Swift Burst Alert Telescope CdZnTe Properties

1
Swift Burst Alert TelescopeCdZnTe Properties

• G. Sato1, K. Nakazawa1, T. Takahashi1, S.
  Watanabe1, M. Suzuki2, M. Tashiro2, Y. Okada3, H.
  Takahashi3, S. Barthelmy4,
• J. Cummings4, N. Gehrels4, D. Hullinger4, H.
  Krimm4, C. Markwardt4, A. Parsons4, J. Tueller4,
  E. Fenimore5, D. Palmer5

1Institute of Space and Astronautical Science
(ISAS), Japan 2University of Tokyo, Japan
3University of Saitama, Japan 4NASA/Goddard
Space Flight Center, USA 5Los Alamos National
Laboratory, USA
Why tailing?
BAT Development
The Burst Alert Telescope (BAT), a large coded
aperture instrument with a wide field-of-view
(FOV), provides the gamma-ray burst triggers and
locations for the Swift Gamma-Ray Burst Explorer.
The BAT detector plane consists of 32,768
individual CdZnTe semiconductor gamma-ray
detectors (4 mm x 4 mm x 2 mm) read out by XA1
ASICs and arranged in an array 1 meter below the
2.6 m2 coded aperture mask.
Electron and hole drift
Output Signal
g-ray
Cathode (-)
Variation in Pulse Height
holes ()
Photo-electron
electrons (-)
Anode ()
Poor charge carrier transport properties low
mobility (m) and short lifetime (t) especially
for holes
15/16 Blocks are installed
57Co
Calibration of 32,768 CZT detectors
Results in Low Energy Tail
Raw Data
NL Corrected Data
Block Level Calibration
Non Linearity of XA1
122 keV
NL Correction
RI Source in lead Anti-Occulter
14 keV
Energy
Escapes
Integral Non-Linearity
Input Test Pulse Voltage V
136 keV
Block (2048CZTs)
Non Linearity Correction
linear function
1 m
CZTs
Low Energy Tail
Raw ADC Bin
Test pulse voltage space
Fitted well with quadratic function
Each CZT detector is calibrated individually in
block sub-array (2048 detectors, 32K/16
blocks) by gamma-rays from radioactive sources
at various incident angles, basis.
Note Energy scale is backward
ADC is nonlinear
Because of the channel by channel variation found
in the XA1 chip (IDEAS), we need to perform Non
Linearity (NL) correction for individual channels.
1
2
ADU Channel
How to model?
Iteration from 0 to D
Charge Induction Efficiency (Hecht Curve)
Variation in Response between Detector Channels
Calculate P(zzdz) (probability of
interaction) Calculate h(z) (charge collection
efficiency) Fill the model histogram at CH_Max
x h(z) with P(zzdz) as a weight
1
2
2
Block Sum Spectrum
TYPICAL 57Co spectra. The clear
differences of spectra 1. the peak position
(Gain), 2. the peak height, 3. the
amount of low energy tail. These features are
produced by the spatial variation of mt products
in the CZT wafer.
3
NL correction works perfectly
FWHM 6keV
Gain Correction
Pulse Height
Weight
Escapes (95, 99 keV)
Energy
3
Interaction Position Distribution
Fill in the Model Histogram
1
Test pulse voltage space
mt-model
Further detailed analysis
Spectrum for individual detector taken in the
calibration are fitted to the analytic mt-model
which takes the induction efficiency and
interaction positions of photons into account.
Extracted mt products are to be used to generate
a response matrix for each detector.
Depth
Characterization
0 z D
Sato et al. Characterization of CdTe/CdZnTe
detectors, IEEE Trans. Nucl. Sci., vol. 49, no.
3, pp.1258-1263,2002
Spectral model (mt-model)
Data at 3 different bias voltages
Conclusion
Hecht equation
100 V
150 V
3 bias voltages
1. BAT Calibration is now under way. 2. We
developed new method to extract fundamental
parameters (mt products) from the spectral
fitting of individual channels.
200 V
Parameters for the mt-model
Fit
Peak shifts Change of the amount of Tail
60 keV (241Am)
22 keV (109Cd)
Determine mt products
18432 CZTs
Distribution of mt electron
mt electron
10-2
With the use of mt products obtained from 57Co
spectra, we can reproduce spectral shapes in
other energy region.
10-3
1 order of magnitude
mt electron
Detector ID
Up to now, mt-fitting have been performed for 9
blocks out of 16 blocks.
Distribution of mt hole
mt hole
3. Although CdZnTe detectors in the BAT are found
to have wide range of mt products, which leads to
the different shape of spectra, our method can be
used to construct response matrices for each
CdZnTe.
10-4
10-5
mt hole
Detector ID
1e-4 lt mt electron lt 2e-2 1e-5 lt mt hole lt 2e-4
Swift
AAS Meeting in Seattle, January 5-9, 2003