A TES detector for ALPS II : characterising a cryogenic, low-background, low energy single photon detector
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Abstract
Axions are Weakly Interacting sub-eV Particles (WISPs) predicted to explain phenomena
like the lack of Charge-Parity (CP) violation in Quantum Chromodynamics (QCD).
With axion-like-particles, they can also contribute to dark matter and explain other
astrophysical phenomena. These particles can couple to photons, a process used in
most experiments attempting to search for them. One such experiment is ALPS II, a
light-shining-through-a-wall experiment. Photons stored in an optical cavity can convert
to axions or axion-like-particles, aided by the presence of a magnetic field. These can
traverse an opaque wall and, in another such optical cavity, reconvert to photons which
can be detected. The rate of these low energy (1.165 eV) 1064nm photons comprising
the signals for ALPS II is O(10^{-5}) cps. A Transition Edge Sensor (TES) can prove to be
a suitable candidate to detect them. This sensor exploits the drastic dependence of the
resistance of a superconducting microchip near its transition temperature, which can
detect the incidence of a photon (as energy) on it from a macroscopic change in its
resistance. The characterisation of a such a TES system to serve as a detection scheme for
ALPS II is covered by this work.
Firstly, the setup and design of this scheme is detailed with its defining features and
readout with SQUIDs (Superconducting Quantum Interference Devices). It is housed
in a dilution refrigerator capable of stably cooling down to temperatures <30 mK. The
working, response and operation of this TES system and SQUID readout is detailed,
readying it for the reception of signals and background alike with a data acquisition and
processing scheme.
Secondly, the analysis of these TES pulses is discussed. Here, the pulses can be fit with a
variety of fitting schemes (as best suited to the TES) and studied with a PCA (Principal
Component Analysis) as well. The application of these procedures yields an energy resolution 7-12% for 1064 nm photons. The fitting scheme which best assimilates
the data in a TES pulse is adopted as the baseline for pulse characterisation and forms
the basis for a pulse selection procedure.
Thirdly, the discussion turns to background events inveigling the detection scheme. For
use in ALPS II, a background rate of 8.1 x 10^{-6} cps over 20 days of data taking is the upper
limit for the detector, assuming a detection efficiency of 50%. The backgrounds are split
into extrinsic and intrinsic backgrounds i.e. those detected with an optical fiber coupled
to the TES and those detected without. It is essential to substantiate the viability of the
TES for use in the ALPS II experiment. In order to do this, a 20 day intrinsic background
dataset is subjected to the pulse selection procedure(s), resulting in a rate of 6.9 x 10^{-6} cps, with a 1064 nm-photon selection 90%. Considering the intrinsic backgrounds, the TES
is thus viable for use in the ALPS II experiment.
A setup to measure the detection efficiency of the system has also been developed and
is also described. This influences the extrinsic background events detected. The first
measurements with this setup have resulted in an efficiency of at least 5% and will be
optimised. The backgrounds can in general originate from a variety of sources including
cosmic rays and radioactivity, but are suspected to be dominated by photons from
blackbody radiation and their pile-ups. To suppress this, hardware options such as an
in-cold filter bench and fiber curling are being investigated for use in the main detection
line to the TES. Testing these suppression methods forms the next series of investigations
for the TES system along with background studies, simulations, and the use of machine
learning techniques for pulse analysis. These forthcoming steps form the efforts to fully
characterise the TES detection system in preparation for ALPS II.