Developments Toward the

Nuclear Compton Telescope

S. E. Boggs, S. Amrose, W. Coburn, G. Holland, R. P. Lin, R. Millan, D. Smith, Space Sciences Laboratory, University of California, Berkeley

M. Amman, M. T. Burks, E. L. Hull, P. N. Luke, N. W. Madden, Lawrence Berkeley National Laboratory

P. Jean, P. vonBallmoos, Centre d’Etude Spatiale des Rayonnements

W. Craig, K. Ziock, Lawrence Livermore National Laboratory

Abstract--Our collaboration has begun the design and development of a prototype high resolution Compton telescope utilizing 3-D imaging germanium detectors. The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2-15 MeV) telescope to study astrophysical sources of nuclear line emission and polarization. NCT will study gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. The instrument has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. We are currently developing a small NCT prototype for a conventional US balloon flight. This flight will critically test the novel instrument technologies, event analysis techniques, and background rejection capabilities we have developed for high resolution Compton telescopes.

I. INTRODUCTION

The Nuclear Compton Telescope will employ a novel Compton telescope design (Figure 1), utilizing twelve 3-D imaging, high spectral resolution germanium detectors (GeDs). The Compton imaging serves three purposes: imaging the sky, measuring polarization, and very effectively reducing background. An overview of the performance characteristics is presented in Table 1. NCT is designed to optimize sensitivity to nuclear line emission over the crucial 0.5-2 MeV range, and sensitivity to polarization in the 0.2-0.5 MeV range. NCT's guiding principle is that high efficiency and excellent background reduction are critical for advances in soft gamma-ray sensitivity. Design of the instrument has been summarized elsewhere [1], and a detailed analysis of the instrument performance and background calculations will be published at a later date.

At the heart of NCT is the array of 3-D positioning GeDs. Our collaboration has developed and tested large volume, 3-D positioning, cross-strip GeDs using LBNL's amorphous Ge contact technology [2]. Timing techniques for measuring the third dimension (depth) have been verified [3,4], and we have fabricated and tested an analog, 3-D readout electronics chain that achieves <1.4 mm FWHM resolution at 60 keV [5]. The system includes a novel charge sensitive preamplifier utilizing predominantly surface mount components. This preamp achieves excellent spectroscopic performance in a small footprint, and at modest cost and low power [6]. In parallel, we have actively developed Compton reconstruction techniques for high-resolution instruments [7], and have extensively simulated Compton telescope performance [8].

Our current prototype 19×19 strip GeD is an 11.0-mm thick, p-type planar detector (Figure 2). Orthogonal strips were deposited on both faces of the GeD, with a strip pitch of 2.00 mm, and a 0.50-mm gap between strips. The strips define an active area of 3.8 x 3.8 cm². A 4-mm wide guard ring surrounds this active area on both faces of the detector. The depletion voltage is –1600 V, and the GeD was operated at -2000 V for these tests. The strips were instrumented on both the ground side (DC coupled) and HV side (AC coupled) with our custom low power, low noise preamplifiers.

Table 1. Performance at 1 MeV (Polarimetry at 0.2-0.5 MeV).

NCT Performance	ConUS Flight (1.5×10⁴s)	LDB Flight (2×10⁵s)
# GeDs	12	12
GeD Volume	1044 cm³	1044cm³
Effective Area	13.9 cm²	13.9 cm²
DE	2.49 keV FWHM	2.49 keV FWHM
ARM (1s)	2.03	2.03
Narrow Line (3s)	5.1×10^-5 g/cm²/s	1.2×10^-5 g/cm²/s
Continuum, DE=E	6.0×10^-4 g/cm²/s/MeV	1.4×10^-4 g/cm²/s/MeV
100% Polarization	32 mCrab	7.5 mCrab

II. Design Overview

The NCT detector configuration is presented in Figure 1. NCT is an array of twelve 3-D GeDs. Our next GeD design has 39×39 2-mm pitch strips, and is 15.0-mm thick, with a total active volume of 87 cm³. The 15 mm thickness has been chosen as a compromise between producing thick detectors to minimize the number of electronics channels per volume, and keeping the detectors thin to optimize the scattering efficiency between layers in the gamma-ray line energy range (0.5-2 MeV).

Figure 1. The heart of NCT is an array of 12 crossed-strip GeDs with 3-D position resolution, excellent spectroscopy, and high efficiency.

Here we have arranged the GeDs in a somewhat unconventional manner for a Compton telescope: the detectors are set vertically with the edge of the detector facing the collimator opening, and hence the astrophysical sources. This configuration optimizes the effective area per unit GeD volume in the 1-2 MeV range. Also, this detector configuration is most efficient to photons that Compton scatter at right angles in the instrument, optimizing sensitivity to polarized radiation. This compact design allows enclosure in an active shield. All of the stopping power required vertically is present, so adding more “modules” horizontally can easily scale the design. The gap between detectors was set at 15 mm. Such a compact, high efficiency geometry can only be achieved by using detectors with fine 3-D imaging capabilities.

The GeDs will be housed in a common cryostat, which is attached to a 50 liter liquid nitrogen dewar to provide >20 days of cooling at 85 K. The GeD array is enclosed on the sides and bottom by a 5-cm thick active bismuth germanate (BGO) anticoincidence shield. A 10-cm thick CsI front shield collimates the detector field-of-view (FOV) to 40º×40º. The active shield is another unconventional component for Compton telescope designs, but plays a crucial role in helping reduce the background induced by the atmospheric gamma-ray emission to a level where the relatively small GeD volume achieves high sensitivity.

In the compact NCT geometry and with GeD timing performance, time-of-flight (TOF) measurements are not able to distinguish the photon interaction order. We have developed several techniques for reconstructing this interaction order independently without the TOF [7]. We also showed that the Compton telescope performance depends on the choice of ‘event cuts,’ such as the minimum permitted distance between the first and second scatter (lever arm), and whether 2-site interactions and photons which backscatter (q₁>90º) are accepted [8]. An overview of the NCT performance characteristics is given in Table 1. Detailed analysis of the event cuts, instrument performance, and background calculations will be presented elsewhere.

III. Technology Developments

When a photon interacts in a GeD, by photoabsorption or Compton scattering, a fast recoil electron is produced which knocks more electrons from the valence band to the conduction band, leaving holes behind. The number of e-h pairs is directly proportional to the energy deposited (1 e-h pair per 2.98 eV). In an applied electric field (bias voltage) these e-h pairs will drift in opposite directions, electrons toward the anode and holes toward the cathode. By segmenting the anode into 2-mm pitch strips, and the cathode into orthogonal 2-mm pitch strips, 2-D positioning is achieved directly through identification of the active anode and cathode strips.

Figure 2. Sketch of our prototype 19×19 orthogonal strip GeD.

A. 3-D Positioning

The e-h pairs have finite drift velocities, ~1 cm per 100 ns. Positioning in the third dimension is achieved directly by the difference between the electron collection and the hole collection times on opposite faces of the detector. We have demonstrated that the difference in collection times provides a very linear measurement of the photon interaction depth in the detector [3,4].

Figure 3 shows the distribution of anode-cathode times for photon interactions using a ⁵⁷Co (122 keV) source held in front of the anode (DC) and cathode (AC). The best-fit exponential attenuation curves are shown for comparison. The attenuation coefficients for both DC-illuminated photons and AC-illuminated photons are consistent with each other, and agree with the mean free path for 122 keV photons in Ge. The linearity of these curves, and their consistency from one side to the other, demonstrates the robust depth measurement provided by the timing signals.

By digitizing the charge collections signals with a fast oscilloscope and analyzing the charge collection times, we have determined that it is possible to achieve < 1 mm depth resolution for 60 keV interactions. This resolution improves at higher energies until the recoil electron stopping length becomes significant.

Figure 3. Demonstration of the expected exponential attenuation curves for 122-keV photons when illuminating the DC and AC sides of the GeD. The e-h collection times provide a robust, linear measurement of the interaction depth.

We want to avoid digitizing the charge signals for flight instruments due to the power and size constraints. Instead, we prefer a simple analog circuit for timing the charge collections. To this end, we have developed a simple analog constant fraction discriminator (see below), which records the time when each channel signal reaches 50% of its maximum amplitude [5]. This system has achieved < 1.4 mm FWHM resolution at 60 keV – not as high as the digitizing technique, but a reasonable trade off for a much simpler, low power system.

B. Amorphous Contacts

Our detectors utilize amorphous Ge contact technologies pioneered by LBNL [2]. In this technique, the blocking electrode is made from a ~0.1-mm thick layer of amorphous Ge which is deposited on the entire detector surface. The strip electrodes are defined by evaporating a layer of metal through a shadow mask on top of the amorphous Ge film. The amorphous Ge film serves as the blocking contact, and fully passivates every detector surfaces not to be used for contact connection. The bipolar blocking behavior of the amorphous contacts allows them to be used on both sides of the detector, replacing both the conventional n-type Li and the p-type ion implanted contacts. The amorphous germanium can be made highly resistive which provides excellent inter-electrode impedance [9], and has been demonstrated to significantly improve the charge collection for events that occur in the gap region, allowing excellent spectroscopy even for events where charge is shared by neighboring strips [3].

The amorphous contact fabrication has proved reliable, providing high yields of successful detectors. Only a few simple steps are involved in the manufacturing process. The amorphous contacts are also robust. Our first prototype, 5-strip detector has operated successfully for 3 years with no measurable degradation of the performance. In addition, the contacts have been shown to be stable with temperature up to 100º C for more than 12 hours, which will permit detector annealing for radiation damage repair in future satellite applications if required.

C. Readout Electronics

Our signal processing chain utilizes conventional GeD quality signal processing electronics. Each detector strip has a compact, low power signal processing chain. Detector signal extraction is accomplished with a unique charge sensitive preamplifier, in which excellent spectroscopic performance is achieved in a small footprint and at modest cost and low power, without sacrificing signal bandwidth [6]. A much-simplified pulse-shaping amplifier, with both a fast channel for timing and a slow channel for spectroscopy, follows each preamplifier. Spectroscopy signals uniquely match the pairs on opposite faces for multiple interactions in the same GeD.

The slow channel consists of a single-stage Sallen-and-Key shaping amplifier with 4 ms peaking time, and is followed by a peak detect and stretch function. Our current generation of electronics has achieved spectral resolutions of 1.1 keV FWHM on the DC-coupled strips, and 1.7 keV FWHM on the AC-coupled strips [5]. The degraded resolution on the AC strips is due to microphonics pickup in the cryostat design, which has already been corrected in later generation cryostats.

The fast channel has a 50% constant fraction discriminator (CFD) that generates a low time walk signal. A simple time to amplitude converter, converts CFD signals to an amplitude that is proportional to the signal charge arrival time at the anode or cathode for the depth determination. This circuit achieves <1.4 mm FWHM depth resolution for 60 keV photons [5].

We are currently prototyping a version of this readout system for flight using conventional surface mount components. The flight electronics will be much smaller in size (~10 cm² per channel), and should maintain the excellent noise performance and positioning resolution of the laboratory system. The expected power for the flight electronics is ~100 mW per channel.

D. Spectroscopy & charge collection

We have been extensively testing the spectral performance of our 19×19 strip prototype detector [10]. The spectral performance is found to be very uniform among the strips (Figure 4). The variation in spectral resolution between DC strips is found to be <10% over the whole detector, when measured with photons at 60 keV, 122 keV, and 662 keV. There is direct evidence of a small amount of charge trapping in the gap between the strips – the resolution of events shared between two neighboring strips is degraded over what is expected by adding the electronic noise in quadrature (Figure 5). The effects of this charge loss are minimal, and should be completely correctable by using the ratio of energy shared between strips and the additional depth positioning information.

Figure 4. Measured spectral resolutions for our 19×19 prototype GeD using bench electronics with optimized peaking times (12 ms). The DC side shows consistent resolutions of 1.0 keV FWHM. Resolution is degraded on the AC side (especially channel 6) due to microphonic pickup in the cryostat, which has been alleviated in our later cryostat designs.

IV. Test Flight

We are developing a 2-GeD prototype version of NCT for a 1-day continental US balloon flight. This prototype flight will test event reconstruction and imaging techniques unique to high resolution Compton telescopes in the space environment, as well as demonstrate the excellent background rejection capabilities and high sensitivity of our compact, GeD design.

V. Scientific Goals

Nuclear astrophysics studies the lifecycle and evolution of matter in our Universe: stellar evolution ending in supernovae, with the ejection of heavy nuclei back into the galaxy to be reborn in new stars. Radioactive nuclei produced through this cycle of creation emit characteristic photons that fingerprint the isotopes themselves, and quantify their abundance, speed, temperature, and attenuation. High spectral resolution measurements of these line emissions probe deep into the center of a supernova explosion, revealing the nuclear burning and dynamics in the core. Nuclear excitations and positron annihilations also reflect the extreme environment on the surface of neutron stars and white dwarfs, and near the event horizon of black holes. NCT is an important advance over current and historical gamma-ray instruments in two regards: its development and flight will provide a testing ground for novel event analysis, background reduction, and imaging techniques for gamma-ray astronomy, and break new ground in the measurement of polarized gamma-ray emission from astrophysical sources.

Figure 5. Charge sharing between strips. Plotted are 122 keV spectra as measured by (left) individual strips in anticoincidence, (middle) nearest neighbors in coincidence, (right) strips separated by at least one strip, in coincidence (Compton scatters). The degradation in nearest-neighbor spectra is direct evidence for slight charge trapping in the gaps [10]. This should be fully correctable using the ratio of energies on the strips, as well as the depth measurements.

VI. Summary

The NCT compact geometry achieves high photopeak efficiencies, increasing the effective area per unit detector volume by a factor of ~100 over COMPTEL/CGRO [11]. This is possible through the use of high 3-D spatial resolution detectors. For a photon or background event, the ability to spatially resolve interactions and measure the energy depositions with high resolution is a powerful new tool for increasing the sensitivity of gamma-ray instruments. Compared to INTEGRAL/SPI [12], we have decreased the background per unit detector volume by factors >30. However, the phase-space for improvement in background rejection techniques is largely unexplored. NCT will provide a crucial testing ground for developing novel background techniques to their full potential. The NCT/ACT sensitivities we presented in Table 1 are conservative upper limits. We believe these sensitivities can be significantly improved through further development of background rejection techniques utilizing the high spectral and spatial resolution of our 3-D GeDs.

VII. References

[1] S.E. Boggs, P. Jean, R. P. Lin, D. M. Smith, P. vonBallmoos, N.W. Madden, et al., "The Nuclear Compton Telescope: a balloon-borne soft gamma-ray spectrometer, polarimeter, and imager," Proc. Gamma 2001 Conf., Baltimore, MD, Apr. 2001.

[2] P.N. Luke, C.P. Cork, N.W. Madden, C.S. Rossington, M.F. Wesela, “Amorphous Ge bipolar blocking contacts on Ge detectors,” IEEE Trans. Nucl. Sci., vol. 39, no. 4, pp. 590-594, Aug. 1992.

[3] M. Amman and P.N. Luke, “Three-dimensional position sensing and field shaping in orthogonal-strip germanium gamma-ray detectors,’ NIM, vol. A452, pp. 155-166, 2000.

[4] S. Amrose, S.E. Boggs, W. Coburn, G. Holland, R. P. Lin, D. M. Smith, “Numerical simulations of 3-D positioning in cross-strips Ge detectors,” this conference.

[5] M.T. Burks, M. Amman, S. E. Boggs, E. L. Hull, P. N. Luke, N. W. Madden, et al., “A Germanium gamma-ray imager with 3-D position sensitivity,” this conference.

[6] L. Fabris, N.W. Madden, H. Yaver, “A fast, compact solution for low noise charge preamplifiers,” NIM, vol. A424, pp. 545-551, 1999.

[7] S.E. Boggs and P. Jean, “Event reconstruction in high resolution Compton telescopes,” Astron. & Astroph. Sup. Ser., vol. 145, pp. 311-321, 2000.

[8] S.E. Boggs and P. Jean, “Performance characteristics of high resolution Compton telescopes,” Astron. & Astroph., vol. 376, pp. 1126-1134, 2001.

[9] P.N. Luke, R.H. Pehl, F.A. Dilmanian, “A 140-element Ge detector fabricated with amorphous Ge blocking contacts,” IEEE Trans. Nucl. Sci., vol. 41, no. 4, pp. 976-978, Aug. 1994.

[10] W.C. Coburn, S. E. Boggs, S. Amrose, R. P. Lin, N.W. Madden, P.N. Luke, et al., “Results of charge sharing tests in a Ge-strip detector,” this conference.

[11] V. Schoenfelder, H. Aarts, K. Bennett, H. deBoer, J. Clear, W. Collmar, et al., “Instrument description and performance of the imaging gamma-ray telescope COMPTEL aboard the Compton Gamma-Ray Observatory,” Astroph. J. Supp. Ser., vol. 86, pp. 657-692,1993.

[12] P. Jean, G. Vedrenne, V. Schoenfelder, F. Albernhe, V. Borrel, L. Bouchet, et al., “The spectrometer SPI of the INTEGRAL mission,” Proc. 5^th Compton Symp., AIP Conf. Proc., vol. 510, p. 708, 2000.