Germanium-based detectors for gamma-ray imaging and spectroscopy - - PowerPoint PPT Presentation

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Germanium-based detectors for gamma-ray imaging and spectroscopy - - PowerPoint PPT Presentation

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Germanium-based detectors for gamma-ray imaging and spectroscopy Mark Amman and Paul Luke Lawrence Berkeley National Laboratory The Sixth International Hiroshima Symposium on the Development and Application of Semiconductor Tracking

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SLIDE 1

Amman 1 September 14, 2006

Germanium-based detectors for gamma-ray imaging and spectroscopy

Mark Amman and Paul Luke Lawrence Berkeley National Laboratory

The Sixth International “Hiroshima” Symposium on the Development and Application of Semiconductor Tracking Detectors Gamma-ray tracking

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SLIDE 2

Amman 2 September 14, 2006

Outline

  • Ge detectors and Gamma-ray imaging
  • Detector fabrication technologies
  • Amorphous-semiconductor contacts
  • Bipolar blocking and barrier heights
  • 3-d position detection
  • Fine electrode segmentation
  • Issues: charge sharing, temperature cycling
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SLIDE 3

Amman 3 September 14, 2006

Detector material: Why Ge?

  • High Z
  • Large commercially available crystals

10 cm diameter boules

  • Large depletion lengths

> 2 cm

  • Near perfect charge collection

µτe, µτh >10 cm2/V → L > 104 cm

  • Favorable charge generation statistics

High efficiency Excellent energy resolution

< 0.2%FWHM @ 1.3MeV

However, cooling to near LN temperatures required because

  • f small band gap

( )

eV 97 . 2 energy creation pair h

  • e

e G eV 08 . factor Fano Ge F , E F 35 . 2 E

2 / 1 Statistics

= ≡ ε = ≡ ε = Δ

γ

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SLIDE 4

Amman 4 September 14, 2006

Multi-hole collimator 2-d position sensitive detector Pin-hole aperture Coded aperture 2-d position sensitive detector #1 2-d position sensitive detector #2

Gamma-ray imaging

Need position as well as spectroscopic information

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SLIDE 5

Amman 5 September 14, 2006

Standard Ge technology

Implanted n+ contacts do not withstand high fields and are not reproducible Metal surface barrier contacts are not rugged and are p-type

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SLIDE 6

Amman 6 September 14, 2006

Segmented electrical contacts

Problems:

  • Thick Li and post fabrication

diffusion limit pitch to ~ 1 mm

  • Interstrip surfaces lack

passivation Fine pitches possible on B- implanted contact

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SLIDE 7

Amman 7 September 14, 2006

Amorphous semiconductor contacts

Advantages:

  • Bipolar blocking contacts
  • Self passivating
  • Simple fabrication process
  • Thin contact dead layer
  • Fine pitches achievable
  • W. Hansen and E. Haller, IEEE TNS 24, 61 (1977).

P.N. Luke, et al., IEEE TNS 39, 590 (1992).

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SLIDE 8

Amman 8 September 14, 2006

Bipolar blocking behavior

Full depletion Full depletion

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SLIDE 9

Amman 9 September 14, 2006

Barrier heights

n+ / p-type Ge / a-Ge device Fit to ΔI = AT2 e-φ/kT

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SLIDE 10

Amman 10 September 14, 2006

Barrier heights

~1011 ~106-108 ρ [Ω-cm] φe +φh [eV] φh [eV] φe [eV] 0.67 0.28 0.39 a-Si (Ar) 0.68 0.39 0.29 a-Ge (Ar+17.5% H2) 0.70 0.34 0.36 a-Ge (Ar) Contact

Barrier heights depend on material and deposition parameters Higher film resistivity of a-Ge (Ar+17.5% H2) over that of a-Ge (Ar) typically desired to obtain high inter-electrode impedance

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SLIDE 11

Amman 11 September 14, 2006

Barrier heights

~1011 ~106-108 ρ [Ω-cm] φe +φh [eV] φh [eV] φe [eV] 0.67 0.28 0.39 a-Si (Ar) 0.68 0.39 0.29 a-Ge (Ar+17.5% H2) 0.70 0.34 0.36 a-Ge (Ar) Contact

Choose this combination to minimize leakage current

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SLIDE 12

Amman 12 September 14, 2006

Barrier heights

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SLIDE 13

Amman 13 September 14, 2006

3-d position detection

  • M. Momayezi, et al., SPIE 3768, 530 (1999).
  • M. Amman and P.N. Luke, NIM A 452, 155 (2000).
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SLIDE 14

Amman 14 September 14, 2006

Detector example

Orthogonal-strip detectors produced for the Nuclear Compton Telescope (Steve Boggs at UC Berkeley Space Sciences Laboratory) 37 strips each side, 2 mm strip pitch, a-Ge (Ar, 17.5% H2) contacts 6 detectors produced to date (12 ultimately required) 8 cm 1.5 cm

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SLIDE 15

Amman 15 September 14, 2006

Detector example

NCT Prototype

60Co (1.173 MeV) source

(50 µCi, 5 m, ~2 hrs.) Compton circle projection Maximum likelihood Courtesy: Steve Boggs, SSL

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SLIDE 16

Amman 16 September 14, 2006

Fine-pitched segmentation

Strip detector produced for synchrotron x-ray applications (Daresbury Laboratory) 1024 strips, 50 µm strip pitch, 1 mm thick

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SLIDE 17

Amman 17 September 14, 2006

Issue: Charge sharing

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SLIDE 18

Amman 18 September 14, 2006

Issue: Charge sharing

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SLIDE 19

Amman 19 September 14, 2006

Other solutions:

  • Smaller gaps
  • Field shaping electrodes: M. Amman and

P.N. Luke, NIM A 452, 155 (2000).

  • Etch away amorphous layer between

electrodes: D. Protic and T. Krings, IEEE TNS 50, 998 (2003). More work to be done …

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SLIDE 20

Amman 20 September 14, 2006

Improvements needed

  • Temperature cycling stability
  • Optimization of a-Ge/Ge/a-Si configuration
  • Charge sharing reduction
  • Side surface state control?