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High-Granularity γ-Ray Tracking Cluster for Background Suppression in Decay Spectroscopy

The .DESPEC. (DEcay.SPECtroscopy) γ-ray tracking cluster utilizes high granularity and pulse shape analysis to improve position resolution and suppress background radiation. The detector module consists of a stack of 3 planar 2D stripe Ge detectors with 8x8 segmentation, providing 1-3 mm 3D position resolution. The algorithm features track the γ-rays back to their origin, improve correlation between implantation and decay events, identify and reject background events, and reconstruct the initial energy. The cluster achieves significant background suppression without the need for additional shielding.

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High-Granularity γ-Ray Tracking Cluster for Background Suppression in Decay Spectroscopy

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  1. DESPEC (DEcay SPECtrocsopy) γ-ray tracking cluster • High granularityto reduce the effect of the “prompt flash” radiation • Pulse Shape Analysisto improve the position resolution • Trackingof the γ-rays back to the origin • Imagingcapabilities for background suppression Stanislav Tashenov GSI, Darmstadt • Polarizationsensitivity

  2. Detector Module • Stack of 3 planar 2D stripe Ge detectors • 68mm2 x 68mm2 x 20mm2 + 2mm guard ring • 6mm gap between crystals • 8x8 segmentation • 1 – 3 mm 3D position resolution with PSA • Energy resolution: 0.2%

  3. Current RISING cluster :128Cd energy time Motivation:background suppression Tasks: • Improve the correlation between the implantation and the decay for long-lived isomers • Identify gamma events from the background sources • Suppress the Compton background • Increase the absolute efficiency by reconstructing the initial energy from the Compton escape events

  4. Tracking algorithm features • Identification of the photopeak events and the escape events • Reconstruction of the initial energy from the escape events • Rejection of the events from background sources

  5. Tracking principle Construction of the “Figure of Merit” • for all possible interactions sequences • for the case of the photopeak and the escape event Selecting the case witch maximizes the Figure of Merit

  6. Figure of Merit for the photopeak event

  7. Figure of Merit for the escape eventinitial energy estimation Energy estimation: Weighted average:

  8. Figure of Merit for the escape event

  9. Results: photopeak events Compton escape suppression without BGO shields. This effect is impossible to achieve without tracking!

  10. Tracking efficiency doesn’t include single hit interactions, pair production, bremsstrahlung etc. Increase of the Peak / Total Significant increase of the Peak/Total ratio (~90%) with respect to the non-segmented detectors without use of BGO shields MGT code: Energy Effic. P/T [MeV] ------------------------------ 2 0.96 0.19 1.2 0.97 0.27 0.6 0.98 0.39 0.2 0.98 0.81 ------------------------------ no increase in P/T

  11. Efficiency gain with respect to non-segmented detectors Results: escape events Tracking Efficiency (escape) 0.7 0.65 0.35 0.02 Tracking Efficiency (total) 1.5 1.5 1.2 0.5 Energy MeV 2 1.2 0.6 0.2 Energy Resolution: FWHM: ~1.5% Lorencian Profile

  12. Optimization - resolution/efficiency

  13. Background suppression via Imaging

  14. Background suppressed spectrum “Ideal” (100% efficient) tracking assumed

  15. Trade-off between background suppression factor and statistics left in spectrum Background suppression by a factor of 10 possible while keeping ≈ 80-90 % of events! (1 mm position resolution)

  16. Outlook • Tracking after PSA (together with A. Kaplanov, KTH Stockholm) • Tracking in realistic experimental situation in presence of Bremsstrahlung photons (together with P. Detistov, Sofia) • Tracking for Multiplicity > 1

  17. End

  18. Germanium shell (AGATA) Rinn = 15 cm Resolution = 3 mm Closest distance = 5 mm Rout = 24 cm E Tracking P/T (keV) efficincy 0.6 1.2 2 0.76 0.79 0.78 0.96 0.96 0.93

  19. Dependance of the background suppression factor on the position resolution

  20. “triangle” Optimization with respect to the Figure of Merit

  21. “Software” anti-Compton shield → no BGO required Simulation with the background radiation: Cleaning of the Compton component of the background

  22. Background suppression via Imaging E ħω

  23. DESPEC γ-tracking array Significant increase of the Peak/Total ratio (~90%) with respect to the non-segmented detectors without use of BGO shields Open symbols: results of MGT tracking code Filled symbols: results of the new algorithm S. Tashenov GSI

  24. DESPEC γ-tracking array Tracking Efficiency (escape) 0.7 0.65 0.35 0.02 Tracking Efficiency (total) 1.5 1.5 1.2 0.5 Energy MeV 2 1.2 0.6 0.2 Energy Resolution: FWHM: ~1.5% Lorencian Profile Significant increase of the photopeak efficiency (by 50%) with respect to the non-segmented detectors due to the reconstruction of the energy from the Compton escape events S. Tashenov GSI

  25. DESPEC γ-tracking array Suppression of the background by factor of 10 while keeping 90% of statistics Compromising statistics it is possible to achieve the factor of 100 S. Tashenov GSI

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