A Si/CdTe Compton Camera for Gamma-ray
Lens experiment

Tadayuki Takahashi a,b,∗
a The Institute of Space and Astronautical Science

3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japanb University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033, Japan

Abstract
Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ-ray detection.
However, a considerable amount of charge loss in these detectors results in a reduced
energy resolution. We have achieved a signiﬁcant improvement in the spectral properties by forming the Schottky junction on the Te side of the CdTe wafer. With
the further reduction of leakage current by an adoption of guard ring structure,
we have demonstrated a CdTe pixel detector with high energy resolution and fullcharge collection capabilty. The detector has a pixel size of a few mm and a thickness of 0.5 − 1 mm. We apply this high resolution detector to a new silicon and
CdTe Compton Camera which features high angular resolution. We also describe
a concept of the stack detector which consists of many thin CdTe layers and provides suﬃcient eﬃciency for hard X-rays and gamma-rays up to several hundredkeV maintaining good energy resolution. A narrow-FOV Compton telescope can berealized by installing a Si/CdTe Compton Camera inside the deep well of an active
shield. This conﬁguration is very suitable as focal plane detector for future focusing
gamma-ray missions.

Key words: gamma-ray astronomy, Compton telescope, CZT, CdTe, Narrow FOV
Compton telescope, CdTe diode
PACS:∗
Email address: takahasi@astro.isas.jaxa.jp (Tadayuki Takahashi).

Preprint submitted to Elsevier Science

30 June 2006

1

Introduction

The hard X-ray to γ-ray range is an important window for exploring theenergetic universe. The Compton Gamma-ray Observatory (CGRO) satellite,
which operated from 1991 to 2000, revealed that there are a variety of gammaray sources[1]. With its four onboard instruments, CGRO covered a wide energy range, from 20 keV to 30 GeV. This wide energy coverage, six decades
in energy, played an important role in studying the realm of non-thermal astrophysics. We can expect a further revolution brought by current operating
missions such as INTEGRAL, Suzaku and other near-future missions (e.g.
GLAST). However, when compared with X-ray astronomy, hard X-ray andγ-ray astronomy are still immature.
While in the 10−100 keV band, current development of new concentrators suchas multilayer mirrors could allow very soon high performance both on sensitivity and angular resolution. Above this energy band, it seems to be very
diﬃcult to apply the technology such as a depth grated multi-layer mirror.
The concept of a Laue lens are currently one of the most attractive candidate to realize the gamma-ray mission with great sensitivity and very highangular resolution in the sub-MeV region. The scientiﬁc output would be verysigniﬁcant such technology becomes available. A Si/CdTe Compton Camera
with a deep and active collimator, which we have been proposed for the NeXT
mission in Japan, will provide high resolution spectroscopy with much higher
sensitivity than present instruments[2,3]. Here, we report results of our prototype Si/CdTe Compton Camera in conjunction with the recent achievementsof high-resolution CdTe detectors.

2

High resolution CdTe detector

Among the range of semiconductor detectors available for γ-ray detection,
CdTe and CdZnTe have a privileged position because of their high density
and the high atomic number of their components, as well as a wide bandgap.
In the 1990’s, the remarkable progress in the technology of producing a highquality single crystal of CdTe and the emergence of CdZnTe have dramaticallychanged the situations of high resolution room temperature detectors based onthese materials. A review of the recent progress of CdTe and CdZnTe detectors
is available in Takahashi & Watanabe [4]. The performance of CdTe for the
IBIS instrument onboard the INTEGRAL mission and CdZnTe for the BAT
instrument on the Swift mission are described in Limousing et al. [5] and Satoet al. [6], respectively.
We have reported a signiﬁcant improvement in the spectral properties of CdTe
2

(a)

(b)

Fig. 1. (a) 57 Co spectrum obtained with the CdTe diode operated at 20 ◦ C .The
active area of the detector is 2×2×0.5 mm3 [15]. The applied voltage is 800 V.
(b) High energy spectrum of 137 Cs above 620 keV with the same dimensions. The
applied bias voltage is 1400 V and the operating temperature is -40 ◦ C . The energy
resolution at 662 keV is 2.1 keV (FWHM)[7].

detectors by means of a Schottky diode (CdTe diode)[4,8–13]. The improvement of the energy resolution by adopting the Schottky junction is dramatic.
For the detector with an area of 2 mm × 2 mm and a thickness of 0.5 mm, theleakage current of the CdTe diode is as low as 10 pA at 20 ◦ C at a bias voltageof 800 V. We have also found that the guard ring structure on Pt side further
reduces the leakage current by one order of magnitude [14]. As demonstrated
in Fig. 1 (a), an energy resolution of 1.1 keV (FWHM) for the 122 keV linefrom 57 Co is obtained [15]. When we cool the detector, the energy resolution
is improved. From 137 Cs, an energy resolution of 2.1 keV (FWHM) for the
662 keV line is obtained with a 2 mm × 2 mm CdTe diode of thickness 0.5mm at a bias voltage of 1400 V [7]. A CdTe diode with an area of 3 mm × 3
mm and a thickness of 1 mm installed in the system manufactured by Amptek[16] has an energy resolution of 260 eV (FWHM) at 6.4 keV when operated
at −40 ◦ C [17].
Another important reason for adopting thin devices is that the very low leakage
current of the CdTe diode enables us to apply a high electric ﬁeld to ensurecomplete charge collection in devices with a thickness < 1 mm. When we
increase the thickness of the CdTe/CdZnTe detector to improve the eﬃciency
for high energy gamma-rays, the eﬀects of incomplete charge collection, thelow energy tail, become signiﬁcant. In some cases, the in active region in thedetector volume increases with the thickness, especially for the region close to
the anode. The 0.5 mm thick CdTe diode becomes fully active, and even holes
generated near the anode face can be completely collected.
3

(a)

(b)

Fig. 2. ( a) The 40 layer CdTe stack detector and one of the layers from the stacked
detector. (b) A spectrum of neutron capture gamma-rays obtained with the stacked
detector. Neutrons from 252 Cf are slowed down by polyethylene and injected to
NaCl. The peaks of 6.1 MeV, 5.6 MeV and 5.1 MeV correspond to gamma-rays
from neutron captures in Cl nuclei, together with single and double 511 keV escapes.
The peaks of 559 keV and 2.2 MeV correspond to gamma-rays produced by neutron
captures of Cd and H, respectively [18,19].

3

Stacked CdTe Detector

In order to extend the application of CdTe diodes to the detection of MeV
gamma-rays, we proposed the idea of a stacked detector, in which several
thin and large CdTe diodes are stacked together and operated as a single
detector[4,18,19]. This is only possible if charge careers generated in each layer
volume are collected completely and there is no inactive region especially atthe bottom region of each layer.
Figure 2 (a) shows a stacked detector consisting of 40 layers of large CdTe
diodes each with an area of 21.5 mm × 21.5 mm and a thickness of 0.5 mm. A
gap of only 0.7 mm has been achieved by utilizing 0.5 mm thick ceramic sheet
for the detector housing. The total volume of the stacked detector amounts
to 9.2 cm3 ( a thickness of 2 cm), which provides a full peak eﬃciency of
20 % at 500 keV and 7 % at 1 MeV. In our stacked detector, the signal fromeach layer is processed independently using an individual analog chain. The
gamma-ray spectrum from the detector is obtained by summing the spectrafrom all layers which record pulse heights above a certain threshold. With thisapproach, the energy resolution could be kept at the same level as that of asingle layer. Although there is still room for improvement for the prototype
detector, we obtain an energy resolution of 1.6–1.7 % (FWHM) for the two
peaks of 1.17 MeV and 1.33 MeV in the energy spectrum of gamma-rays from60
Co. The advantage over conventional scintllation counters is demonstrated
in 2 (b) in which gamma-ray lines up to 6 MeV from neutron captures in Na
4and Cl are shown.
In addition to the increase of eﬃciency, the stack conﬁguration and individual
readout provide informations of the interaction depth. This depth informationis very useful to reduce the background in space applications, because we can
expect that low energy gamma-rays interact in the upper layers and, therefore,
we can reject low energy events detected in lower layers. Moreover, since thebackground rate is proportional to the detector volume, low energy events
collected from the ﬁrst few layers in the stacked detector have a high signal
to background ratio, in comparison with events obtained from a monolithic
detector with a thickness equal to the sum of all layers.

4High resolution CdTe Gamma-ray pixel module

High energy resolution is very attractive for applications in astrophysics,
where precise determination of the central energy and the proﬁle of X-ray
and Gamma-ray lines are crucial. If we can form pixels on the electrode of thelarge area CdTe layer used in the stacked detector, the small capacitance ofthe pixel leads to the improvement of energy resolution. However, the number of channels to be handled in the system becomes very large and we needto have very low noise analog circuits implemented as multi-channel ASIC
(Application-Speciﬁc Integrated Circuit). The energy resolution around 1−2
keV is challenging if we have to rely on highly compact CMOS based ASICs.
A new low noise front-end ASIC, VA32TA, has been developed to realize thisgoal in collaboration with Ideas ASA in Norway [20].

(a)

(b)

Fig. 3. (a) 8x8 CdTe pixel detector module. The detector has 8×8 = 64 pixels witha pixel size of 1.4 × 1.4 mm2 . The thickness of the detector is 0.5 mm. (b) large
area CdTe detectors consists of four pixel moduless

In order to demonstrate the performance of pixel detectors based on CdTe
Schottky diode, we have developed 8×8 CdTe pixel detector modules with the
5

2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
0

20

40

60

80

100

120

140

Fig. 4. Summed spectra 57 Co taken with the detector shown in Fig. 3 at 30 min and
60 min after the bias voltage is turned on.The operating temperature s 5 degree
and the bias voltage is 600 V. Energy resolution from the summed spectrum is 1.5
keV (FWHM) at 122 keV.

pixel size of 2.0 × 2.0 mm2 and 1.4 × 1.4 mm2 with a thickness of 500 µm.
The indium side is used as a common electrode and is biased with a voltage
of 400 V. The platinum side is divided into 8 by 8 pixels surrounded by aguard ring with a width of 1 mm. Each pixel is connected to a fanout board
by using In/Au stud bump bonding technology [7]. The signal from each pixel
is fed into the VA32TAs on-board the FEC. Capacitance and leakage currentof each pixel are 1 pF and a few pA at 0 °C. Low leakage current is realized by
employing a guard ring to absorb leakage current from the detector edge[14]Fig. 3 (a) shows a photo of the CdTe pixel module. As shown in Fig. 3 (b),a large area can be obtained by buttering the modules. Fig. ?? shows spectra
obtained from 64 pixels in the gamma-ray module. It is clear that the spectra
are quite uniform all over the detector plane. Fig. 4 shows summed spectra
57
Co taken at 30 min and 60 min after the bias voltage is turned on. No
degradation due to possible charge accumulation is seen on this time scale.
The energy resolution (FWHM) is 1.5 keV for gamma-rays of 122 keV. Theenergy threshold is measured to be as low as 7 keV.

5

Semiconductor Compton Telescope

By utilizing the technique of the Compton telescope, the COMPTEL instrument on-board the CGRO satellite has achieved pioneering results in this energy region. The energy band between 0.1 MeV and 10 MeV is poorly explored
due to diﬃculties associated with the detection of such photons. The Comp6

ton telescope COMPTEL[3] on board CGRO demonstrated that a gamma-ray
instrument based on the Compton scattering is useful for the detection of the
gamma-rays in this energy band. Although COMPTEL performed very wellas the ﬁrst orbit-based Compton telescope for MeV gamma-ray astrophysics,
it suﬀered from large background, poor angular resolution, and complicatedimage decoding.
Taking advantage of signiﬁcant progress in CdTe technology, we are developing
a new generation of Compton telescope, the semiconductor Compton telescope[8,22,29,31,30]. In addition to the low background capability by utilizing theCompton kinematics, it features high spectral resolution (2 keV (FHWM) at
100 keV) and high angular resolution close to the theoretical limit deﬁned by
the Doppler broadening. Capability to measure gamma-ray polarization fromthe directional information of scattered gamma-ray photons is also attractive
feature of a Compton camera.

Fig. 5. Conceptual design of semiconductor Compton telescope consisting of layers
thin silicon and CdTe imaging layers.

The concept of the semiconductor Compton telescope, which we have beenworking on, is shown in Fig. 6. The telescope is based on a hybrid semiconductor gamma-ray detector consisting of layers of thin Si and CdTe layers todetect photons in a wide energy band (0.05 - 1 MeV); the Si layers are required
to improve the performance at a lower energy band (< 0.3 MeV). In the stackof Si and CdTe layers, we require require each event to interact twice, once
by Compton scattering and then by photo-absorption. Once the locations andenergies of the two interactions are measured, the Compton kinematics allows
us to calculate the energy and direction (as a cone in the sky) of the incident
γ-ray following the Compton equation. The incident direction of a γ-ray is
calculated from the position and the energy information of the interactions.
Therefore, ﬁne energy resolution is the key requirement for the semiconductor
Compton telescope to retain a low background level with Compton kinematics.
The energy resolution around 1 keV is required for both Si and CdTe detec7

(a)

(b)

Fig. 6. (a): The double-sided Si strip detector (DSSD). (b): A shadow picture obtained with the DSSD using 22 keV X-rays.

Fig. 7. The six layered DSSDs. The pitch between DSSD layers is 5.4 mm.

tors to approach the physical limit of the angular resolution due to Doppler
broadening.
Regarding the uncertainty of the order of scattering in an event, we can use therelation that the energy deposition by Compton scattering is always smaller
than that of the photo-absorption for energies below Eγ = 255 keV (Eγ =me /2). This relation holds above this energy and there is only one solution, if
e 2
the scattering angle θ is smaller than cos−1 (1 − 12 ( m) ) [3].
Eγ
The scattering part is very important in the Compton telescope. In order tocover a wide energy band and also to cover the wide scattering angle of theCompton interaction in the detector, the energy threshold of the detector must
be low. Furthermore high Compton scattering probability is desirable to obtaina high eﬃciency as a Compton telescope. To this end, we have developed a new
0.3 mm thick double strip Si detector (DSSD) with Hamamatsu Photonics. For
the ﬁrst step as a high resolution DSSD, we have chosen a strip length of 2.56
cm, a strip pitch of 0.4 mm and a strip gap of 0.1 mm. The Current-Voltage
measurement indicates a depletion voltage of 65 V. The leakage current is 0.5
nA/strip at 20 °C and 0.05 nA/strip at 0 °C. The strip capacitance is measured
to be 6.3 ± 0.2 pF. Intrinsic noise performance with a silicon strip detector ismeasured to be 1.0 keV (FWHM) at 0 °C, which is in good agreement with
8

(a)

(b)

Fig. 8. (a) Energy spectrum of 57 Co for a DSSD measured at −10 ◦ C . (b) Energy
spectrum of 241 Am measured at 0 ◦ C . The 122 keV peak and 60 keV peaks are
magniﬁed in the vertical axis in inset plot for 57 Co and 241 Am, respectively.

the analytically calculated noise value of 0.9 keV[20].
Fig. 6 shows a picture of the DSSD and its peripheral circuit. The signals fromthe strips are processed by front-end-cards (FECs) on which two low noise
analog ASICs, VA32TAs are mounted. The strips on the p-side are connected
directly to the input of the ASIC, while the strips of the n-side are connected
via coupling capacitors. A shadow image by collecting 22 keV X-rays from a
109
Cd source is shown in Fig. 6 [27]. The shadow mask is a bookmark madeof brass covered with gold, whose thickness is 0.8 mm.
Figure 8 (a) shows the sum of the energy spectrum from 57 Co for all channels
at -10 °C. Fits yield energy resolutions of 1.1 keV (FWHM) for the 14 keV line
and 1.3 keV for the 122 keV line. Figure 8 (b) shows the energy spectrum from241
Am at 0 ◦ C . A ﬁt yields an energy resolution of 1.3 keV at 60 keV[28].
By combining the stacked DSSDs and three CdTe pixel detectors, we constructed a prototype SI/CdTe semiconductor Compton telescope. A pictureof the stackjd DSSD in the prototype is shown in Fig. 7. Two CdTe pixel
detectors are placed underneath the DSSDs, and another detector is arrangedon the side of the DSSDs [29].
Fig. 9 (a) shows the Compton reconstructed images a of 22 Na 511 keV gammaray source obtained with the prototype. The selected energy regions are 500–
520 keV. From the calculated scattering angle and the two-hit positions, aCompton cone is drawn on the sky on an event by event basis. We project thecone onto the plane at the distance of 350 mm, and obtain the image of the
gamma-ray source. A circle with a radius of 5 degree is drawn together witheach image as reference. We note that the images are asymmetric because
the arrangement of CdTe absorbers is not symmetric. Fig. 9 (b) shows thereconstructed spectrum for 511 keV gamma-rays the 22 Na gamma-ray source.
The measured angular resolution is plotted in Fig. 10, together with the esti9

800

radius 5°

0.25

600
400

0.2

y [mm]

200
0.15
0

-200

0.1

-400
0.05
-600
-800
-800 -600 -400 -2000200
x [mm]

400

600

800

0

(a)

(b)

Fig. 9. Compton reconstructed image (left) and spectrum (right) of the 511 keVgamma-ray obtained with a prototype Si/CdTe Compton telescope. The angularresolution is 3.9 degree. The energy resolution is 14 keV (FWHM) at 511 keV[29].
A circle with a radius of 5 degree is drawn as reference in the image.

mated values (∆θDB ) from the simulation. The scattering angle is determinedwith a resolution of 3.9 degree (FWHM) at 511 keV. The eﬀect of the Doppler
broadening becomes smaller as the incident energy becomes higher, because
the binding energy and momentum of electrons in materials become relatively
small for gamma-ray photons with higher energies. In the same ﬁgure, theeﬀect of ∆θposition , ∆θenergy and ∆θDopplerBroadening :√∆θtotal =(∆θposition )2 + (∆θenergy )2 + (∆θDopplerBroadening )2(1)

are plotted together with the experimental data as ∆θtotal [29]. The experimental values can be explained by the three contributions. In most cases, theangular resolution is limited by the eﬀect of Doppler broadening, except thatthe contribution of position resolution is comparable to that of Doppler broadening in the higher energy region (> 500 keV). According to the analysis basedon Monte Calro detector simulation, we conﬁrmed the contribution of Doppler
broadening is 2.6 degree, and the contributions from position resolution andenergy resolution are 2.6 degree and 2.7 degree, respectively[29] Additionally,
CdTe layers can also be used as scatterer if the energy of incident gamma-rays
is high. According to our measurements, the obtained angular resolution is35.9 degree at 122 keV, and 12.2 degree at 511 keV, which is mainly dominated by the eﬀect of Doppler broadening[31].
When kinetic energy of an electron is above ∼ 250 keV, the range of electronsin silicomn exceeds the thickness of one layer of the DSSD (300 µm). In thiscase, the recoil electron deposits energy on the successive layers of the DSSDs
and the direction of the electron could be tracked from the hit positions. Then,
we can fully solve the Compton kinematics from the information obtainedwith the detectors. Although the information of the direction of electrons are10

Fig. 10. Relation between incident gamma-ray energy and angular resolution. Filled
circles show the experimental data. Open triangles, squares and diamonds are estimated contributions of detector energy resolution, detector position resolution and
Doppler broadening, respectively. The totals obtained by adding the three estimatedvalues in quadrature are plotted by open circles[29].
smeared out by the eﬀect of multiple scattering, we can limit the direction ofincident gamma-rays as an arc, not a cone.
In the calculation of Compton kinematics, the angles between the directionof recoil electrons and that of scattered gamma-rays can be used to rejectbackground events, especially for those in which the scattered gamma-rays are
not fully absorbed in CdTe. Fig. 11 shows the reconstructed image of 662 keVgamma-rays from 137 Cs from events that satisfy the consistency of the angle
between the electron and the scattered photon, which can be calculated either
geometrically from hit positions or from the deposited energy [30]. In drawing
the image, the width and the length of each arc are evaluated from the energy
resolution and position resolution of the component detectors.

6

Narrow FOV Si/CdTe Compton Telescope

In the energy range above hundred keV (the sub-MeV region), the photon
ﬂux is much smaller than that in X-ray region, therefore shielding against
background photons becomes much more important. In this situation, thereduction of background becomes critical for performing “precise” experimentsin this energy band. In the Hard X-ray Detector (HXD) onboard the Suzaku
satellite, the well-type shield made of BGO scintillators, which surrounds themain detector part (Si PIN detector and GSO scintillator), is expected toreduce the background to the limit achieved by the conﬁguration of active
11

800

radius 5°
5

600
400

4
y [mm]

200
3

0

-200

2

-400
1
-600
-800
-800 -600 -400 -2000200
x [mm]

400

600

800

0

Fig. 11. An image of 137 Cs obtained from the Compton reconstruction with electron
tracking[30].

shield and collimator [23,24].
In order to further reduce the background and thus to improve the sensitivity,
we have proposed a new detector called the Soft Gamma-ray Detector (SGD)
for the NeXT mission. The SGD is a Si/CdTe semiconductor Compton telescope mounted inside the bottom of a well-type active shield[8,25,17,21,3]. As
shown schematically in Fig.12, the telescope consists of 24 layers of DSSDs(double-sided silicon strip detectors) surrounded by CdTe pixellated detectors. The opening angle provided by the BGO shield is 4 degrees at 500 keV.
For the SGD, an additional copper collimator restricts the ﬁeld of view of thetelescope to 30’ for low energy photons (<100 keV) to minimize the ﬂux due
to the Cosmic X-ray Background from within the FOV. These modules arethen arrayed to provide the required area.
The major advantage of employing a narrow FOV is that the direction of incident γ-rays is constrained to be inside the FOV. If the Compton ring does notintercept the FOV, we can reject the event as background. Most backgrounds
can be rejected by requiring this condition (albeit with a corresponding reduction in instrument eﬀective area). Background photons from the BGO andcopper collimator for which the reconstructed Compton ring intersects the
FOV cannot be eliminated if there is no signal detected in the active shield,
however this source of background contributes only within a limited range ofscattering angle. Combining background suppression techniques available in
the SGD, we expect to achieve background levels of 5×10−7 counts/s/cm2 /keVat ∼ 100 keV and 2×10−7 counts/s/cm2 /keV at ∼ 500 keV. The eﬀective areaof the SGD in the Compton mode is 120 cm2 at 200 keV and 50 cm2 at 400
keV, if we use 25 units (total geometrical area of 625 cm2 ).
The concept of a narrow FOV Compton telescope is expected to reduce dramatically the background from radio-activation of the detector materials,
12Fig. 12. Conceptual design of the SGD module [3]. A stack of 24 Si DSSDs and
CdTe pixel detectors are assembled to form a semiconductor Compton Telescope.
The telescope is mounted inside the bottom of a well-type active shield. The opening angle provided by the BGO shield is 4 degrees at 500 keV. An additional copper
collimator restricts the ﬁeld of view of the telescope to 30’ for low energy photons (<100 keV). With this narrow FOV, events are rejected as background if thereconstructed Compton ring does not intercept the FOV.

which is a dominant background in the case of the uzaku HXD [24]. The
30’ FOV of the ﬁne collimator is required to improve the sensitivity which islimited by source confusion below a few hundred keV. It should be noted that
we can also measure polarization of incident gamma-rays from the azimuthal
distribution of Compton scattered photons. The demonstration of the polarization measurement was performed by using a fully polarized synchrotron
beam in the Spring 8 facility [?,32].

7

A narrow FOV Si/CdTe Compton Camera for the Gamma-raylens mission

Focusing instruments have two tremendous advantages: ﬁrst, the volume of
the focal plane detector can be made much smaller than for non-focusing
instruments, and second, the residual background, often time-variable, can bemeasured simultaneously with the source, and can be reliably subtracted. The
MAX mission, which is based on the concept of Laue gamma-ray lens is very
promising approach to extend the focusing capability above several hundred
keV to reality.
In order to achieve the ultimate sensitivity for the MAX mission, the focal
13

plane detector should be designed very carefully. A Compton telescope is agood solution, because the direction of incident gamma-rays could be determined by the Compton reconstruction and therefore eﬀective in discriminating
gamma-rays coming from the lens. However, the angular resolution limited by
the Doppler broadening is 2.5 degrees for 511 keV and also the ordering ofinteractions has to be solved before we determine the direction of incident
gamma-rays.
The concept of the narrow FOV Compton telescope realized with an active
collimator made of BGO scintillator and a stack of Si/CdTe detector is avery attractive solution as the focal plane detector of the gamma-ray lens
experiment. Fig. 13 shows the conceptual design of the focal plane detector.
The FOV of the active collimator is determined such that it only sees the
gamma-ray lens. With this, all we can assume is that the signal gammarays come from the FOV of the collimator, and others can be rejected as
background. Furthermore a thick BGO scintillator shields gamma-rays fromother directions and also reduces the background due to activation in the BGO
scintillator itself. This is quite important, because the 511 keV line from theβ + decays becomes a major background source for the study of annihilationlines from the celestial objects. The fact that the Si/CdTe Compton telescope
can be operated at moderate temperatures of 0 to -20 degrees makes the designof the shield easy for satellite applications. The compact design of a stack of
Si and CdTe detectors also helps, because, if the volume of the shield becomeslarge, the dead time caused by the shield becomes signiﬁcant.
The eﬀective area of the scattering part could be a concern. As shown in Fig.
14, a stack of Si detectors with a total thickness of ∼ 5 cm corresponds toτ =1 for Compton scattering. This number is ∼ 2 cm for CdTe. Although
CdTe detectors also act as the scatterer above 300 keV, the angular resolution
is larger than 10 degrees. To have a suﬃcient number of layers with DSSD
would be preferable in terms of the angular resolution. Depending on theenergy of interest, the number of Si and CdTe layers and their ratio should bechosen. Fig. 15 shows the peak detection eﬃciency of events which undergo
multi Compton scattering and ﬁnal photo absorption in a 80 layer detector.
The eﬃciency for the diﬀerent numbers of Si and CdTe layers are shown as
a function of photon energy. The dimensions of the detector are 25 cm × 25
cm and the thickness of each layer is 0.5 mm. Gamma-rays are injected to thecenter of the top layer.

8

Conclusion

As gamma-ray energy increases to the MeV region, the detection of photons,
the determination of the incident direction, and the rejection of background14

Fig. 13. Conceptual drawing for the focal plane detector for a gamma-ray lensmission.

Fig. 14. Thickness of the detector which corresponds to τCompton =1 for silicon andCdTe.

become very diﬃcult. Gamma-ray focusing technology should change this situation considerably.
We have proposed a new detector called the Soft Gamma-ray Detector (SGD)
for the NeXT mission, which utilizes the idea of a narrow FOV Si Compton
telescope. In the SGD, we combine a stack of Si strip detectors and CdTe pixel
detectors, to form a Compton telescope, which is mounted inside the bottomof a well-type active shield. We have demonstrated Compton reconstruction
by using a prototype Si/CdTe Compton telescope, consisting of six-layered
DSSDs and three 8 × 8 CdTe pixel detectors. By using this prototype, wehave obtained Compton reconstructed images and spectra of gamma-rays from15

Fig. 15. Simulated peak detection eﬃciency for events which interact more than
twice in the detector volume (more than one Compton scattering and ﬁnal photo-absorption). The detector consists of 80 layers of thin Si or CdTe detector. The
results are shown for CdTe layers only (ﬁlled circles), Si layers only (ﬁlled triangls) and mixture (open circles). Dimensions of each layer are 25 × 25 cm2 with athickness of 0.5 mm2 . Gamma-rays are injected at the center of the top layer.

122 keV to 662 keV. The achieved energy resolutions are 9.1 keV and 14 keV
at 356 keV and 511 keV, respectively. The angular resolution is 3.9 degrees(FWHM) for 511 keV gamma-ray photons.
The concept of the narrow FOV Compton telescope becomes a very attractive
solution for the focal plane detector of the gamma-ray lens experiment. In
order to obtain suﬃcient eﬃciency for 511 keV lines for the future mission, we
are now working on the larger scale model of the Si/CdTe Compton telescopewith several tens of layers of DSSD and CdTe pixel detectors.

Acknowledgement

This research was carried out in collaboration with K. Nakazawa, S. Watanabe, G. Sato, T. Mitani, T. Tanaka, K. Oonuki, K. Tamura, T. Kishishita, S.
Takeda, M. Ushio, J. Katsuta (Institute of Space and Astronautical Science,
JAXA), H. Tajima (Stanford Linear Accelerator Center), M. Nomachi (Osaka
University) and Y. Fukazawa (Hiroshima University), and supported by the
Ministry of Education, Science, Sports and Culture, Grant-in-Aid in Japan(13304014, 14079207).
16

References[1] N. Gehrels and J. Paul, Physics Today, 51, 2, 26 (1998).
[2] T. Takahashi, K. Mitsuda, H. Kunieda, “The NeXT Mission”, Proc. SPIE, inpress, 2006
[3] T. Takahashi, et al., “Wide band X-ray Imager (WXI) and Soft Gamma-ray
Detector (SGD) for the NeXT Mission,” Proc. SPIE, vol. 5488, pp.549-560,
2004.
[4] T. Takahashi and S. Watanabe “Recent Progress on CdTe and CdZnTe
detectors,” IEEE Trans. Nucl. Sci., vol. 48, pp. 950-959, 2001.
[5] O. Limousing, C. Blondel, J. Cretolle, H. Dzitko, P. Laurent, F. Lebrun, J.P.
Leray, et al. “The ISGRI CdTe gamma-ray camera ﬁrst steps,” Nucl. Instr.
Meth., vol. A442, pp. 244-249, 2000.
[6] G. Sato, A. Parsons, D. Hullinger, M. Suzuki, T. Takahashi, M. Tashiro, K.
Nakazawa et al. “Development of a Spectral Model Based on Charge Transport
for the Swift/BAT 32K CdZnTe Detector Array,” Nucl. Instr. Meth., vol. A541,
pp. 372-384, 2005.
[7] T. Takahashi, S.Watanabe, G.Sato, Y.Okada, S.Kubo, Y.Kuroda, M. Onishi, R.
Ohno, “High Resolution CdTe Detector and Applications to Imaging Devices,” IEEE Trans. Nucl. Sci., vol. 48, No. 3, pp. 287-291, 2001.
[8] T. Takahashi, K. Nakazawa, T. Kamae, H. Tajima, Y. Fukazawa, M. Nomachi,
and M. Kokubun , “High resolution CdTe detectors for the next generation
multi-Compton gamma-ray telescope ,” Proc. SPIE, vol. 4851, pp. 1228-1235,
2002.
[9] C. Matsumoto, T. Takahashi, K. Takizawa, R. Ohno, T. Ozaki, K. Mori,“Performance of a New Schottky CdTe Detector for Hard X-ray Spectroscopy,”
IEEE Trans Nucl. Sci., 45, 428, 1998.
[10] T. Takahashi, K. Hirose, C. Matsumoto, K. Takizawa, R. Ohno, T. Ozaki, K.
Mori, Y. Tomita, “Performance of a New Schottky CdTe Detector for Hard
X-ray Spectroscopy,” Proc. SPIE, vol. 3446, pp. 29-37, 1998.
[11] M. Funaki, T. Ozaki, K. Satoh, and R. Ohno, “Growth and Characterization of
CdTe Single Crystal for Radiation Detectors,” Nucl. Instr. Meth. , vol. A436,
pp. 120-126, 2000.
[12] T. Takahashi, B. Paul, K. Hirose, C. Matsumoto, R. Ohno, T. Ozaki, K. Mori, Y.
Tomita, “High-resolution Schottky CdTe Detectors for Hard X-ray and Gammaray Observations ,” Nucl. Instr. Meth. vol. A436 pp.111-119, 2000.
[13] T. Takahashi, T. Mitani, Y. Kobayashi, M. Kouda, G. Sato, S. Watanabe, K.
Nakazawa, Y. Okada, M. Funaki, R. Ohno, “High Resolution Schottky CdTe
Diodes,” IEEE Trans. Nucl. Sci., vol. 49, No. 3, pp. 1297-1303, 2002.

17

[14] K. Nakazawa, K. Oonuki, T. Tanaka, Y. Kobayashi, K. Tamura, T. Mitani,
G. Sato, S. Watanabe, T. Takahashi, R. Ohno, A. Kitajima, Y. Kuroda and
M. Onishi, “Improvement of the CdTe Diode Detectors using a Guard-ringElectrode,” IEEE Trans. Nucl. Sci, vol.51,No.4,pp.1881-1885, 2004.
[15] T. Tanaka, Y. Kobayashi, T. Mitani, K. Nakazawa, K. Oonuki, G. Sato, T.
Takahashi and S. Watanabe, “Recent achievements of the high resolution
Schottky CdTe diode for gamma-ray detectors,” New Astronomy Reviews, 48,
pp. 269-273, 2004.
[16] Amptek Inc., http://www.amptek.com[17] T. Takahashi, K. Nakazawa, S. Watanabe, G. Sato, T. Mitani, T. Tanaka,
K. Oonuki, K. Tamura, H. Tajima, T. Kamae, G. Madejski, M. Nomachi, Y.
Fukazawa, K. Makishima, M. Kokubun, Y. Terada, J. Kataoka, M. Tashiro,“Application of CdTe for the NeXT Mission”, Nucl. Instr. Meth. A, 541, pp.
332-341, 2005 .
[18] S. Watanabe, T.Takahashi, Y. Okada, G. Sato, M. Kouda, T. Mitani, Y.
Kobayashi, K. Nakazawa, Y. Kuroda, M. Onishi, “CdTe stacked detectors for
Gamma-ray detection”, IEEE Trans. Nucl. Sci.,vol. 49, No. 3, pp.1292-1296,
2002.
[19] S. Watanabe, T. Takahashi, K. Nakazawa, Y. Kobayashi, Y. Kuroda, K. Genba,
M. Onishi, K. Otake , “A stacked CdTe Gamma-ray Detector and its application
as a range ﬁnde,” IEEE Trans. Nucl. Sci., vol. 49, No. 3, pp. 1297-1303, 2002.
[20] H. Tajima, T. Nakamoto , T. Tanaka , S. Uno , T. Mitani , E. do Coutoe Silva, Y. Fukazawa, T. Kamae, G. Madejski, D. Marlow, K. Nakazawa, M.
Nomachi, Y. Oakada, T. Takahashi, “Performance of a low noise front-end ASIC
for Si/CdTe detectors in Compton gamma-ray telescope,” IEEE Trans. Nucl.
Sci., 51, pp. 842-847, 2004.
[21] H. Tajima, T. Kamae, G. Madejski, T. Mitani, K. Nakazawa, T. Tanaka, T.
Takahashi, S. Watanabe, Y. Fukazawa, T. Ikagawa, J. Kataoka, M. Kokubun, K.
Makishima, Y. Terada, M. Nomachi and M. Tashiro, “Design and Performanceof Soft Gamma-ray Detector for NeXT mission”, IEEE Trans. Nucl. Sci., inpress, 2005.
[22] T. Mitani,T. Tanaka, K. Nakazawa, T. Takahashi, T. Takashima, H. Tajima,
H. Nakamura, M. Nomachi, T. Nakamoto, Y. Fukazawa,“A Prototype Si/CdTe
Compton Camera and the Polarization Measurement , ” IEEE Trans. Nucl.
Sci., pp.2432-2437, 2004.
[23] T. Kamae et al., “Astro-E hard X-ray detector,” Proc. SPIE, vol. 2806, pp.
314-328, 1996[24] M. Kokubun et al.,“Improvements of the Astro-E2 Hard X-ray Detector (HXDII), ” IEEE Trans. Nucl. Sci., v51,pp.1991, 2004[25] T. Takahashi, K. Makishima, F. Fukazawa, M. Kokubun, K. Nakazawa, M.
Nomachi, H. Tajima, M. Tashiro, and Y. Terada, “Hard X-ray and Gamma-ray

18

detectors for the NeXT mission,”, New Astronomy Reviews, 48, pp. 309-313,
2004.
[26] H. Tajima, T. Kamae, S. Uno, T. Nakamoto, Y. Fukazawa, T. Mitani, T.
Takahashi, K. Nakazawa, Y. Okada, and M. Nomachi, “Low noise double-sided
silicon strip detector for multiple-Compton gammaray telescope,” Proc. SPIE,
vol. 4851, pp. 875-884, 2002[27] T. Tanaka, T. Mitani, S. Watanabe, K. Nakazawa, K. Oonuki, G. Sato, T.
Takahashi, K. Tamura, H. Tajima, H. Nakamura, M. Nomachi, T. Nakamoto,Y. Fukazawa, “Development of a Si/CdTe semiconductor Compton telescope ”,
Proc. SPIE, vol. 5501, pp. 229-240, 2004.
[28] Y. Fukazawa, T. Nakamoto, N. Sawamoto, S. Uno, T. Ohsugi, H. Tajima, T.
Takahashi, T. Mitani, T. Tanaka, K. Nakazawa, “Low-noise double-sided silicon
strip detector for soft gamma-ray Compton camera,” Proc. SPIE, vol. 5501, pp.
197-207, 2004[29] S. Watanabe, T.Tanaka, K. Nakazawa, T. Mitani, K. Oonuki, T. Takahashi,
T. Takashima, H. Tajima, Y. Fukazawa, M. Nomachi, S. Kubo, M. Onishi, Y.
Kuroda, “A Si/CdTe Semiconductor Compton Camera,” IEEE Trans. Nucl.
Sci., 52, pp.2045-2051, 2005.
[30] T. Tanaka, S. Watanabe, S. Takeda, K. Oonuki, T. Mitani, K. Nakazawa, T.
Takashima, T. Takahashi, H. Tajima, N. Sawamoto, Y. Fukazawa, M. Nomachi,“Recent results from a Si/CdTe semiconductor Compton telescope,” NIM A, inpress, 2006.
[31] K. Oonuki, T. Tanaka, S. Watanabe, S. Takeda, K. Nakazawa, T. Mitani,
T. Takahashi, H. Tajima, Y. Fukazawa, M. Nomachi, “Results of a Si/CdTe
Compton telescope,” Proc. SPIE, vol. 5922, pp. 78-88, 2005[32] H. Tajima, T. Mitani, T. Tanaka, H. Nakamua, T. Nakamoto, K. Nakazawa,T. Takahashi, Y. Fukazawa, T. Kamae, M. Kokubun, G. Madejski, D. Marlow,
M. Nomachi, E. do Couto e Silva, “Gamma-ray polarimetry with Compton
Telescope,” Proc. SPIE, vol. 5488, pp. 561-571, 2004

19