Beta-spectroscopy of long lived nuclides with a PIPS detector-setup

Several applications in modern nuclear physics, research and engineering are limited by a lack of precise knowledge in spectral shape data for beta-decays. Specifically the interest aims to study spectral data for forbidden decays with respectively long half-lives, which is one of the central activities of our group. For the investigation of those rare beta-decays the group operates a setup of six PIPS detectors in a vacuum chamber built out of low-radioactivity materials. In the long term the setup will be used as low-backgrounddetector for the investigation of rare beta-decays. In order to reduce the measuring-background a muon veto was installed. The characterization of the setup in the energy-range from 20..1000 keV using conversionelectrons is described. A set of useful calibration-nuclides was established to determine energy calibration and efficiencies.


Introduction
The study of beta-spectral-shapes was of great interest in the 1950 to the 70-ties.The present work aims for new precision measurements, especially for forbidden decays.On the one hand the relevance of the exact spectral-shape rised up again due to huge improvements in experiments like Ultra-Low-Background (ULB) experiments -on the other side theoretical models came to a state where experimental data of spectral shapes is needed as input for new achievements [1,2].
At the 'Institute of Nuclear-and Particle physics' (IKTP) at TU-Dresden a Low-Background PIPS-detector (Passivated Implanted Planar Silicon) setup was installed for precise spectral shape measurements especially for investigation of forbidden decays with a maximum Beta-Endpoint-Energy of E β ≈ 1 MeV.Thus forbidden beta decays with the corresponding Q-values have a comparable high log-ft-value those are associated to long-lived nuclides [3].The condition for the necessary sufficient counting statistics is fulfilled with an activity above the detector-specific detection-limit and can be served by a reasonable number of source-nuclei.In order to get minimal statistical uncertainties due to the energy-loss in the sample itself the sample-layer also needs to be as thin as possible.The bigger the area of the sample-surface and the detector-efficiency, the better both requirements can be met.A high-resolution-detector with a sufficient efficiency is achieved with an array of PIPS-detectors.In comparison to magnetic spectrometers, which achieve a brilliant resolution in this field, an array of PIPS-detectors enables the capability to measure a necessarily extendedarea-sample with the coverage of the full energy range at once. a e-mail: alex do@gmx.net

Experimental setup
PIPS-detectors are flat, disc-shaped semiconductor-diodes, which are commercially available as detectors for direct ionising particles with an active area of up to 2 000 mm 2 .In the active-volume with thicknesses of up to d = 1 mm electrons with an energy up to E e− ≈ 860 keV deposit their full energy [4].Thus the full-energy-peak-efficiency (FEP-efficiency) decreases for higher electron energies, hence PIPS-detectors are limited to a maximum energy of ∼ 1500 keV for precise shape-measurements, as retrieved from estimations with Monte-Carlo Simulations.
To provide a good efficiency for a big sample-surface the discussed setup provides six CANBERRA (PD-1000-300) PIPS-detectors with an active surface of 300 mm 2 each.The active volume has a nominal thickness of 1000 µm (Fig. 1).For a proper spectroscopy of direct ionising particles one wishes an active detector-volume without entrance-window to prevent any energy-loss of the investigated particles.The ion-implanted front-contact with a maximum thickness of 50 nm is a special feature of the used PIPS detectors in order to provide a minimal entrance radiation-window.Thus the present detectors are not protected with a light-tight foil, the thickness of the entrance-window is given by the implantation contact only.
For a given detector-surface and detection-efficiency the detection-threshold is mainly influenced by the measuring-background.In order to achieve a minimal background-level the setup was shielded actively and passively for ionising radiation from natural radioactivity and cosmic rays.Particular attention was paid on a low intrinsic radioactivity of construction materials to reach a possibly low BG-impact.
Thus the PIPS-detectors were installed inside a counting chamber made of ultra-pure copper, which enables the opportunity to lower the air-pressure in the Figure 1.The PIPS-detectors are installed into an evacuated counting-chamber, made from ultrapure copper.To reduce ionising radiation from natural, environmental radioactivity the chamber is housed into 10 cm of lead.Underneath a plastic-scintillation-detector is installed in order to reduce the cosmic-ray-background, which is mainly given by atmospheric muons.
space between sample and detector down to ∼ 1 mBar.The chamber with the direct coupled pre-amplifiers was housed into 10 cm of lead.Underneath the setup a 10 mm plasticscintillator was installed in order to suppress BG which is caused due to atmospheric muons (Fig. 1/Fig.2).
All of the PIPS-detectors are coupled to a mesytec MSI-8 pre-& main-amplifier-system with 5 cm cables almost directly on the outside of the chamber.The modular system biases the connected detectors with the high-voltage (iseg 203M), amplifies and shapes the detector-signal where the output signals have an energy-proportional pulse-height.Those signals from the PIPSs and the veto-detector are acquired with a FAST-Comtec MPA-4 multiparameter-system, which allows a coincidence-/ anticoincidence analysis.

Detector characterisation
Precision measurements of spectral shapes need a very good knowledge of detector properties and characteristics, not least to get a reliable response matrix and to give estimations for uncertainties.
With the use of mono-energetic conversion-electrons the detectors energy-and resolution-calibration as well as the Full-Energy-Peak-efficiency were determined.Therefore a set of suitable calibration sources was evaluated.As PIPS-detectors are sensitive to electron and X-rays due to their thin entrance-window, a proper attention was paid on non-interfering X-ray and conversion-lines.In the end a set of 137 Cs, 207 Bi, 133 Ba and 242 Pu was assorted to calibrate the detectors with the corresponding K-conversion-lines in the energy-range from 23.158 keV to 1059.805 keV.
Moreover the detectors were characterised for the size of their active volume and the entrance-window.The latter was investigated with an X-ray-Fluorescence-Analysis (XFA) scan where the front-contact was analysed as implanted silver-ions.The thickness-profile of the active volume was scanned with a collimated 241 Amsource (Fig. 3).The results show that PIPS-detectors have a well defined active volume, which can be precisely manufactured by ion-implantation.
Based on the input from the manufacturers datasheets and the results from detector-characterisation a Monte-Carlo-model was created with the use of PENE-LOPE 2014 [5].The FEP-values from the experimental characterisation were reproduced by the simulation.

Summary and outlook
In order of precise beta-spectral-shape-measurements a low-background counting-chamber, equipped with 10.1051/epjconf/201714610014 ND2016 PIPS-detectors was set up at IKTP / TU-Dresden.With detectors of the maximum bulk-size, which is commercially available.A calibration technique with conversion-electrons was successfully evaluated and used for energy-, FWHM-and FEP-efficiency calibration.
Moreover the detectors were characterised for the shape of their active volume and their radiation entrancewindow.The results from these measurements in combination with the data from the manufacturers datasheets were used as input for a Monte-Carlo-model.The latter will be used for unfolding of beta spectra in future experiments.

Figure 2 .
Figure 2. Passive and active BG-reduction lead to a peak-sensitivity of 1.3 mBq (E ≈ 1200 keV) to 2.7 mBq (E ≈ 350 keV).Whereas the passive shielding mainly reduces the BG in the lower energy-region up to 300 keV, the muon interaction which shows up in the region around 350 keV with a Landau-shaped peak is efficiently reduced by anti-coincidencecuts.The conversion-electron-spectra were retrieved for energy-, resolution-, and FEP-efficiency-calibration.

Figure 3 .
Figure 3.To retrieve a relative thickness profile of the active volume the detector was scanned with a collimated 241 Amsource.A relative intensity-profile N /N 0 was retrieved for different scanning-points in the distance r to the detectorcenter.The intensity-profile which is shown is influenced by the clear cylindrical-shape of the active-volume and Gaussian beamprofile with an FWHM of approx.2.5 mm.