Appendix A Event selections

The results on the COMPTEL instrumental lines given in the main body of this paper have been derived using two different sets of event selections, described below.

  
A.1 CDG analysis

In the COMPTEL analysis of the cosmic diffuse gamma-ray background (CDG), the intense atmospheric $\gamma$-ray background is eliminated by selecting data from times when the Earth was sufficiently far outside a circular field-of-view defined by event selections. Detailed descriptions of these selections can be found in, e.g., Kappadath (1998) and Weidenspointner (1999).

The standard set of CDG event selections used in this paper are as follows:

 

$$70~{\rm keV} \le {E}_{1} \le 20~{\rm MeV}$$

$$730~{\rm keV} \le {E}_{2} \le 30~{\rm MeV}$$

$$800~{\rm keV} \le {E}_{\rm tot} \le 30~{\rm MeV}$$

$$110~{\rm ch} \le {\rm ToF} \le 130~{\rm ch}$$ (A.1)

$${\rm PSD}_{\rm low}({E}_{1}) \le {\rm PSD} \le {\rm PSD}_{\rm high}({E}_{1})$$

$$6^\circ \le \bar{\varphi} \le 38^\circ$$

$$\zeta + \bar{\varphi} = \xi \le 40^\circ$$

where $\bar{\varphi}$ is the photon scatter angle (see Eq. (1)), and $\zeta$ is the angle between the direction of the scattered photon and the telescope z-axis. The selection on $\xi$ defines a field-of-view of half opening angle 40$^\circ$. The values of PSD $_{\rm low}$ and PSD $_{\rm high}$are 40 ch and 110 ch at 70 keV, 55 ch and 95 ch at 1 MeV, 55 ch and 92 ch at 10 MeV, and 40 ch and 90 ch at 20 MeV. At intermediate E₁ energies, these boundaries are linearly interpolated. In addition, the horizon of the Earth, as seen by the instrument, is required to be at least 5$^\circ$ outside the field-of-view.

  
A.2 Imaging analysis

In COMPTEL imaging analyses such as that of the ²⁶Al 1.8 MeV line emission from the galaxy, $\gamma$-ray albedo photons from the Earth's atmosphere are rejected by determining for each event the minimal angular distance between its possible origins in the sky and the Earth horizon. This angular distance, denoted $\epsilon$ below, is required to exceed a minimum value (see e.g. Schönfelder et al. 1993). Hence, the effective field-of-view in imaging analysis changes with time as the instrument orbits the Earth.

The standard set of imaging event selections used in this paper are as follows:

 

$$70~{\rm keV} \le {E}_{1} \le 20~{\rm MeV}$$

$$650~{\rm keV} \le {E}_{2} \le 30~{\rm MeV}$$

$$750~{\rm keV} \le {E}_{\rm tot} \le 30~{\rm MeV}$$

$$115~{\rm ch} \le {\rm ToF} \le 130~{\rm ch}$$ (A.2)

$$0~{\rm ch} \le {\rm PSD} \le 110~{\rm ch}$$

$$0^\circ \le \bar{\varphi} \le 50^\circ$$

$$\epsilon \ge 5^\circ.$$