Position sensitivity of the proposed segmented germanium detectors for the DESPEC project

Position sensitivity of the proposed segmented germanium detectors for the DESPEC project

A. Khaplanov, S. Tashenov, B.Cederwall

2008 11 11

DESPEC project at FAIR

Facility for Antiproton and Ion Research Low-energy branch of the SuperFRS

HISPEC/DESPEC

A. Khaplanov, 2008 11 11

DESPEC Ge array

GSI

FAIR

DESPEC Ge array

24 planar triple modules

16 planar triple modules

6 clovers

γ-ray tracking spectrometer for experiments with stopped exotic beams.

Nuclei implanted in a highly segmented silicon detector, AIDA at the center of Ge array


Compton background suppressionenergies of photons interacting in more than one crystals can be added back, while Compton-escape events can be identified and suppressed with no anti-coincidence shields required.

Recoil identificationin case of relatively long-lived nuclei or isomers, it is essential to correlate the γ -rays with an earlier implantation location in the silicon detector.

Background radiation suppressionusing γ-ray imaging, background photons and those originating in other parts of the experiment and in the environment, can be rejected.

Advantages of position sensitive detectors


16 pixels 17 x 17mm 8+8 strips 68 x 8.5mm

2 mm guard ring

6

20

72

Proposed detectors

Pixel detector Strip detector

Triple planar detector module

90

75

53.5

1030 29

24.5

Clover detector

depth segmentation of the outer contact, mm

25 – 25 – 40

20 – 40 – 30

20 – 25 – 25 – 20

15 – 25 – 30 – 20

15 – 25 – 25 – 25

12- and 16-fold segmented crystals


Similar detectors currently in use


KTH planar (25 pixel)

SmartPET (12+12 strips)

EXOGAM (4-segment clovers)

Net and transient signal pulse shape variation as a function of interaction depth in a planar detector.

far

close

net signal

transient signal

interactions

net signal transient signal

Pulse shapes

E


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

0.5

1

1.5

2

2.5

3

segment number

po

sitio

n e

rro

r, m

m

x-errory-errorz-error

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

0.5

1

1.5

2

2.5

3

segment number

po

sitio

n e

rro

r, m

m

x-errory-errorz-error

1, 5, 9, 13 2, 6, 10, 14

3, 7, 11, 154, 8, 12, 16

1, 2, 3, 4

5, 6, 7, 8

9, 10, 11, 12

13, 14, 15, 16Segment numbering

Position uniformity tests for Clovers

15-25-30-20 15-25-25-25

front middle 1 middle 2 back


Monte Carlo eventsPosition resolution for realistic interactions:

250, 662, 1332 keV photons simulated in GEANT4

~3000 for each energy from the implantation point (source events)

~10000 for each energy from all directions isotropically (background events)

Interaction points for source events:


Pulse shape analysis

measured/ simulated

signal

energy deposit

distribution

time-matched meta-signal

-20

0

20

-30-20

-100

10

2030

0

10

20

XY

Z

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

-0.6

-0.4

-0.2

0

0.2

segment number

0 50 100 150 200 250

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

time, ns

Position reconstruction:

•interactions generated in GEANT4

•pulses calculated for simulated interactions

•preamp response and noise added

•pulses used to reconstruct positions

•original and reconstructed positions compared

•mean position errors as function of energy, type and multiplicity of interaction


Detector comparison

Detector

(sensitive volume)

16-strip

planar

(68x68x20)

16-pixel

planar

(68x68x20)

15 – 25 – 30 – 20

clover

(<53.5x53.5x90)

physical voxel size 8.5x8.5 mm 17x17 mm different

basis grid 5x5x14x16 5x5x14x16 64x(14to17)x44

Single 1.14 2.13 3.41

Multiple 2.20 3.92 6.63

Single, merged 2.11 3.23 4.89

Multiple, merged 2.58 4.24 7.39

Single, non-merged 0.99 1.79 2.85

Multiple, non-merged 2.05 3.68 6.04

Mean position errors in mm

Multiple interactions in the same segment:

-- merged into 1 effective interaction

-- when resolved, both position and energy resolution are very poor.

This affects 20% (strip16) 30% (pixel16) 40% (clover16) 50% (clover12) of events.

Single – one segment in crystal hit

Multiple – more than one segment hit


0 0.2 0.4 0.6 0.8 1 1.2 1.4

0

1

2

3

4

5

6

Energy, keV

Pos

ition

err

or,

mm

Strip detector

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1

2

3

4

5

6

Energy, keV

Pos

ition

err

or,

mm

Strip detector

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1

2

3

4

5

6

Energy, keV

Pos

ition

err

or,

mm

Pixel detector

0 0.2 0.4 0.6 0.8 1 1.2 1.40

1

2

3

4

5

6

Energy, keV

Pos

ition

err

or,

mm

Pixel detector

mul

tiple

inte

ract

ions

StripPixel

sing

le in

tera

ctio

nsError vs energy

Clover

0 0.2 0.4 0.6 0.8 1 1.2 1.40

2

4

6

8

10

12

Energy, keVP

ositi

on e

rror

, m

m

Clover 15-25-30-20 detector

0 0.2 0.4 0.6 0.8 1 1.2 1.40

2

4

6

8

10

12

Energy, keV

Pos

ition

err

or,

mm

Clover 15-25-30-20 detector

note – the scale is doubled



γ-ray tracking

θ1

γ

γ’γ’’

θ2

E1

E2

E3

γ-ray tracking

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

662 keV strip 24 planar strip 16 planar 6 clover

P/T

efficiency0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.081

10

100

662 keV strip 24 planar strip 16 planar 6 clover

back

grou

nd s

uppr

essi

on fa

ctor

efficiency


Compton suppression Ambient Background suppression Imaging

0.5 1.0 1.5 2.0100

101

102

103

104

105

coun

ts

energy, MeV0

-20-10

010

20

-20

-10

0

10

20

0

10

20

Conclusions

Planar strip good position resolution,

little merging

64 voxels readout through 16 signals – pile-up in a strip presents problems,

both sides must be instrumented,

guard rings (dead material)

Planar pixel small contacts – little pile-up,

one side needs to be readout,

least data stored for PSA

lower position resolution,

more merging,

guard rings (dead material)

Clover best raw (before tacking) efficiency,

best active/passive material ratio, no guard rings,

only 6 cryostats required

lowest position resolution,

most merging,

large segments – most PSA data required


Position sensitivity of the proposed segmented germanium detectors for the DESPEC project

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