I5 : Gamma-ray instrumentation development

Introduction

Supernovae, the end point of the massive stars evolution, occur at a rate of 2-3 per century in a Galaxy such as the Milky Way. The radioactive elements synthesized in these explosive events decay subsequently and emits gamma-rays. In the days following the explosion, the gamma-ray photons loose energy through inelastic scatterings in the thick supernova envelope and emerge as visible light revealing the event. In the following months, as the envelope extents and get thinner, gamma-rays can escape. This gamma-ray line emission provides a direct measure of the amount of synthesized species.

The atmosphere being opaque to gamma-rays, large space telescopes have been developed to measure the gamma-ray lines (0.5 – 1.3 MeV) emitted by the unstable nuclei forged during supernova explosions. However despite the numerous supernovae observed in the visible light during these high-energy observatory lifetimes, in only one occasion, gamma-ray lines from a supernova have been clearly detected by a telescope on board a satellite. All the other supernovae were too distant to allow for a detection of the emitted gamma-ray lines. One way forward to ensure these detections in a mission lifetime is to build more sensitive and more expensive detectors but, before funding them, a clear demonstration of their capabilities is required.

One can then conceive developing new gamma-ray detectors in a continuous R&D program and prove the capability and ruggedness of the products by performing observations from a high altitude (40 km) balloon. After each successful demonstration flight, the payload (or a copy) can be stored until a better detection payload has demonstrated its superiority and will replace the previous one. That way a qualified payload is always ready to fly and continuously improved. Should a supernova explode in our Galaxy or in the local group, a flight can readily be designed to observe the supernova a few months after the explosion with the best possible detector technology. In fact most of the detections of the ⁵⁶Co line from a supernova (SN1987a) were made with modest gamma-ray detectors on board balloons (see figure aside). Should the progress of the detectors be such that it is proven (with margins) that any supernova in the Virgo cluster (the nearest one) will be clearly detected by a large satellite experiment, a proposal to develop such a mission can be submitted in response to the first ad hoc announcement of opportunity of a space agency. Meanwhile regular flights can be scientifically dedicated to specific objectives either in astrophysics or atmospheric physics.

Since the Comptel experiment on board the CGRO mission, little progress has been made in the observation of the sky between 1 MeV and 50 MeV. The Compton scattering dominating the other photon-matter interactions up to 8 MeV, the experimental community has been very active to study various concepts of Compton telescopes using scintillators or/and semi-conductor detectors. However the exercise is very difficult and many of the bright ideas based upon Monte-Carlo simulations were revised after an unsuccessful balloon flight. Here clearly the balloons have proven to be very useful in avoiding the building of a large, expensive but inefficient satellite experiment. We think that the progress in microelectronics should permit today the building of an original concept, a “pure” silicon Compton telescope, without heavy calorimeter. Testing on a balloon a breadboard of such a telescope would be very useful. Moreover, an interesting astrophysical observation seems at reach for this breadboard. The linear polarization of the Crab nebula has been observed at visible, X-rays and gamma-ray wavelengths. The surprising result is that while the polarization angles observed in the visible and in gamma-rays (E>200 keV) agrees (~ 123°), that observed in X-rays (2--5 keV) differs significantly (156°). There should be transition spectral regions around the X-rays. Compton telescopes are naturally very good polarimeters. A measurement in the range 50 – 200 keV will refine the position of the change of regime between the X-rays and the gamma-rays and set constraints on the synchrotron nebula model.

AIM and APC have in common a long history of space gamma-ray detectors (COS-B, GRANAT/SIGMA, INTEGRAL/ISGRI) and are still actively developing new ones (TARANIS/XGRE).

One can note that the total duration of a balloon project is of the order of a few years, i.e. close to the nominal duration of a PhD. A student involved in the detector development, the instrument and mission design, the payload system aspects, the attitude stabilization and/or restitution, the data flow, the telemetry and the data analysis will develop the skills that are necessary to drive or contribute to scientific projects. In other words it is a very good Principal Investigator and Instrument Scientist school. On the technical side, students from engineering schools can be also involved in such a project, dealing with the mechanics, the thermal control, the electronics and the project management, all under the planning pressure. This is a way to educate future System Engineers and Project Managers.
 

Description of the measurement technique

Taking advantage of the Compton scattering formula to constrain the incoming photon direction was the original idea of Compton telescopes. Provided the scattered photon is absorbed, the incoming photon energy is the sum of the two energy deposits. The interaction positions (X1, Y1), (X2, Y2) give the scattered photon  direction and θ the angle between the incoming and scattered photon directions is given by the energy deposits  through the Compton formula. The incoming photon direction belongs therefore to a cone of opening angle θ and whose axis is the scattered photon direction. In a classical Compton telescope, two position sensitive spectrometers are used: a scatterer and a calorimeter.

The scatterer should have a high scattering efficiency but should allow for only one interaction, otherwise the interaction positions and the energy deposits may be confused. This double requirement is a difficult one. Using detectors with a low atomic number helps reducing the probability of an absorption in the scatterer. The thickness is then an issue. A thin detector is not efficient enough and in a thick detector there will be always more than one interaction. One way out is to use a stack of thin detectors, thin enough to exclude the possibility of a second interaction in the same detector. The scatterer efficiency is then governed by the number of detectors and could be as large as resources allow. Ideally, we would like to have it large enough to ensure a full absorption, i.e. the scatterer would be itself the calorimeter.

Figure 2: Schematics illustrating the principle of a Compton telescope made with a stack of thin detectors

a) Case of a scattering followed by an absorption

b) Case of three scatterings without absorption

However, in space, electric power is limited and a compromise should be made implying probably the presence of a classical imaging calorimeter. The calorimeter must optimize the absorption and will use therefore high atomic number elements. It should also be a fast and good spectrometer and allow for a decent spatial resolution.

Given the constraints of space experiments, fulfilling the scientific requirements is not an easy task and this explain why no other Compton telescope followed COMPTEL (1991-2000), the first Compton telescope onboard a satellite. However, technological progresses allow today the design of much more sensitive experiments.
 

The scatterer

The best spectrometer is high purity cooled germanium. However, the scatterer should minimize the dead mass, i.e. non-detecting material in which photon can interact without notice. The cooling requirement is not very good from this point of view. To stay with semiconductors, silicon has a lower Z, is a good spectrometer and do not require active cooling.

It can be readily realized that for a square meter class instrument the number of pixels per layer may be very large (10⁶) and that imply an excessive power consumption. To give an idea, lets assume a power consumption of 1 mW/channel, a hundred layers would require 100 kW. For that reason, the use of strip detectors with 2N independent channels for N² pixels is required. Double Side Stripped Detectors (DSSDs) have been widely used in particle tracking experiments (e.g. CMS)

Figure 2: Schematic of a Double Side Stripped Detector (DSSD)

In space, Fermi uses couples of Single Side Stripped Detectors (SSSDs) for monitoring the electron and positron tracks because there is no need to measure the energy deposits. It is not possible to use these detectors if an energy deposit measurement is required as in a Compton telescope. SSSDs relates energy deposits with only one coordinate while DSSDs give the two coordinates related to an energy deposit.

The R&D program