Depletion Region Dynamics of an AGATA Detector

Steven MoonUniversity of Liverpool

UNTF 2010, University of Salford14th-16nd April 2010

• Overview

Background What is AGATA?

• Gamma Ray Tracking• Pulse Shape Analysis• Case for AGATA

Role of the University of Liverpool AGATA Group Characterisation of AGATA detectors Scan modes Experimental setup

• Overview (cont.)

Some Scan Results Experimental

Simulated

Experimental vs Simulated

Next Steps....

AGATA – Advanced GAmma Tracking Array 180 Coaxial HPGe Detectors, tapered to asymmetric

hexagonal end → 36-fold Segmentation

3 types of AGATA detector (all asymmetric)• RED most asymmetric• GREEN• BLUE least asymmetric

Arranged into ‘ball’, i.e. 4π ‘Spherical Honeycomb’ structure, around beam-target interaction position

Final array will consist of 60 ‘Triple-clusters’

• Background - What is AGATA?

(Images adapted from M. R. Dimmock, PhD Thesis, 2008)

1

2

3

4

5

6

A

B

C

D

E

F

Established technology → Compton Suppression

Currently in use in facilities world wide→ e.g. GAMMASPHERE @ ANL, USA

Good, but not that good....→ discards many valuable events→ Only accept events occurring completely

within single HPGe detector volume....

• Background – Established technology

• Background – Gamma-Ray Tracking

Next-generation technology → Gamma-Ray Tracking

ɣ-ray Source

θ

• Background – Pulse Shape Analysis

1

2

To accurately obtain θ, need accurate interaction positions...→ Pulse Shape Analysis

→ Use core and segment charge pulses to determine (x, y, z) of interaction

→ Core and segment pulses give a unique ‘fingerprint’ for a given interaction position

2 ns samples(Images adapted from C. Unsworth, Private Comm., 2010)

• Background – Case for AGATA

Compared to current arrays, AGATA will: Dramatically increase access to weakest

signals from exotic nuclear events→ up to factor of ≈1000 improvement in sensitivity→ allow access to unseen channels in previously

studied reactions Complement new RIB facilities

→ Smaller reaction cross-sections→ Higher levels of background

• Role of the UoL AGATA Group

Different aspects of AGATA project handled by different institutions across Europe

University of Liverpool AGATA Group:→ Characterisation & Acceptance Testing

Scan AGATA detectors using various techniques 137Cs Coincidence Scans 137Cs Singles Scans

• Front-face → Front-face Bias• Side

Aim is to provide confidence in Electric Field Simulations of AGATA detectors

• Experimental Setup• Acquire in singles

mode using mono-energetic 137Cs source

• Scan detector on 2mm2 grid @ 30s per scan position

• Demand fold-1 (i.e. 1 hit seg.) events of full (662keV 137Cs) energy

(Image adapted from M. R. Dimmock, PhD Thesis, 2008)

• Compress 2mm scan data

• Examine detected gamma-ray intensity (for Rings 1-6) at each x-y scan position

• Repeat for various HV Bias Voltages(4500V, 4000V, 3000V, 2000V, 1500V, 1000V, 750V, 500V, 250V, 100V, 50V)

• Results

• Results

• Results (cont.)

• Compare intensities to those at full (4500V) bias...

• MGS Simulations

• Detector simulated for all experimental bias voltages using MGS (Multi-Geometry Simulation)

• Impurity concentrations (supplied by Canberra)

- Front: 0.65 x 10-10 cm-3

- Back: 1.4 x 10-10 cm-3

• Image results in similar fashion...

• MGS Simulations (cont.)

Detector at 4000V

100% depleted

• MGS Simulations (cont.)

Detector at 3000V

94.8% depleted

• MGS Simulations (cont.)

Detector at 2000V

76.6% depleted

• MGS Simulations (cont.)

Detector at 1500V

62.2% depleted

• MGS Simulations (cont.)

Detector at 1000V

44.0% depleted

• MGS Simulations (cont.)

Detector at 750V

33.8% depleted

• MGS Simulations (cont.)

Detector at 500V

22.6% depleted

• MGS Simulations (cont.)

Detector at 250V

10.4% depleted

• MGS Simulations (cont.)

Detector at 100V

1.7% depleted

• MGS Simulations (cont.)

Detector at 50V

0.3% depleted

• Experiment vs Simulation

• Experiment vs Simulation

• Next Steps

Continue comparison of experimentally derived depletion volumes and MGS simulation

Compare experimental & simulated pulse shapes Compare with other depletion simulations

(e.g. using JASS, Maxwell 3D) Compare with other measurements, e.g. C-V

measurements conducted at University of Cologne (B. Birkenbach & B. Bruyneel, to be published)

Does this method of scanning allow a practicable derivation of the crystal impurity concentration?

Questions & Comments...Steven Moon1, D. Barrientos2, A.J. Boston1,

H. Boston1, S.J. Colosimo1, J. Cresswell1, D.S. Judson1

P.J. Nolan1, C. Unsworth1

1Department of Physics, University of Liverpool, Liverpool, L69 7ZE, UK2Laboratorio de Radiaciones Ionizantes, Universidad de Salamanca, 37008 Salamanca, Spain

• Background – Compton Suppression Established technology

→ Compton Suppression Positive Core

HPGe Detector Volume

BGO Shielding

ɣ-ray Source

Established technology → Compton Suppression

• GAMMASPHERE @ Argonne N.L., USA

• JUROGAM @ Uni. of Jyvaskyla, Finland

→ Only accept events occurring completely within HPGe detector volume....

• Background – Compton Suppression

• Experimental Setup (cont.)

• Repeat for various HV Bias Voltages(4500V, 4000V, 3000V, 2000V, 1500V, 1000V, 750V, 500V, 250V, 100V, 50V)

Core Energy at Varying Bias Voltage