Geant4 Simulation of HPGe Clover Array -

This post presents an educational Geant4 simulation of a hypothetical HPGe clover detector array. The geometry is not based on any real running experiment; it is designed purely for learning detector modeling, addback, and anti-Compton concepts.

1) What is being simulated?

A hypothetical array of 9 HPGe clover modules arranged on a sphere (radius ~25 cm)
Each clover consists of 4 HPGe crystals (clover segmentation)
Surrounded by BGO anti-Compton shields and an active collimator ring (for veto/background suppression)
A disk-like gamma source placed at the origin (for testing response and spectra)

2) Total Energy Spectrum (baseline)

This is the basic gamma energy spectrum produced by the simulation (without zoom). It shows the overall response up to ~1800 keV and the presence of photopeaks over a Compton background.

Total energy spectrum up to 1800 keV — **Figure 1.** Total deposited energy spectrum (0–1800 keV). Photopeaks sit on top of Compton continuum and background.

3) Compton Continuum (zoom view)

This plot zooms into the Compton region to show how partial-energy events form a continuum. These are events where gamma rays scatter (Compton) and escape after depositing only part of their energy.

4) Photopeak Region (zoom view)

This zoom highlights photopeaks (full-energy peaks). These occur when the gamma deposits its full energy inside the detector (via photoelectric absorption or multiple interactions summing to full energy).

Photopeak zoom — **Figure 3.** Zoom of photopeak region. Peaks correspond to full-energy events (good spectroscopy signature).

5) Angular Response / Coverage

This plot represents the response as a function of angle (theta). It helps check whether the detectors are distributed across angles as intended and whether the acceptance is reasonable.

Counts vs theta — **Figure 4.** Counts versus polar angle (θ). Useful to validate detector placement and angular coverage.

6) Single-Crystal Spectra (segmentation check)

Each clover has 4 crystals and we can record spectra crystal-by-crystal. These plots verify segmentation and help study how energy sharing happens across crystals.

Examples from the set:

Singles crystal 00 — **Figure 5.** Single-crystal energy spectrum (crystal channel 00). Confirms crystal-wise response and segmentation.

Singles crystal 01 — **Figure 6.** Single-crystal spectrum (crystal channel 01). Differences reflect geometry/angle/statistics effects.

7) Addback Spectra (the main clover advantage)

Addback means: within one event, sum energies deposited in multiple crystals of a clover. This recovers full-energy events when a gamma scatters between crystals, improving photopeak efficiency.

7.1 Addback per Clover

These plots show addback reconstructed spectra for individual clovers (index 00, 01, …). This is useful to verify that each clover behaves similarly.

Addback clover 00 — **Figure 7.** Addback spectrum for Clover 00 (sum of its 4 crystals per event). Photopeaks should strengthen vs singles.

Addback clover 01 — **Figure 8.** Addback spectrum for Clover 01. Similar structure expected if array is symmetric.

7.2 Total Addback over the full array

This is the most “summary” plot: addback (event-wise) across all clovers to show the overall performance of the array.

Addback sum over all clovers — **Figure 9.** Addback spectrum summed over all 9 clovers (event-wise). This demonstrates improved full-energy peak recovery.

8) Key Learning Outcomes

Geometry validation: overlap checks confirm the model is physically consistent.
Segmentation: crystal-wise spectra verify proper hit assignment.
Addback physics: addback strengthens photopeaks by recovering multi-crystal energy sharing.
Background understanding: Compton continuum and peak regions become very clear.

Disclaimer

This detector array is hypothetical and built only for educational demonstration of Geant4 detector simulation and gamma spectroscopy concepts.

References:

S. Agostinelli et. al., Nucl. Instrum. Meth. A 506, 250 (2003).