Room-Temperature Semiconductors: From concepts to applications Zhong He Nuclear Engineering and Radiological Sciences Department The University of Michigan, Ann Arbor, Michigan August 19 th, 2011, Beijing Summer School

Principle of Gamma-Ray Spectroscopy Gamma-ray spectroscopy = Electron spectroscopy “inside” the detector volume. (1)Gamma rays are detected only through the secondary electrons generated in gamma-matter interactions. (2)The detector must: (a) Promote gamma to e – conversions! (b) Measure the kinetic energy of electrons Gamma e-

Principal Gamma – Matter Interactions Photoelectric absorption – Full energy conversion (   Z 4.5  Strongly enhanced in high-Z materials) – Low energy predominant (< few hundred keV) Compton scattering – Partial energy transfer (   Z  Roughly proportional to density of material) – Medium energy predominant (few hundred keV to few MeV) Pair production (   Z 2  Enhanced in high-Z materials) – High energy predominant (above several MeV)

0 0.2% (High Purity Ge) 2.5-3% (LaCl 3 ) (CdZnTe) % Theoretical limit of scintillators NaI Semiconductors Scintillators 2% 7% Limit of semi. Why Interested in Wide Band-Gap Semiconductors for  -Ray Detection? (1)Superior energy resolution (smaller w-values and Fano factor) (2) Higher stopping power (higher Z and density) HgI 2 (80-53, 6.4), CdZnTe( , 6.0), Ge(32, 5.32), Si(14, 2.33) (3) Room-temperature operation (no cryogenic cooling)  Wide band-gap Technical challenges (1)Severe hole trapping & electron trapping cause signal deficit (2)Crystal yield (cost) and non-uniformity

Effect of charge trapping ne 0 nhnh QQ VV (1) That is  Q if all electrons reach the anode and all holes reach the cathode? (2) That is  Q if all electrons reach the anode and holes did not move? D D/4

 Q = +(ne 0 )  (z/D) Effect of charge trapping ne 0 QQ  V=  Q/C D Induced charge Q Z Z or Time t D or electron collection time +ne 0 C

Conventional detectors using planar electrodes Could the pulse amplitude depend only on electrons?

Experimental result Measured 137 Cs Energy spectrum using conventional (cathode-anode) readout Detector #4E-1 (single-interaction events incident from the cathode) 662 keV cut-off 32 keV X-ray Cathode Anode 15mm CdZnTe e- Signal amplitude = gain  (n  e 0 )  (normalized electron drift length z)  Energy  z 137 Cs Baseline offset

The Shockley-Ramo Theorem The induced charge Q q on an electrode by a moving point charge q is given by: where  w (x q ) is the weighting electric potential that would exist at qs instantaneous position x q under the following circumstances: the selected electrode at unit potential (no dimension), all other electrodes grounded and all charges removed. The change of the induced charge on the electrode of interest if the charge q moves from the initial position X q to the final position X q is

The Frisch grid technique (1944) Cathode Anode Frisch grid Ions e- The anode signal depends only on electrons (single polarity charge sensing) (e 0 )

Principle of 3-D Position-Sensing e-h+e-h+ C = ne 0  z A = E  ne 0 z z A single anode is replaced by an array of pixel anodes  Simultaneously readout from each pixel anode and the cathode Z. He, et al. NIM-A 422 (1999)

3-D Position-Sensing Photo-peak amplitude Depth of interaction Anode Cathode 20 mm 15 mm Detector # 4e-1 (CZT) Single polarity (e-) charge sensing 3-D correction (1) Depth (z) correction (2) Align pixels (x & y) 11  11 anodes Cathode Energy Cathode Anode 662 keV

Single-Pixel 137 Cs Spectrum of CZT #4E-1 (121 Pixels) Cathode =  3 kV; Grid =  40V; ASIC (BNL H3Dv1); Dynamic Range = 3 MeV FWHM = 0.48 % (3.2 keV) 662 keV (No collimator, room-temp. operation) 32 keV Ba K  36 keV Ba K   1.5 cm 2 cm Res.(FWHM in %) of 11  11 Pixels

137 Cs Spectrum of All-Events (4E-1 + BNL-H3Dv1) 0.69% (4.5 keV) FWHM 662 keV E1E1 E2E2 Anode Cathode  CdZnTe E3E3 15 mm From all 121 anode pixels

Source: Eu D CZT (#4E-1) single-pixel events Resolution = 0.7% FWHM Comparing to Other  Spectrometers The (3-D) reconstruction process is linear with respect to energy deposition

228 Th Energy Spectra (Detector #4E-1, whole volume, 25 o C, source uncollimated) 2614 keV D.E. S.E.

Performance Goals  E/E  1% FWHM (at 662 keV) Real-time  Imaging + isotope I.D. Eighteen 2  2  1.5 cm 3 CdZnTe detectors (108 cm 3, 648 grams = 1.43 lb) Applications Number of photons: E1E1 E2E2   

Alternative HgI 2 Array systems 662 keV 2.32% FWHM Eighteen 18  18  10 mm 3 detectors (Active volume: 14  14  10 mm 3 ) Single-pixel spectrum Energy (keV)

3-D Readout on TlBr Detectors Gold Anode Cathode Pixel: 1 mm x 1mm TlBr 4.2 mm Keitaro Hitomi et al. IEEE NSS, Oct. 2007

Overlaid Optical and  -Ray Image 60 Co 22 Na 133 Ba

214 Bi 609 keV Natural Background  -Ray Images (using 2, 3- and 4-interaction events in keV) 90° 180° 0° 90°

 -Ray (550 – 650 keV) Image Viewed by one 2  2  1.5 cm 3 CZT inside a Lead Cave Pb

Energy-Imaging Integrated Deconvolution (EIID) Eu-152 Cs to 666 keV 778 to 782 keV 1.8-cm steel shielding D. Xu et al. NIM-A 574 (2007)

Detection of Shielded Source Cs-137 no shielding 137 Cs behind 3.7-cm steel Shielded sources have unique signatures Also identified a 60 Co source behind 2.7-cm Pb

Detect K-salt in Natural K Background (400 keV – 1.6 MeV, EIID 5-iterations) All events Raw Two-pixel decon. in 4-  511 Tl Bi Cs Bi Bi Tl Ac Ac Bi Bi K Half angle 30 degrees 2-pixel event image at potassium energy

Tracking Moving Targets An array of 3  3 (nine) 2  2  1.5 cm 3 CdZnTe tracks a moving 137 Cs

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