The IPHC scanning table
Pulse Shape Comparison Scanning (PSCS)#
The IPHC scanning table is using the Pulse Shape Comparison Scanning (PSCS) method. In this technique, two singles scans are performed: one with the detector positioned horizontally relative to the collimator and one positioned vertically. In singles scanning, the position of the collimator constrains the position of the \(\gamma\)-ray interaction in the detector. Horizontal and vertical scans are therefore used to determine the (X, Z) and (X,Y) positions of the interaction, respectively. The signals from both scans are then compared using the PSCS method.
The scanning table setup is shown below. The 170 kg heavy collimator sits on two motorized perpendicular axes \(X_T\) and \(Y_T\) that allow it to move within a range of 300 mm in both directions with a precision of about 10 \(\mu m\). Above the collimator, two fixed plates allow the placement of the crystal in the vertical and horizontal positions. Most importantly, a laser alignment system is mounted on the collimator’s mechanical support to maintain the detector’s relative alignment when switching between vertical and horizontal positions.
Collimator#
The collimator consists of a metallic cylindrical block, 189 mm high and 220 mm in diameter, made from iron, lead, and tungsten to absorb γ rays not passing through the central hole. The base of the collimator, which holds a cylindrical capsule containing the γ-ray source, can slide down to allow for source replacement. A channel with a 1.6 mm diameter runs along the central axis of the capsule to the source, and a 165 mm long channel pierces the collimator’s central axis, aligning with the capsule’s channel. A rendered section of the collimator is shown in below. The central part of the collimator can be mechanically swapped, allowing the use of three different central parts with diameters of 1.0 mm, 0.5 mm, and 0.2 mm. GEANT4 simulations of the collimator beam profile at different heights of interest above the collimator were done. It was shown that most of the γ rays are still within a lateral spread of 2 mm at 95 mm above the collimator.
Data acquisition system (DAQ)#
The data acquisition (DAQ) for this work uses TNT2 cards developed at IPHC, which handle both digitizing and pre-processing of signals. Each TNT2 card has 4 channels, requiring 10 cards for scanning an AGATA detector unit: 9 for the 36 segments and 1 for the core signal used as a trigger. These cards sample the incoming signals at 100 MHz with 14-bit resolution. Energy readings are obtained using the moving window deconvolution (MWD) algorithm. For this work, signals were sampled over a 1.2 μs window (120 samples) with a 5.95 μs shaping time for the MWD process.
References#
The developments that were made on the IPHC scanning table lead to various publications and theses. More details on the scanning table can be found in the following references:
-
PhD theses:
-
Thesis work of Michaël Ginsz (2015): Characterization of high-purity, multi-segmented germanium detectors
-
Thesis work of Bartolomeo De Canditiis (2020): 3D characterization of multi-segmented HPGe detectors : simulation and validation of the PSCS technique and its application for different energies with a 152 Eu source
-
Thesis work of Mojahed Abushawish (2020): New Machine Learning-Based Approaches for AGATA Detectors Characterization and Nuclear Structure Studies of Neutron-Rich Nb Isotopes
-
-
Articles:
- De Canditiis B. and Duchêne, G., Simulations using the pulse shape comparison scanning technique on an AGATA segmented HPGe gamma-ray detector, Eur. Phys. J. A 56, 276 (2020)
- De Canditiis, B., Duchêne, G., Sigward, M.H. et al., Full-volume characterization of an AGATA segmented HPGe gamma-ray detector using a \(^{152}\)Eu source, Eur. Phys. J. A 57, 223 (2021)