4 Identified radioactive isotopes

  

Table 1: A summary of the isotopes identified in the COMPTEL instrumental line background. For simplicity, only the photon energies of the most frequent decay modes are listed. If $\beta$-decays are involved, the $\beta$-particles have been included in the response simulations. The identification of ²⁰⁸Tl has to be considered tentative. The label "prompt'' for the half-life of the stable isotope ²D refers to the time-scale for the emission of the 2.22 MeV photon

Isotope	Half-Life	Decay Modes and	Main
		Photon Energies [MeV]	Production Channels
2D	prompt	2.224	1H(n $_{\rm ther}$,$\gamma$)
22Na	2.6 y	$\beta^+$ (91%): 0.511, 1.275	27Al(p,3p3n),
		EC (9%) : 1.275	Si(p,4pxn)
24Na	14.96 h	$\beta^-$: 1.37, 2.75	27Al(n,$\alpha$),
			27Al(p,3pn)
28Al	2.2 min	$\beta^-$: 1.779	27Al(n $_{\rm ther}$,$\gamma$)
40K	1.28 109 y	EC (10.7%): 1.461	primordial
52Mn	5.6 d	EC (64%): 0.744, 0.935, 1.434	Fe(p,x), Cr(p,x),
		$\beta^+$ (27%): 0.511, 0.744, 0.935, 1.434	Ni(p,x)
57Ni	35.6 h	$\beta^+$ (35%): 0.511, 1.377	Ni(p,x), Cu(p,x)
		EC (30%): 1.377
208Tl	1.4 1010 y	$\beta^-$ (50%): 0.583, 2.614	primordial
	(232Th)	$\beta^-$ (25%): 0.511, 0.583, 2.614

The major components of the COMPTEL instrumental line background can be attributed to eight individual isotopes, namely ²D, ²²Na, ²⁴Na, ²⁸Al, ⁴⁰K, ⁵²Mn, ⁵⁷Ni, and ²⁰⁸Tl (Weidenspointner 1999). Identification of these isotopes was achieved in an iterative process, starting from the most prominent lines. The $E_{\rm tot}$ and E₂ spectra (see Fig. 6) are particularly useful for the identification of isotopes that give rise to single photon (type A) or multiple photon (type C) events, respectively. The diagnostic power of E₁ spectra is limited since they do not exhibit line features at the energy of the incident photons, but only rather broad features at the corresponding Compton edges. Due to the correlated signatures in E₁-E₂ space, individual spectral features can be accentuated by applying suitable event selections. Viable isotope identifications are required to account self-consistently for spectral features in selected regions of the E₁-E₂ dataspace (see the detailed explanations in Appendix B), as well as for their variation with time and/or incident cosmic-ray intensity (discussed in Sect. 5). The telescope response to individual isotopes was modelled through Monte Carlo simulations. The isotopes, their half-lifes, most important decay channels, and main production channels are summarized in Table 1. Below, a review of the isotope identifications is given (a more detailed account on the COMPTEL instrumental line background can be found in, e.g., Weidenspointner 1999).

  
4.1 ²D

Thermal-neutron capture on hydrogen results in the production of a stable ²D nucleus and the emission of a single 2.22 MeV photon, as seen in $E_{\rm tot}$ (see top panel in Fig. 6). The liquid scintillator NE 213A in the D1 detector modules consists of 9.2% H and 90.8% C by mass, making it an efficient moderator for neutrons. The instrumental 2.22 MeV photons are isotropically emitted from the D1 scintillators and constitute single photon (type A) background events. The average moderation and absorption times in the D1 scintillator for an incident neutron are a few $\mu$s and about 3 10^-4 s, respectively, hence the instrumental 2.22 MeV line is a quasi-prompt background component that follows the local, instantaneous cosmic-ray intensity (Weidenspointner et al. 1996). This quasi-prompt nature of the production/emission of the 2.22 MeV line is the reason for treating, in the following, the stable isotope ²D as if it were unstable, and for referring to it as "short-lived''. In principle, the initial scatterings of the incident neutrons could be identified by their PSD values. However, the time-scale for the moderation and absorption of an incident high-energy neutron is too long to associate the initial scatterings with the delayed neutron capture producing the $\gamma$-ray. The instrumental 2.22 MeV events therefore cannot be rejected by a selection on PSD.

The bulk of the neutrons producing ²D are expected to be of atmospheric origin, with secondary neutrons produced in the spacecraft being of minor importance. This follows from measurements of the fast-neutron flux in the D1 detector and Monte Carlo simulations of the production of secondary neutrons in cosmic-ray interactions (Morris et  al. 1995a), as well as from an estimate for the neutron absorption efficiency of the D1 detector.

  
4.2 ²⁴Na

The main production channels for ²⁴Na are neutron-capture reactions such as ²⁷Al(n, $\alpha$)²⁴Na and proton reactions such as ²⁷Al(p, 3pn)²⁴Na. The Al structure of the D1 detector system is the primary source of ²⁴Na activation. As described in Sect. 3, spectral features of the ²⁴Na background in both the $E_{\rm tot}$ and E₂ spectra can be understood by considering its cascade nature (type C events). In particular, the two photons with energies of 1.37 MeV and 2.75 MeV produce line features at about 1.3 MeV and 2.7 MeV in E₂ (see bottom panel in Fig. 6).

In addition, the time-variation of the ²⁴Na event rate is consistent with expectations for an isotope with a half-life of 15 h that is produced during SAA passages (see Sect. 5.1). Also, ToF distributions summed using data selections that favour ²⁴Na events have the characteristics of type C background events, i.e. they peak slightly below a ToF value of 5 ns (see Fig. 7), corresponding to an average distance of the location of the photon emission from the D1 module of about 20-30 cm.

Figure 7: The measured ToF distribution of 24Na events from the vicinity of the D1 detector

  
4.3 ⁴⁰K

⁴⁰K is one of the contributors to the spectral line feature at about 1.4 MeV in $E_{\rm tot}$ (see top panel of Fig. 6). A prominent line at this energy was present before launch in some of the calibration data, and at that time was tentatively identified with primordial ⁴⁰K radioactivity, contained e.g. in the concrete of the buildings. The 1.4 MeV line was still present after launch, however, and did not vary with any orbital parameter, indicating an instrumental origin as well.

A viable explanation for the instrumental 1.4 MeV line was the potassium in the glass of the D1 PMTs (van Dijk 1996). Electron capture by ⁴⁰K results in the emission of a single 1.46 MeV photon (type A event). This origin is supported by the fact that a disproportionate fraction of the photons at 1.46 MeV interact in the outer sections of the D1 scintillator, indicating that the photons enter the D1 detectors from the sides, consistent with the location of the D1 PMTs. Also, the observed $\bar{\varphi}$distribution at 1.46 MeV peaks at high $\bar{\varphi}$ values, again indicating that the photons enter from the sides. These characteristics were reproduced in Monte Carlo simulations. Furthermore, the potassium content in the D1 PMTs derived from the observed 1.4 MeV line rate is consistent with the manufacturer's specifications, as was confirmed by radiological measurements of a PMT in the laboratory. The mass of the PMT front window (made of Corning France 801.51) is about 24.9 g, with a potassium mass fraction of 6%; the mass of the PMT side and back glass (both made of Schott 8245) is about 40.7 g and 9.1 g, respectively, both with a potassium mass fraction of 0.14%. The front window contains more than 95% of the potassium in the PMTs and is closest to the detectors. The ⁴⁰K background therefore is dominated by the front window. The ⁴⁰K activity of the front window, normalized to its mass, is about 0.2 decays s^-1 g^-1.

  
4.4 ²²Na

Routine data processing and the investigation of galactic 1.8 MeV line emission from radioactive ²⁶Al indicated the build-up of a broad background component at about 1.5 MeV in $E_{\rm tot}$ (see top panel of Fig. 6). Investigations of the energy distribution of this background feature, as well as of its ToF dependence, led to its identification with the $\beta^+$-decay of ²²Na produced in the D1 detector material (Oberlack 1997). This background component is produced by a two photon cascade (type C event) with energies of $\sim$500 keV and $\sim$1.3 MeV, consistent with the absorption of the 1.275 MeV photon in D2, and the scattering of an annihilation 511 keV photon in D1 (the reverse process, involving absorption of the 511 keV photon in D2, is suppressed because 511 keV is below the D2 threshold). The ToF distribution of the 1.275 MeV ²²Na line in E₂ is consistent with the ToF signature of the ²⁴Na cascade. The build-up of the feature is consistent with an isotope of half-life of 2.6 y (see Sect. 5.1).

The most important process for the production of ²²Na in the D1 detector are proton interactions in the aluminium structure, the main channel being ²⁷Al(p, 3p3n)²²Na. Based on the production cross-sections, some ²²Na should also be produced in proton interactions with silicon, found in the glass of the D1 PMTs, such as Si(p, 4pxn). It is expected that most of the ²²Na production occurs during SAA passages (see Sect. 5.1).

  
4.5 ²⁸Al

The isotope ²⁸Al is produced by thermal-neutron capture on aluminium, the most abundant element in the D1 detector. The existence of the strong ²D line due to thermal-neutron capture on hydrogen indicates the presence of a thermal-neutron flux in the D1 scintillators. The cross-section for thermal-neutron capture on aluminium has a value of 0.28 mb (cf. 0.33 mb for hydrogen). Upon its $\beta^-$-decay, ²⁸Al can generate a type C background event: the 1.78 MeV photon is absorbed in D2, with a bremsstrahlung photon (from the $\beta^-$) scattering in D1. A small fraction of the ²⁸Al decays result in type A background events from the double scattering of the 1.78 MeV photon.

From the beginning of the mission a weak, albeit significant, line feature at about 1.8 MeV has consistently been present in the E₂ distribution (see bottom panel of Fig. 6). The position of this feature is independent of E₁, as expected for a type C event. Additional support for the identification of ²⁸Al comes from the fact that, as expected, the 1.8 MeV feature is short-lived (see Sect. 5.2).

  
4.6 ⁵²Mn and ⁵⁷Ni

The above mentioned isotopes were not sufficient to account entirely for the pronounced and broad $\sim$1.4 MeV feature in E₂ that built up over the mission. After subtracting the effects of the other isotopes, there was clear evidence for a build-up of the 1.4 MeV residual, whose half-life was estimated to be between that of ²⁴Na and that of ²²Na (assuming that it is due to a single isotope). The E₁ dependence of the 1.4 MeV residual in E₂ suggested that the line is due to a $\beta^+$-decay and therefore arises from type C background events. After detailed modelling of the E₂ and $E_{\rm tot}$spectra, taking into account the material composition of the D1 detector, it was concluded that the 1.4 MeV residual is due to a blend of two different isotopes, namely ⁵²Mn (1.43 MeV) and ⁵⁷Ni (1.38 MeV).

The isotope ⁵²Mn is produced in SAA-proton interactions with the Fe, Cr, and Ni in the D1 detector, found mainly in the electronics. Matters are complicated by the fact that ⁵²Mn can be produced in either its ground state or an isomeric state. These two states have different half-lifes and decay schemes. The time variation of the ⁵²Mn event rate suggests that this isotope is more likely produced in the ground state ( T_1/2 = 5.6 d) than in the isomeric state ( T_1/2 = 21.1 min), as described in Sect. 5.1. In this case, most ⁵²Mn events are due to the absorption of a 1.43 MeV photon in D2, accompanied by the scattering of another photon, from the radioactive decay or positron annihilation, in D1.

The isotope ⁵⁷Ni is expected to be mostly produced by proton interactions with Ni and Cu in the D1 detector during SAA passages. The ⁵⁷Ni background is mainly produced by the absorption of the 1.38 MeV photon in D2, with one of the 511 keV annihilation photons scattering in D1 (type C event). Similar to ⁵²Mn, the identification of ⁵⁷Ni, which has a half-life of about 36 h, is supported by modelling the observed long-term variation of its background contribution (see Sect. 5.1).

  
4.7 ²⁰⁸Tl

Similar to the 1.4 MeV feature, the 2.7 MeV feature in E₂represents a blend of lines from more than a single isotope. In addition to ²⁴Na, at least one component with a very long half-life (exceeding that of ²²Na) is present. The position of the residual is about 2.6 MeV. ²⁰⁸Tl, which is part of the natural ²³²Th decay chain, is the most viable candidate.

The isotope ²⁰⁸Tl undergoes $\beta^-$-decay through several channels, all of which involve the emission of at least two photons, implying that the ²⁰⁸Tl multiple photon (type C) events are efficient in triggering the telescope. The half-life of ²⁰⁸Tl is only 3.1 min. If the isotope is part of a natural decay chain, however, then its effective half-life is equal to the longest half-life of any of its parent isotopes, which in this case is the isotope ²³²Th with a half-life of 1.4 10¹⁰ y. The main channel for the production of background events by ²⁰⁸Tl is the absorption of the 2.61 MeV photon in D2, with another photon scattering in D1.

It was assumed that ²⁰⁸Tl is distributed like ⁴⁰K and is only present in the front glass of the D1 PMTs. This was tested by recording $\gamma$-ray spectra of a spare D1 PMT using a Ge spectrometer. Taking the COMPTEL results for the ⁴⁰K and ²⁰⁸Tl lines at face value, and assuming that both isotopes are equally distributed in the D1 PMTs, one expects that the ²⁰⁸Tl activity is about 10^-2 decays s^-1 g^-1 in the front window or about 6% of that of ⁴⁰K, based on the simulated efficiencies, corresponding to a ²³²Th mass fraction of a few 10^-8. Unfortunately, the laboratory measurement of ²⁰⁸Tl was inconclusive. The 1.46 MeV ⁴⁰K line is detected at the 12$\sigma$ level above the general background ( $1700 \pm 140$ counts). The strongest ²⁰⁸Tl lines at 2.61 MeV and 0.58 MeV, however, with intensities of 100% and 85.5%, were only detected at the 0.9$\sigma$ ($50 \pm 56$ counts) and 1.4$\sigma$ ( $128 \pm 92$ counts) level, respectively. Although consistent with expectations, these numbers are insufficient to test the assumptions. Much longer integration times are needed.