Eur. Phys. J. C (2018) 78:392
https://doi.org/10.1140/epjc/s10052-018-5852-7
Regular Article - Experimental Physics
Improving sensitivity of a BEGe-based high-purity germanium
spectrometer through pulse shape analysis
M. Misiaszek
1
, K. Panas
1,a
, M. Wojcik
1
, G. Zuzel
1
,M.Hult
2
1
M. Smoluchowski Institute of Physics, Jagiellonian University, Lojasiewicza 11, 30-348 Kraków, Poland
2
European Commission, DG JRC, JRC-Geel, Retieseweg 111, 2440 Geel, Belgium
Received: 23 December 2017 / Accepted: 26 April 2018
© The Author(s) 2018
Abstract We performed Pulse Shape Analysis to sepa-
rate single-scattered gamma energy deposition events from
multiple-scattered photons in a high-sensitivity γ -ray spec-
trometer. The spectrometer is based on a Broad Energy High
Purity Germanium detector and the developed technique uses
multivariate analysis by an application of the Multi-Layer
Perceptron Neural Network. A very good separation of the
single-site- and multi-site events was achieved leading to a
significant reduction of the background level of the investi-
gated spectrometer – the double escape peak, rich in single-
site events, was reduced by 95%, while the full energy peaks
lost at most 25% of their counts. The peak to Compton ratio,
calculated for the 2614.5 keV gamma line from
208
Tl, was
improved by 114.3%.
1 Introduction
For germanium semiconductor detectors a single energy
deposition by γ -ray is often described as a single-site event
(SSE). Multi-site events (MSEs), like e.g. multiple Comp-
ton scatterings, have several interaction sites separated by a
distance of about 1 cm. The differences and discrimination
between SSEs and MSEs is of primary interest for exper-
iments like Gerda [1,2] looking for the neutrinoless dou-
ble beta (0νββ) decay with application of the High Purity
Germanium (HPGe) detectors. Events from the hypothetical
0νββ decay would ionize the detector’s active volume by
means of two electrons (with a range of less than 1 mm in
germanium) and therefore belong to the first category. The
background due to external γ -rays is typically of multi-site
type, because γ -rays with energies in the range of ∼ 1MeV
deposit their energy mainly via multiple Compton scattering
with a mean free path of a few centimeters.
a
e-mail: krzysztof.panas@doctoral.uj.edu.pl
For applications in γ -ray spectrometry the situation is
reversed: SSEs should be rejected and MSEs preserved. The
SSEs are of background type, because they are mostly single
Compton scattered events with the scattered photon escap-
ing the crystal, while the full energy peaks (FEPs) regis-
tered by the detector (and used to evaluate the activities of
radionuclides) contain mainly MSEs.
1
The developed proce-
dure shall therefore make peaks more distinguished by reduc-
ing the flat Compton continuum and make possible to eval-
uate peaks from radioisotopes, which would be otherwise
under spectrometer’s minimal detectable activity (sensitiv-
ity).
Discrimination between SSEs and MSEs for the Broad
Energy Germanium (BEGe) detectors was extensively stud-
iedintheframeofGerda experiment [3–7], also with respect
to applications in ultra-low background γ -ray spectrometry
[8]. The approach that was worked out was to look for a ratio
of the maximal current signal amplitude (A) to the corre-
sponding event energy (E) – the so-called A/E cut. This is
a one-parameter method, where the cut value is determined
according to the calibration data, obtained usually by irradi-
ating the spectrometer with a
228
Th source (due to the emitted
high energy γ -ray from the decay of
208
Tl – a more detailed
explanation can be found in Sect. 3.2). Measurements car-
ried out for various BEGe detectors demonstrated very good
performance of the A/E method, however, its applications to
the coaxial detectors did not provide satisfactory results.
The main goal of our work was to find an alternative, effi-
cient and stable Pulse Shape Analysis (PSA) procedure to
distinguish between MSEs and SSEs in BEGe type detectors.
As it will be shown, our method does not need any corrections
like e.g. the ones related to the dependence of the A/E classi-
1
For typical detectors used nowadays in γ -ray spectroscopy (crystal
volume of several hundreds of cm
3
), and for energies of some hundred
keV and higher, the full energy peaks contain mostly events, which were
caused by a few Compton scatterings followed by the photoelectric
effect.
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