Imaging Hard X-Ray Compton Polarimeter
SOI Sensor Prototype Specification
Hiro Tajima1 and Gary S. Varner2
1

Contact: htajima@slac.stanford.edu
Stanford Linear Accelerator Center
Stanford University, Palo Alto, CA 94025

2

Contact: varner@phys.hawaii.edu
Instrumentation Development Laboratory
Department of Physics and Astronomy, University of Hawaii at Manoa

15 June 2005
ABSTRACTThis article describes the design specifications and proposed readout architecture for a future generation ofsilicon pixel detector for measuring hard x-rays for use in a future imaging Compton scattering polarimeter.
Keywords: X-ray detector, Compton Polarimeter, SOI detector

1. Introduction
The hard X-ray and gamma-ray bands have long been recognized as important windows for exploring theenergetic universe. It is in these energy bands that non-thermal emission, primarily due to accelerated highenergy particles, becomes dominant. However, by comparison with the soft X-ray band, where thespectacular data from the XMM-Newton and Chandra satellites are revolutionizing our understanding of thehigh-energy Universe, the sensitivities of hard X-ray missions flown so far, or currently under construction,
have not dramatically improved over the last decade. Clearly, the scope of discovery expected with much
improved sensitivity for both point and extended sources is enormous.
Polarization measurements play key roles to study particle acceleration mechanisms in supernovae
remnants, pulsars and back holes since synchrotron radiation, Compton scattering and bremsstrahlung aremain photon processes and they produce distinct photon polarization features. An imaging polarimeter
enables detailed polarization mapping in these objects and allows us to locate particle acceleration sites.
Furthermore a measurement of polarization feature caused by General Relativity effect around Kerr backholes would provide THE definite proof of the back hole.

The HXICP will consist of a hard X-ray mirror and a
focal plane detector. The basic IHXCP focal plane
detector module’s design is based on a Compton
kinematics telescope[1,2]. The hybrid design of thismodule, illustrated in Figure 1, incorporates both silicondetectors and pixelated CdTe detectors. The siliconlayers are required to cause at least one Compton
scattering before a photo-absorption and also improves
the angular resolution because of the smaller effect of thefinite momentum of the Compton-scattering electrons(Doppler broadening) than CdTe. The Compton
telescope consists of 10~20 layers of silicon detectors
Figure 1: The Imaging Hard X-ray Compton(double-sided silicon strip detectors or silicon monolithic
Polarimeter (IHXCP) concept.
pixel detectors) and 2 layers of thin CdTe pixelated
detectors surrounded by 5 mm thick CdTe pixelated
detectors. The telescope is surrounded by a BGO (Bi4Ge3O12) shield units to reject backgrounds. The width ofsilicon strip or pixel depends on the requirement on the point spread function of the X-ray mirror. For atypical hard X-ray mirror, 0.5 arcminutes or better can be expected. We require each IHXCP event to interact
twice in the stacked detector, once by Compton scattering in the Si or thin CdTe part, and then by photoabsorption in the CdTe part. Once the locations and energies of the two interactions are measured as shown in
Figure 1, the Compton kinematics allows us to calculate the angle between the telescope axis and the incidentdirection of the event using the formulacos θ = 1 +me c 2m c2− e ,E2 + E1
E2

where θ is the polar angle of the Compton scattering, and E1 and E2 are the energy deposited in each photon
interaction. The direction of the incident photon can be confined to be on the surface of a cone determinedfrom θ and the two interaction positions. The fineenergy resolution of the Si and CdTe devices help
reduce the width of these “Compton rings”. We candetermine the location of point sources asintersections of multiple rings. The angularresolution is limited to ~8° at 100 keV due to thefinite momentum of the Compton-scattering
electrons, which is comparable to the FOV of theBGO collimators. Although the order of the events
can be uncertain, we can use the relation that theenergy deposition by Compton scattering is always
smaller than that of the photo absorption forenergies below Eγ = 256 keV (E γ = mec2/2).
The polarization can be measured by an azimuth
scattering angle distribution as shown in Figure 2.
Figure 2: An example of polarization determination
as a function of scattering angle.

In order to maximize the energy resolution, a lownoise pixel sensor is a natural choice.
.

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1.1.

Sensor specifications

The specification for a pixel sensor to perform the energy measurements while operating in a self-triggered
mode are listed in Table 1 below. Of note is the desire to minimize the power and to provide a low-noise,data-driven measurement of the Compton energy deposition.
Table 1: Specification for the IHXCP pixel sensor.
Pixel Size
Pixel Array (Detector) Size
Noise
Global Trigger Rate
Single Pixel Rate
Trigger Threshold
Trigger Latency
PowerTotal Array PowerADC precision

200 x 200 µm
2.1 x 2.1 cm<=10 e500 Hz
10? mili-Hz
0.5 keV1-2 µs200 µW/pixel
2W
12 bits

2. Architecture
A schematic view of the proposed architecture is found in Figure 3. The base pixel is 200 x 200 um. An
array of 100 x 100 of these (2cm x 2cm) form the x-ray focal plane. In order to simplify the readout andprovide efficiency in readout, the detector is segmented into 4 quadrants as shown. Within each of thesequadrants, which are identical from a layout point of view, are a set of column and row electronics which
perform the following key functions:
1.
2.
3.
4.
5.

Hit detection and hit time encoding
Sample selection
Analog to Digital Conversion
Event hit word build
Queue and transfer data to DAQ

3basic pixel
200um
200um

100 x 100pixel arraysegmented into
4 separate (identical)
50 x 50 pixel
arraysTrigger out1) hit detect

Data buffer

timestamp/event sequencer3) sample hold (delayed)row hit/priority encode2) send trigger (isochronous)8) collect hit[or abort ifno trigger]4) assert col hit
comparator bank5) select comparator to convert6) digitize sample(s)7) build hit event

Col hit decode/MUX
Wilkinson ADC(6)Figure 3: Overview of the readout architecture, illustrating the data-driven nature of the proposed scheme. A hit
is encoded and collected as per the enumerated steps shown and described in more detail in the text.

2.1.

Pixel level DetailKey to obtaining the desired performance is the functionality contained within each pixel. A schematic representation ofthe simplest, baseline design is shown in Figure 2. Hold is common to an entire row, so column decoding is used toresolve actual hit channels. The basis of the signal sensing and storage is similar to that being developed for charged
particle tracking applications [3].

4+

set baseline

Vbase

−

Row enable

VDD
threshold

8−12

DAC

ThreshVDD

Collection electrode−Row hit+

VIsrc

Hold sample
AnalogbufferColumn Hit

Sel Column

Analog sample (bussed)

Figure 4: Essential elements of each pixel cell. A comparator is used to determine the crossing of a threshold set
with an in-pixel DAC. A feedback loop is used to compensate for leakage current and restore the baseline aftercollection of an induced signal.

2.2.

Data format

Each hit pixel is expected to produce a 48-bit data word: x-position (6-bits), y-position (6-bits), hit time (24bits – 16s wrap-around to avoid timing ambiguities), and ADC value (12-bits). The details of this are not soimportant at this stage, merely to indicate that some type of serial shift-out of the data is possible given thelow event rate and desire to minimize cabling and power.

53. Implementation Details
The choice of number of pixels is driven to match the expected reticle size. However this expectation is for
the TSMC 0.25um process and may need to be revised. The quadrant architecture scheme shown can beextended to cover discrete die, in which 4 die could be fashioned together to form a larger array – or if there
are issues with maximum reticle size.

3.1.

Prototype proposal

At least a small array of pixels should be fabricated to explore the noise levels that can be expected. Also, it
is desirable to implement as much of the peripheral control/sampling logic as will fit, commensurate with theshared real estate available on the first 5x5mm device. The primary issues to be addressed in evaluating theperformance of this process are:1. Noisea. Due to leakage current
b. kT/C noise through storage capacitorsc. fixed pattern noise/cross-talk
2. Uniformity
a. Collection efficiency
b. Shaping time
Basic test structures to address these issues seem an obvious choice. We would like to propose to make avery modest array of at least 3 different geometries:
1. 200 x 200 um – basic design
2. 100 x 100 um – to help understand leakage current scaling
3. 10 x 10 um – to evaluate architecture for low-energy “entrance window” x-rays
As a first check, it doesn’t seem necessary to provide very extensive arrays, and perhaps a few variants willallow tests of systematic effects.

64. Open Questions
A component of the design and prototype study will be the obtainable performance. Certain design issues
have been raised during the conceptual design process and are listed below.

4.1.

Size of Pixel Sense Transistor?

The observed voltage shift due to the collection of electrons from e-h pair creation depends upon the size ofthe readout transistor chosen. One would like to make as small as possible to permit the maximum sensitivity(largest voltage shift) per collected electron. However, this needs to be matched to the needed dynamic range
in a way that does not lead to saturation. This will be studied.

4.2.

Force nearest-neighbor triggers?

For best energy resolution, it is desirable to collect small, but non-zero signal from neighbor pixels that may
have been below threshold for forcing themselves to be read out. This becomes more critical if under realistic
operating conditions, it is impossible to maintain a uniform and low threshold level. Such a forced readout ispossible, at the expense of added complexity. The tentative plan is to implement this feature.

4.3.

Correlated Double Sampling?

Optimal noise/SNR performance is always obtained with these type of Monolithic Active Pixel Sensor
detectors if one takes advantage of multiple samples, correlated tightly in time (pre-sample and post-sample,
separated by a us or two). It adds some complexity, but may be very much well worth doing or evennecessary to attain the desired performance. The tentative plan is to implement this feature.

5.References
1. H. Tajima, G. Madejski, T. Mitani, T. Tanaka, H. Nakamura, K. Nakazawa, T. Takahashi, Y. Fukazawa,T. Kamae, M. Kokubun, D. Marlow, M. Nomachi, and E. C. Silva, “Gamma-ray polarimetry withCompton telescope,” in UV and Gamma-Ray Space Telescope Systems, ser. SPIE, vol. 5488, 2004, pp.
561–571.
2. H. Tajima, “Gamma-ray polarimetry,” Nucl. Instrum. Methods A, vol. 511, pp. 287–290, 2003.
3.

G. Varner et al., Nucl. Instrum. Methods A 541, 166 (2005).

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