2 Instrument description

COMPTEL is the first double-scattering Compton telescope designed for $\gamma$-ray astronomy to operate on a satellite platform. A detailed description of the COMPTEL instrument, which is sensitive to $\gamma$-rays in the 0.8-30 MeV range, can be found in Schönfelder et al. (1993). Briefly, the instrument consists of two planes of detector arrays, D1 and D2, separated by 1.58 m (see Fig. 1). The D1 detector consists of seven cylindrical modules filled with NE 213A organic liquid scintillator. The D2 detector consists of 14 cylindrical NaI(Tl) crystals. The D1 scintillator material has a low average atomic number to optimize the occurrence of a single Compton scatter, while the D2 scintillator crystals have a high density and average atomic number to maximize their photon absorption properties. Each detector array is surrounded by a pair of overlapping anti-coincidence domes, manufactured of NE 110 plastic scintillator, to reject charged particle triggers of the telescope. The in-flight performance of the instrument is monitored with two calibration (CAL) units, each composed of a ⁶⁰Co-doped scintillator viewed by two 1/2 inch photomultiplier tubes (PMTs), that provide tagged photons for in-flight energy calibration (Snelling et al. 1986). The COMPTEL instrument accepts and registers coincident triggers in a single D1-D2 module pair within the coincidence time window of $\sim 40$ ns in the absence of a veto signal from the four charged particle shields as valid events. These interactions can be caused by a single photon or by multiple photons and/or particles. Among other parameters, a time-of-flight (ToF) value and a so-called pulse shape discriminator (PSD) value in D1 are recorded for each event. The ToF is a measure of the time difference between the triggers in the D1 and D2 detectors and is used to discriminate forward scattered (D1 $\rightarrow$D2) events, such as celestial photons with a ToF value of about 5 ns, from backward scattered (D2 $\rightarrow$D1) background events which cluster around a ToF value of about -5 ns. The PSD is a measure of the shape of the scintillation light pulse in the D1 detector. The energy loss characteristics of recoil electrons resulting from Compton scattering and of recoil protons resulting from neutron scattering are different, allowing one to reject many neutron induced events. The summed energy deposits in the two detectors, E₁ + E₂, are a measure of the total energy of the incident photon, $E_{\rm tot}$, while the photon scatter angle $\bar{\varphi}$ is determined from E₁and E₂ through the Compton-scatter formula:

$$\cos \bar{\varphi} = 1 - \frac{m_{\rm o} c^2}{{E}_{2}} + \fr... ...1} + {E}_{2}}, \; {\rm with} \; m_{\rm o} c^2 = 511~{\rm keV}.$$ (1)

Figure 1: A schematic view of the Compton telescope COMPTEL (from Schönfelder et al. 1993)