner detector and on increasing the gas flow
(3) the effect of cleaning the central cathode using a nitric acid wash
The main results are summarised in Table 1.
Run
(1) DIIa June 2005
(2) DIIb Feb 2007
(3) DIIb July 2007
(4) DIIb Feb 2008
(5) DIIb Mar 2008
(6) DIIb Aug 2008
Detector
configuration
Original state
RG58, teflon cables
removed and inner
detector sealed
As above
As (2) (with slight
cuts change)
As (4) but cathode
nitric cleaned
After MWPC nitric
clean
Gas flow rate
(chg/day)
1
1
GPCC rate
(m-3day-1)
8000+/-1000
820+/-40
RPR rate
(day-1)
500+/-20
40+/-2
10
1
110+/-30
-
51+/-4
55+/-8
1
-
3.4+/-2
awaited
(do we have data pre/post cathode clean showing that the GPCC rate stayed the same?)
The main results from this work are then:
(1) Emanation studies: Internal detector components were removed and tested for radon
emanation using a sensitive purpose-build facility. The sum of the radon emanation
rates measured from all these internal detector materials was found to be consistent with
the result of paper 2 in which the total observed radon rate of ~1Hz was obtained through
measurements of the GPCC rate in the detector itself. By this means we can account for
the sources of >95% of the radon observed in the detector.
(2) Material replacement: Following replacement of the radon producing materials both
the GPCC rate and RPR rate reduced by similar proportions of ~x10 (compare run 1 and
2). This confirms again the relationship between the radon alphas in the gas and the
RPRs as deduced in paper 1 where the rise in the rates of both following the start of a
new run with fresh gas was observed to be of the expected form. Further evidence is
illustrated by a plot showing simultaneously an alpha on one side of the detector and a
recoil on the other (Johanna). It suggests that the material replacement procedure
successfully reduced the short-lived daughter products of the decay chain, 218Po and 214Po
(see paper 2).
(3) Gas flushing: The high flush run (see run 3) resulted in a further reduction in
contained alpha GPCC rate of x8 but no further reduction in the RPR rate leaving the
RPR rate at ~50 cts/day. This is consistent with the hypothesis that there is a second
population of long-lived radon progenies, notably 210Po, that results from the decay of
210
Pb (22 year half-life) accumulated on the surfaces of the wires from plate-out that has
naturally occurred over the years since manufacture of the wires. The rate of these RPRs
is not expected to be influenced by any changes in the current radon environment, as
observed. (note: flushing will remove uncharged progeny components of the decay chain,
typically 20% are produced uncharged (see paper 2), and any charged components that do
not reach the cathode).
(4) Plate-out of 210Pb: It is difficult to estimate the expected rate of 210Po RPRs as this
depends on the integrated radon-exposure history that results in 210Pb plate-out.
However, using data from Samuelsson et al. that gives typical implanted activity of 210Pb
on materials vs. radon exposure, it is possible to deduce an expected RPR rate from the
central cathode wires of 10-100 cts/day. This is consistent with the observed rate.
(5) Nitric acid cleaning: To investigate this further the central cathode was removed,
carefully washed using a nitric acid process known to be effective at removing plate-out
(see EXO) and replaced. Following this process there was observed no effect on the
GPCC rate, as expected. However, the RPR rate reduced by a further factor x16 (see
results for runs 4 and 5). This is entirely consistent with removal of 210Pb plate-out and
hence of 210Po RPRs.
(5) The final RPR rate: A low rate RPR nevertheless remains present. This is most likely
due to a residual population of short-lived RPRs and possibly also a small population
RPRs generated in the WMPC that escapes the high field region there. This hypothesis is
the subject of further investigation.
Conclusion: The sources of both short and long-lived radon progeny recoil events have
be separately identified in the DRIFT II detector by a combination of radon emanation
studies, gas flushing and nitric acid cleaning. By this means techniques have also been
established to mitigate the RPR rate that have resulted so far in a factor x160 reduction in
the remaining background in DRIFT. The prospect of further application of these
techniques and development of additional data cuts, not implemented here, leaves DRIFT
well on track to achieving the goal of zero background in 1 year of operation.
Paper Sections
1. Introduction
As above, from paper 1 and 2, review of identification of radon and RPR events from the
radon decay chain; contained alphas (Tim's plot); short lived RPR components; long
lived RPRs (210Pb)
2. Radon emanation tests
Description of facility and results
3. Radon and RPR reduction tests
Gas flow; nitric acid cleaning of central cathode
4. Discussion
Results, achieved background and implications