Speaker
Dr
Krzysztof Pelczar
(Jagiellonian University/Institute of Physics)
Description
Noble (argon, xenon) or inert (nitrogen) gases are extensively
utilized in experiments looking for rare nuclear events at low
energies, for which reduction of backgrounds is crucial. Highly
radio-pure detectors (e.g. HPGe diodes in LAr, LAr/LXe TPCs) are in
direct contact with the gases, being exposed to the intrinsic
impurities during assembly, handling and operation. Therefore $^{222}$Rn
daughters plating out on the detectors' surfaces (especially long-lived
$^{210}$Pb) may significantly contribute to the overall background index.
A measurement method of mobility and ionic lifetime of alpha emitters
from the $^{220}$Rn decay chain is presented, based on a teflon-made, 20 cm tube,
instrumented on one end with a large area (1 cm$^2$) Si-PIN
diode as alpha-particle detector. Opposite to the detector a $^{228}$Th surface
alpha-source is placed on a movable holder. The high voltage divider
resistors, located in the groove outside the tube, form the
electric field. The alpha-activity registered by the diode
varies with the field strength and the source distance for each
gas tested. Opposite polarities of ions produced in energetic
alpha and beta decays were tested.
The choice of $^{220}$Rn chain over $^{222}$Rn is based on the fact that
the shorter nuclear life-time of $^{220}$Rn and $^{216}$Po allows for easy
determination of their mobility and ionic lifetime in the setup described.
A typical mobility of positively charged Po-216 measured is on the
order of 1.3 cm$^2$s$^{-1}$V$^{-1}$, while the ionic lifetime
is approximately 10 s (and much longer for ultra-pure gases).
Foreseen are measurements of the ionic properties in cryogenic liquids,
being highly relevant for such setups as DarkSide (LAr TPC) or
Gerda (HPGe in LAr), where energetic alpha or beta decays present
in the $^{222}$Rn products entering the experiments' active volumes may mimic
the signal of interest for these experiments. Properties of the ionized
$^{222}$Rn daughters deduced from the conducted measurements
are therefore outlined.
Primary author
Dr
Krzysztof Pelczar
(Jagiellonian University/Institute of Physics)
