3 Measuring Galactic Gamma-Ray Signals with INTEGRAL/SPI
3.2 INTEGRAL and its Spectrometer SPI
d
l 2
. (3.9)
In fact, it is most ecient to construct a coded-mask instrument for which the transparent and opaque elements have the same size (Skinner 2008), so that the angular resolution is only depends on the ratio d/l. Decreasing the pixel elements is similar to constructing a pinhole camera but suers from detection eciency.
Increasing the separation between mask and detector ("focal length") is only possible as far as the instrument ts on a satellite to be launched into space. Typically, the angular resolution of MeV coded-mask telescopes is thus of the order of degrees.
In order to resolve ambiguities on the detector plane, due to the fact that there are more "sky pixels" than "detector pixels", the instruments are "dithered" from pointing to pointing around the object in the sky, so that there will be also temporal coding in addition to the spatial coding of the mask. The specications of the SPI instrument will be discussed further in Sec. 3.2.2.
3.2 INTEGRAL and its Spectrometer SPI
3.2.1 The INTEGRAL Mission
The INTEGRAL satellite is the INTErnational Gamma-Ray Astrophysics Laborat-ory of the European Space Agency. The spacecraft was planned during the Compton Gamma Ray Observatory (CGRO) mission as a follow-up instrument to observe the
"violent" high-energy universe. INTEGRAL was planned as a three-year mission with possible extension up to ve years (Winkler et al. 2003). Due to its great suc-cess in deciphering the messages from gamma-ray sources throughout the Milky Way and beyond, the mission duration was extended several times and is still operating and taking data (year 2016).
INTEGRAL was launched on October 17, 2002, from the spaceport in Baikonur,
Figure 3.5: The INTEGRAL spacecraft. Dimensions are(5×2.8×3.2) m. The deployed solar panels are 16 metres across. The mass is 4 t(at launch), including 2 t of payload. The main instruments, the gamma-ray spectrometer telescope SPI and the soft gamma-gamma-ray imager IBIS, as well as the complementary instruments, JEM-X for soft X-rays and OMC for visible light, are shown; picture from Winkler et al.
(2003).
Kazakhstan, by a PROTON rocket into a high-inclination and high-eccentricity orbit. It carries four co-aligned instruments, the gamma-ray spectrometer telescope SPI (Sec. 3.2.2), the soft gamma-ray imager IBIS, the soft X-ray monitor JEM-X (two identical units), and the optical monitoring camera OMC. The two latter instruments are complementary to the main telescopes SPI and IBIS.
Figure 3.6: Chosen trajectory for the INTEGRAL spacecraft. The Earth is shown as the blue sphere with the launch point in Baikonur, surrounded by the inner and outer Van Allen radiation belts in red and orange, respectively. The low-Earth parking orbit and the nal upper stage boosting for the transfer to the high-inclination, high-eccentricity orbit are indicated, as well as the apogee injection towards the nal orbit.
The orbit, shown in Fig. 3.6, was chosen to avoid enhanced radiation near Earth, because the planet is surrounded by tori or energetic charged particles, the Van Allen radiation belts. The particles which build the belt are mainly from the solar wind or cosmic rays, captured by the Earth magnetic eld, and bound to it due to a magnetic bottle eect of the terrestrial magnetic dipole eld. The inner belt is located between ≈ 3000 and 6000 km above the surface and mainly consists of high-energy protons. The outer belt reaches up to altitudes of ≈ 25000 km, thereby bracing the inner belt, and is built up of electrons. Due to the enhanced particle densities, increased electronic malfunctions can occur, and also the radiation damage to the instrument detectors (cameras) would degrade the sensitivity and measurement quality quickly. Therefore, the orbit was chosen highly eccentric with a perigee height of≈9000 kmand an apogee height of≈154000 km, at an inclination of52.5◦ with respect to the rotation axis of the Earth. This leads to a≈3 dayorbital period1 in which the instruments typically take data between orbital phases ≈ 0.1 and ≈0.9, corresponding to nominal altitudes of ≈50000 km, thereby avoiding the radiation belts.
For this thesis, data from the spectrometer SPI were used. SPI is described in detail in Sec. 3.2.2. Nevertheless, the other instruments aboard INTEGRAL will be introduced briey below.
IBIS is the Imager on-Board the INTEGRAL Satellite, dedicated to observe in the energy range from 15 keV to 10 MeV. In addition to its coded mask aperture, it is using two detector array layers to function also as a Compton telescope. The one layer, called ISGRI (INTEGRAL Soft Gamma-Ray Imager), consists of 128× 128 pixels made of CdTe, building an area of ≈ 2600 cm2. Due to the fact that these detectors are very small, they provide an excellent spatial resolution, but their thinness makes them only suitable for energies between 15and 150 keV. The other layer is called PICsIT (Pixellated Caesium-Iodide Telescope), and is made of64×64 CsI scintillators, doped with Tl, with a total area of2890 cm2. The mask is situated 3.4 m above the detector layer, and enables a fully-coded eld of view of9◦×9◦, and a partially-coded eld of view of 19◦ ×19◦. Due to the large number of detectors, combined with the Compton telescope principle, the spatial resolution is around 12 arcmin; the point source location accuracy is 30 arcsec. The energy resolution of IBIS is9-10%(∆E =E/2), and thus focusses on sources with a spectral continuum rather than spectral gamma-ray lines (Ubertini et al. 2003).
The Joint European X-Ray Monitor (JEM-X) is a complementary instrument, provid-ing additional information on the identication of gamma-ray sources. It is made of two co-aligned telescopes, each of them using its own coded mask. Each detector is an imaging microstrip gas chamber and has an area of ≈ 500 cm2, providing an angular resolution of3 arcminover a fully-coded eld of view of10◦×10◦. JEM-X is recording spectra in the energy range3-35 keV, with an energy dependent resolution of ∆EF W HME = 0.40q
1
E[keV] +601 (Lund et al. 2003).
The Optical Monitoring Camera (OMC) is rounding o the INTEGRAL spacecraft, being a classical camera which is using refraction of optical light around a wavelength
1INTEGRAL will be deorbitted safely in 2029. For this manoeuvre, the boosters have been used in 2015 to bring the satellite into a lower and thus faster orbit around Earth in order to decelerate again by residual atmosphere at these altitudes. In particular, this leads to a shorter orbital period after INTEGRAL's 1500th revolution around Earth.
Table 3.1: SPI instrument characteristics; values taken from Vedrenne et al. (2003), Winkler et al. (2003), Roques et al. (2003), and Siegert et al. (2016d).
Parameter
Energy range 20 keV-8 MeV
Detectors 19 high-purity Ge detectors, cooled to≈85 K Detector area 508 cm2geometrical;10−100 cm2 eective Detector thickness 7 cm
Spectral resolution (FWHM) 2.1 keVat 511 keV;3.1 keVat 1809 keV 3σContinuum sensitivity 1.5×10−6ph cm−2 s−1keV−1 at 511 keV 3σLine sensitivity 5×10−5 ph cm−2 s−1 at 511 keV
Field of view 16◦×16◦(fully coded);31◦×31◦(partially coded) Angular resolution (FWHM) 2.7◦
Source location (radius) 10 arcmin 3σAbsolute timing accuracy ±52µs
Total mass 1228 kg
Power (max/avg) 385/110 W
of 550 nm, and focussing it on a CCD (charge-coupled device). It is supporting the INTEGRAL mission by observing the optical emission from the prime targets of gamma-ray emission. It provides the brightness and position of the optical coun-terpart of any gamma- or X-ray transient taking place within the eld of view of 5◦×5◦. The angular resolution amounts to24.5 arcsec, and the point source location accuracy to ≈ 6 arcsec. The "1-Megapixel" camera can identify sources down to magnitudes V = 18 (Mas-Hesse et al. 2003).
3.2.2 The Spectrometer SPI
SPI is the high spectral resolution gamma-ray spectrometer aboard INTEGRAL and uses a coded-mask aperture to "focus" gamma-rays in the energy range between 20 keV and 8 MeV onto its 19-segmented Ge detector array (Vedrenne et al. 2003).
The angular resolution, which SPI can achieve in its fully-coded eld of view of 16◦ × 16◦, is 2.7◦. Due to the solid state Ge detectors, SPI obtains a spectral resolution of ≈ 2.1 keV at 511 keV, depending also on the temperature, and the current state of degradation of SPI due to cosmic-ray bombardment, Sec. 3.2.3.1. In the following sections, the sub-systems of SPI will be described, that are necessary for high-precision spectroscopy in space. Table 3.1 provides an overview of the key parameters of the instrument.
3.2.2.1 Camera
The centrepiece of SPI is its camera, consisting of 19 high-purity Ge detectors, working as reverse-biased n-type diodes. Each of the detectors has a hexagonal shape with a side length of3.2 cmand a height of 69.42 mm, for a total geometrical area of 508 cm2. The 19 detectors are ordered in a honeycomb structure, and numbered from 00 to 18, beginning with detector 00in the centre (Fig. 3.7). This design was chosen to provide a large eective area for incoming photons, and at the same time to have a compact setting. In Fig 3.8b, one of the detectors is shown, encapsuled in Al, working as the cathode for the external high-voltage of 4 kV.
Figure 3.7: Schematic drawing of the detector numbering of the SPI camera. Symmetry-axes and the direction to the IBIS-instrument, Sec. 3.2.1, are shown.
The gradual deterioration of the Ge detectors, due to cosmic rays hitting the space-craft, is countered by heating up the whole camera twice a year in a so-called "an-nealing" period. In particular, high-energy cosmic-ray particles interact with the crystalline structure of the Ge detectors and spoil the regular structure. If a re-lativistic particle hits a Ge atom in the lattice, the recoil momentum may be so large that the atom may be left in another position in the lattice, building either vacancies or interstitials. These irregularities in the lattice structure lead to a de-crease in the charge collection eciency of the detectors, because electrons or holes can be trapped in these imperfections. The event readout time in the instrument is calibrated to be nite, but the trapped charges would invoke a time delay which lengthens the processing time. The resulting pulse shape is consequently dierent compared to a regular lattice structure. The eect on the spectrum of a mono-energetic source will be a shift of the peak energy, and an asymmetric broadening of the instrumentally resolved line shape (Kretschmer, K. A. 2011).
SPI exhibits a Stirling-cycle cooling system in order to cool the entire Ge detector array down to≈85 K, so that less thermally excited electrons populate the conduc-tion band of Ge for an almost leakage current free semiconductor detector. Because of the detector degradation, the entire array is heated up to 105 ◦C for about two weeks twice a year. This annealing process, which is related to tempering, repairs the lattice structure and resets the instrumental resolution back to an acceptable value.
In order to function as a gamma-ray telescope, the SPI mask is situated 1.71 m above the detector array with a diameter of 0.72 m which results in a fully-coded eld of view of16◦×16◦ and a partially coded eld of view of 31◦×31◦. It consists of 127 hexagonal elements, 63 being opaque to gamma-rays, and 64 transparent.
The opaque pixels are made of W, are 30 mm thick, and have otherwise the same dimensions as the detectors. The mask pattern is 120◦-rotational symmetric about its centre, see Fig. 3.8d. The limiting factor of the spatial resolution is the number
(a) Single Ge detector. The central bore is0.6 cm
in diameter and5.5 cmin length. (b) Ge detector ight mode, encapsuled and wired.
(c) Complete detection cold plate assembly in a vacuum
box for thermal tests. (d) SPI mask with opaque W elements.
Figure 3.8: The SPI camera system; pictures from Vedrenne et al. (2003) and the SPI team.
of mask pixels that are used in reconstructing the shadowing pattern in relation to the number of camera "pixels"; in the case of SPI, the angular resolution is≈2.7◦. Because the mask is shadowing about 50% of the incoming celestial photons, the geometrical area is reduced. In addition, as high-energy photons are more likely to pass through the Ge detectors without interaction, the area is reduced even further, leading to an eective area between≈10and≈100 cm2. The point-source location accuracy of SPI is ≈10 arcmin.
3.2.2.2 Anticoincidence systems
Satellite-based nuclear astrophysics telescopes suer from both the problem of focus-sing gamma-rays, and cosmic-rays steadily exciting the instrument and spacecraft material. Therefore, the camera system has to be protected against photons from
"other" directions than what the mask is trying to focus, and also cosmic-rays. SPI has several anticoincidence systems which try to reduce the photon and particle background.
The SPI "anticoincidence system" (ACS, Figs. 3.9a and 3.9b) is made of 91 separ-ate BGO crystals, arranged in four sub-units, surrounding the camera system, and shielding it from all directions. The upper collimator ring, lower collimator ring, and the side shield assembly, are each made of 18 BGO crystals, and the lower veto shield is made of 36 crystals. The BGO blocks have thicknesses of 16 mm at the top, to 50 mm at the bottom. The additional spacecraft mass due to BGO only is 512 kg.
Scintillating BGO crystals emit photons in the range between ≈375-650 nm when exposed by high-energy gamma-rays. Each of the SPI ACS BGO crystals is viewed by two (for redundancy) photomultiplier tubes in order to trigger a veto-signal, if SPI is hit from the "wrong" direction. Because only the total event rates over all SPI ACS crystals are recorded, there is no spectral and no directional information available from this anticoincidence system. However, because it has a large eective area of up to≈1 m2, and fast timing, it is also used for studying gamma-ray bursts (GRB, e.g. von Kienlin et al. 2004; Larsson et al. 2004).
During a typical SPI observation of a 5×5pointing grid (see Sec. 3.2.4.3), the SPI ACS count rate varies a few percent from its mean value, which ranges between 5×104 and 105 ph s−1, depending on the state of the solar cycle, and can reach much higher values during solar ares.
(a) SPI ACS BGO blocks in hexagonal
arrangement. (b) SPI ACS bottom shield viewed by
PMTs. (c) PSAC in a light diusion chamber,
viewed by PMTs.
Figure 3.9: SPI anticoincidence systems; pictures from Vedrenne et al. (2003) and the SPI team.
For further reduction of the instrumental background near the511 keVline, another veto unit was installed in SPI, directly below the mask, the plastic scintillator an-ticoincidence subassembly (PSAC, Fig. 3.9c). The PSAC is 5 mm thick and0.8 m in diameter, and viewed by four photomultiplier tubes. Before launch, Jean et al.
(1997) simulated a plastic scintillator veto system below the mask, and estimated a gain of sensitivity in measuring the511 keV line of factor of 1.4. However, the eect of the real PSAC is only "modest" (Vedrenne et al. 2003), and the sensitivity of the 511 keV line is now (15 of 19 detectors working) around5-6×10−5 ph cm−2 s−1 for an exposure of1 Ms (Siegert et al. 2016d).
Another system that was implemented in SPI, trying to reduce the background originating in the energy deposition of β±-particles into the detectors, is a pulse-shape discriminator circuit. In contrast to photons, charged particles do not deposit their whole energy at one point in the solid state detector, but rather create a trace of ionisation (Knoll 2010). This changes the pulse shape of the collected charges and should therefore ideally be distinguishable from photons, further reducing particle induced backgrounds by sending veto signals. Mandrou et al. (1997) suggested a
pulse-shape discriminator system for SPI, and Jean et al. (1997) simulated the eect of such a system in association with SPI. The estimated factor of2(Jean et al. 1997) was not reached in reality, because there seems to be a relatively low fraction of localisedβ-decays in the target energy range, between 0.4 and 2.0 MeV, with respect to model predictions (Vedrenne et al. 2003). However, in the energy range between
≈1400 keV and 1700 keV, there are background features, called "electronic noise", which can be "ltered" eciently by pulse-shape selections. Although the raw, background-dominated, spectrum is reduced by≈15%, the systematic uncertainties in this energy range is increased because the instrumental response function, and the real eciency using these selection criteria, are not well known (see e.g. Appendix of Siegert et al. 2016a).
Figure 3.10: Cut-out perspective of the SPI spectrometer viewed from the side. The main systems are shown and indicated with arrows: Ge detector array with front end electronics (FEE), cooling systems, BGO shields, PSAC, and mask; picture from Vedrenne et al. (2003); see text for details.
3.2.2.3 Calibration and Performance
The sub-units of SPI have been tested, calibrated, and compared to simulation predictions on ground before launch. The instrumental imaging response function (IRF), i.e. how does a source look like on the Ge detector array if it shines through the mask from a particular direction, has been determined by a combination of Monte Carlo simulations and a ray-tracing method (Sturner et al. 2003). A full Monte Carlo calculation of the response, at that time, was too CPU-intensive, be-cause a complete coverage for each detector, at each incident photon direction and each photon energy, needs large statistics during such a simulation. As a con-sequence, the number of simulated photons was reduced by partly performing a
ray-tracing algorithm2. A sketch of the used method is shown in Fig. 3.11a, and an example of the SPI imaging capability is shown in Fig. 3.11b.
(a) Schematic illustration of the decomposi-tion of the response generadecomposi-tion process into ray tracing and Monte Carlo parts.
(b) SPI IRF at508.33 keVfor the photopeak eective area as a function of direction for detector 00. The mask pattern is clearly seen and the hexagonal boundary is a con-sequence of the shape of SPI's BGO col-limator.
Figure 3.11: Imaging response generation process on ground; pictures taken from Sturner et al. (2003).
Four of the 19 Ge detectors failed for unknown reasons during perigee passages of INTEGRAL. Detector number 02 (D02) failed in December 2003, D17 in July 2004, D05 in September 2009, and D01 in May 2010. Consequently, there have to be additional sets of IRFs for each camera conguration. In fact, the number of single events and multiple events3 in neighbouring detectors changes signicantly after a failure. When a detector turns out, photons can still scatter in it, and can be detected by adjacent detectors afterwards. This increases the number of single events in adjacent detectors while the number of multiple events decreases.
These IRFs can then be used to determine the point spread function (Fig. 3.12b) of the SPI telescope on ground, by testing dierent sources with known energy, intensity, and location. Part of the IRFs are also the eective area of the SPI camera as a function of energy and conguration (see Fig. 3.12a).
The spectral resolution of each Ge detector as a function of photon energy depends sensitively on its temperature. Attié et al. (2003) determined the energy resolution (FWHM) of SPI during the ground calibration as
2In general, ray-tracing is the determination of the visibility of a "ray" originating from a three-dimensional object at a particular point in the environment.
3Photons which are scattered not only in the volume of one detector, but in more than one are called multiple events. The number of detectors participating in one multiple events varies from two to many, whereas the interaction in two detectors, "double event", is the most probable.
(a) Full-energy peak eective area of SPI telescope. The red dots represent the measured in-ight eective area for an on-axis source. It is compared to the response matrix before launch (IRF release 1, Nov. 2002, dotted line) and after the launch (IRF release 3, July 2003, solid line).
(b) The SPI point-spread-function determined from meas-urements of four dierent sources at a distance of 125 m. Each curve is the mean of cross-sections through the response in two orthogonal directions;
(b) The SPI point-spread-function determined from meas-urements of four dierent sources at a distance of 125 m. Each curve is the mean of cross-sections through the response in two orthogonal directions;