Gamma-ray astrophysics in the MeV energy range plays a crucial role for the understanding of many exciting cosmic explosions and cosmic accelerators (e. g. Supernovae, Novae, Supernova Remnants (SNRs), Gamma-Ray Bursts (GRBs), Pulsars, etc.). Gamma rays can be only gathered from space, with satellite missions like ESA’s INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory), launched in 2002 and now in orbit. The relevant radioactive isotopes in supernovae and novae emit in the energy range between 122keV and 2MeV (e. g. 56Co at 0.847, 57Ni at 0.122, 44Ti at 1.157, 7Be at 0.478, 22Na at 1.275 MeV). In order to detect these gamma-ray lines, an instrument with high sensitivity and high-energy resolution is needed.
The detection of photons in the MeV range is particularly challenging, since at this energy matter-radiation interaction occurs mainly through Compton scattering, which has the lowest cross-section (as compared to photoelectric absorption and pair creation, at lower and higher energies, respectively) and also is hard to handle. Instrumental noise related to electronics and to activation of the instrument materials (in space) is high in general, which adds an extra difficulty to detect the lines of interest. Another problem is that lines from explosive phenomena are broad, so that an increase in the sensitivity is required to detect them.
Imaging capability is also important in order to localise sources in crowded regions as the galactic centre and also to reduce the background. Different techniques to determine the position of a gamma-ray cosmic source are coded aperture system (coded mask), Compton camera and focusing Laue lens.
The main aim of this experimental activity is the design, development and test of new instruments based on radiation semiconductor detectors, to address the demanding performances of sensitivity, spatial and energy resolution. Cadmium Zinc Telluride (Cd(Zn)Te) and CdTe is the material selected for our detector due to its high detection efficiency and good energy resolution, besides the advantage of operating at room temperature. Silicon (Si) is also employed to manufacture detectors since it is a mature technology and its low cost.
One of the instrumental concepts that would achieve the aforementioned requirements is an advanced Compton camera. It is built by stacking CdTe pixel/strip detector modules. We have performed the CdTe pixel detector (with ohmic and Schottky contacts) hybridisation and developed the front-end electronics (FEE) associated with the detector (see Fig. 1) taking advantage of the expertise of our collaborator institutes IMB-CNM (CSIC) and IFAE. A measurement set-up (see Fig. 2) located at the Radiation Laboratory of the Institute, allows us to perform spectroscopic measurements (energy and spatial resolution, efficiency) at room and low temperature (see Fig. 3) under a controlled environment inside a vacuum chamber. The set-up consists of a customized Aluminium vacuum chamber, vacuum pump and cooling control equipment in order to cool down the detector.
In parallel with the instrumentation development, it is mandatory to understand the detection processes themselves, in order to improve the design and the performance of the detectors (see Fig. 4). This task is carried out by Monte Carlo simulations with Geant4. The Geant4 toolkit is used to simulate the photon and secondary particles transport in the active elements of the detector. In order to compute the energy resolution and angular resolution, Compton kinematics reconstruction is computed with the MEGALIB toolkit. An accurate mass model of the Cd(Zn)Te detector prototype, that includes passive material in the detector and its surroundings, true energy thresholds and energy and position measurement accuracy, is crucial to determine the energy deposited in the detectors of the prototype and predict its performance (energy resolution, angular resolution, efficiency and sensitivity).
Simulations of the space radiation environment are also required for radiation detectors suited for space missions (e.g. Fig. 5), using for instance SPENVIS and CREME96.
Gálvez, J., Hernanz, M., Álvarez, L., Artigues, B., Ullán, M., Lozano, M., Pellegrini, G., Cabruja, E., Martínez, R., Chmeissani, M., et al, Hard-X and gamma-ray imaging detector for astrophysics based on pixelated CdTe semiconductors, Journal of Instrumentation, 11, pp. C01011, 2016, jan, 10.1088/1748-0221/11/01/C01011.
Gálvez, J., Hernanz, M., Álvarez, L., Artigues, B., Álvarez, J. M., Ullán, M., Lozano, M., Pellegrini, G., Cabruja, E., Martínez, R., et al, Development of a pixelated CdTe detector module for a hard-x and gamma-ray imaging spectrometer application, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 9905, \procspie, pp. 99054B, 2016, jul, 10.1117/12.2231720
Álvarez, J. M., Gálvez, J., Hernanz, M., Isern, J., Llopis, M., Lozano, M., Pellegrini, G., Chmeissani, M., Imaging detector development for nuclear astrophysics using pixelated CdTe, Nuclear Instruments and Methods in Physics Research A, 623, 1, pp. 434 - 436, 2010, 1st International Conference on Technology and Instrumentation in Particle Physics, CdTe pixel detector, 10.1016/j.nima.2010.03.027
Gálvez, J., Hernanz, M. et al., “Pixel CdTe semiconductor module to implement a sub-MeV imaging detector for astrophysics”, Journal of Instrumentation, 12, pp. C03048, 2017, mar, 10.1088/1748-0221/12/03/C03048
Senior institute members involved
M. Hernanz, JL. Gálvez