AstroMeV
M4
proposal
Orbit
selection
–
activation
of
gamma‐ray
detectors
on
various
orbits
Version
1
–
June
2,
2014
–
by
Jürgen
Kiener,
Clarisse
Hamadache
and
Vincent
Tatischeff
1. Introduction
The selection of the AstroMeV mission orbit must be based on various criteria: the constraints
it imposes in terms of payload mass, telemetry, and mission cost, the desired instrument fieldof-view and sky coverage, as well as the instrument background induced by the radiation
environment. This document presents some results on the latter aspect. We study in particular
the proton-induced activation of common detection materials on various orbits.
2. Radiation
environment
On a low-inclination (i < 5°) low-Earth orbit (LEO; typical altitude of 550 km), a gamma-ray
instrument is exposed to a variety of background components (see Fig. 1): Galactic cosmic
rays (mainly protons, α-particles and electrons), whose fluxes are strongly modulated by the
geomagnetic field; secondary protons, neutrons, electrons and positrons produced in the Earth
atmosphere; and atmospheric gamma rays. The latter have two origins: reflection of cosmic
diffuse X- and gamma-ray photons, which is the dominant component below ~100 keV, and
high-energy emission induced by cosmic-ray particles impinging the Earth atmosphere.
Figure 1: Background environment of a spacecraft on an equatorial LEO (550 km) that would avoid
the South Atlantic Anomaly. The satellite is still exposed to semi-trapped, secondary protons and
leptons, as well as to albedo neutrons and atmospheric gamma rays (see text). Noteworthy, the
cosmic diffuse X- and gamma-ray radiation (in green) is not always a background; it can also be an
interesting science topic for gamma-ray astronomy.
In addition, even on a low-inclination LEO a satellite can be exposed to a high flux of
energetic particles from the South Atlantic Anomaly (SAA). This was clearly evidenced by
the BeppoSAX mission, which measured the radiation environment on a low-inclination (~4°),
500 – 600 km altitude orbit almost uninterruptedly during 1996 – 2002. The data recorded by
the Particle Monitor (PM) onboard BeppoSAX show that the high-energy particle intensity in
the SAA is strongly dependent on altitude, decreasing by about an order of magnitude
between 600 km and 550 km, as well as on the solar cycle and the local magnetic field
properties1.
The radiation environment in a high-Earth orbit is somewhat simpler, the most important
component being due to Galactic cosmic rays. However, the flux of sub-GeV particles is
much higher in a high-Earth orbit than in a LEO, because the geomagnetic shielding is not
operating at altitudes above that of a geosynchronous orbit2.
INTEGRAL was launched into a high elliptical, high inclination (52°) orbit, with an initial
perigee altitude of about 9000 km and an apogee altitude of about 153 000 km. On such an
orbit, the satellite spends most of the time (>80%) above an altitude of 60 000 km, well
outside the Earth’s radiation belts. But INTEGRAL‘s orbit has evolved considerably
throughout the mission. Thus, the perigee altitude reached a maximum of over 13 000 km in
2006, then dropped to a minimum of only 2756 km, which was reached on 25 October 2011
(Fig. 2). The evolution of INTEGRAL’s high elliptical orbit (HEO) strongly affects the
radiation environment encountered by the spacecraft near Earth. In particular, the exposure to
energetic (>10 MeV) proton irradiation has increased substantially after 2009, when the
satellite started to pass through the inner Van Allen radiation belt.
Figure 2: Evolution of INTEGRAL’s perigee altitude and orbital inclination from the launch date up
to 2020. From Hübner et al. (2012), in proc. of the SpaceOps 2012 Conference, paper n° 1291272.
1
Campana, R., et al. 2014, Experimental Astronomy in press, arXiv:1405.0360
By definition, the altitude of a high-Earth orbit is entirely above that of a geosynchronous orbit
(35 786 km).
2
-1
10 6
10 4
: IREM in May 2010
(Zp ! 5000 km)
10 2
iv
iii
ii
i
1
10
-2
Interplanetary spectrum
a)
10
-4
1
10
10
2
10
3
10 9
10 8
High Elliptical Orbit
-1
Apogee Za=153 000 km
Inclination i=52!
Arg. of perigee w=0!
-2 -1
-2 -1
Proton flux (m s MeV sr )
High Elliptical Orbit
-1
-1
Proton flux (m s MeV sr )
10 9
10 8
10
4
Apogee Za=153 000 km
Inclination i=52!
Arg. of perigee w=0!
10 6
10 4
: IREM in Dec 2003
(Zp ! 10000 km)
10 2
iv
1
10
10
iii
ii
i
-2
Interplanetary spectrum
b)
-4
1
10
10
Energy (MeV)
2
10
3
10
4
Energy (MeV)
Figure 3: Orbit-averaged proton flux endured by a spacecraft on an INTEGRAL-like HEO as a
function of perigee altitude Zp. Dashed curves (i) to (iv) correspond to Zp = 5000, 10 000, 15 000
and 20 000 km, respectively. The red solid curve shows the proton flux in the interplanetary medium
near Earth. The green data points show the solar-quiet (no flare), orbit-averaged proton fluxes
measured by the INTEGRAL Radiation Environment Monitor (IREM) at two epochs: May 2010 (Zp ~
5000 km) and December 2003 (Zp ~ 10 000 km). Left panel: conditions of minimum solar activity, for
which CREME96 uses the trapped proton model AP8MIN; right panel: conditions of maximum solar
activity, for which CREME96 uses AP8MAX.
Figure 3 shows the mean proton flux endured by a satellite on an INTEGRAL-like HEO as a
function of the perigee altitude. The simulated spectra were obtained with the CREME96 set
of tools3. The IREM data were extracted from the SEPF tool4. We see that the trapped proton
model used in CREME96 (AP8) is in reasonable agreement with the INTEGRAL data taken in
May 2010, i.e. near solar minimum and when the perigee altitude was around 5000 km. But
the agreement is not good for Zp ~ 10 000 km. We see in Fig. 3b that IREM measured much
more protons of about 100 MeV in December 2003 than predicted by the AP8MAX model
(trapped proton spectrum ii).
3. Material
activation
We calculate the activation of common detection materials in space considering a block of
100 kg of matter exposed to a constant flux of energetic protons for one year. We consider
two cases for the irradiation setup: either that the detection material is directly exposed to the
radiation environment, or that the detectors are shielded from the proton flux by a C layer of
1.3 g cm-2. The latter is representative of an anticoincidence system made of ~1-cm thick
plastic scintillator panels covering the main instrument. The radioisotope production is
calculated by taking into account both the nuclear reactions induced by the primary protons,
and the reactions from the secondary protons and neutrons produced in the detectors.
The cross sections of the proton- and neutron-induced spallation reactions are obtained from
the TALYS nuclear reaction code5 below 250 MeV and from the latest version (INCL4.6) of
the Liège intranuclear cascade (INC) code coupled to the ABLA07 nuclear de-excitation
model6 at higher energies. More details on the cross sections will be given elsewhere.
3
https://creme.isde.vanderbilt.edu/
Solar Energetic Proton Flux tool, see http://proteus.space.noa.gr/sepf_tool/
5
See http://www.talys.eu/
6
See Boudard, A., et al. 2013, Phys. Rev. C, 87, 014606, and references therein
4
-1
Effective radiation activity (Bq kg )
10 3
High Elliptical Orbit
10 2
Apogee Za=153 000 km
Inclination i=52°
Si, no shield
Arg. of perigee w=0°
Ge, no shield
Ge
shielded
10
Si shielded
Ge, equatorial LEO
1
Si, equatorial LEO
10
3
10
4
Perigee altitude (km)
Figure 4: Effective radiation activity of Si and Ge after 1 yr of proton irradiation as a function of the
perigee altitude of an INTEGRAL-like HEO. Two irradiation conditions are considered: either the
semiconductors are directly exposed to the proton flux or they are shielded from the radiation
environment by a C layer of 1.3 g cm-2 (see text). The hatched areas reflect the uncertainties arising
from the solar activity (the activation is higher at solar minimum due to the higher flux of Galactic
cosmic rays, see Fig. 2). Also shown is the activity of these materials in an equatorial LEO at an
altitude of 550 km. In this case, the results with and without the C shielding are almost identical,
because the proton spectrum is relatively hard.
Once the production of the radioisotopes has been evaluated, we compute the decay radiation
from these nuclei, as well as from their daughter isotopes, using the NuDat 2.6 library7. We
then calculate the “effective radiation activity”, which we define as being the rate of
radioactive events that can be confused with Compton events from celestial gamma-ray
photons. It includes the gamma and β+ radioactivity, but not the decay of radioactive nuclei
by β- emission, electron capture, or internal conversion, without the concomitant emission of
a gamma-ray photon. All these single events can easily be rejected and will not contribute to
the background.
Figure 4 shows calculated effective radiation activities of silicon and germanium for various
orbits. For the equatorial LEO, we have considered both primary and secondary, semi-trapped
protons, but not albedo neutrons (see Fig. 1). Calculations for the HEO were done with the
proton fluxes shown in Fig. 3.
We see that the activation of Ge is higher than that of Si, by a factor of about 3.1 for the highEarth orbit (Zp > 20 000 km), 3.3 for the LEO, and reaching about 17 for the HEO with Zp =
5000 km and no shielding of the detectors. The main species contributing to the Si activity are
30
P (half-life T1/2=2.50 m), 29P (4.14 s), 28Al (2.24 m) and 27Si (4.15 s). Those for the Ge
activity are 74As (17.77 d), 72As (26.0 h), 73Gem (0.50 s), 73As (80.3 d), 76As (26.3 h) and 70As
(52.6 m). It is remarkable that the radioisotopes produced in Si have much shorter lifetimes
than the main radioisotopes produced in Ge. As a result, the activity of Si detectors on an
HEO passing through the inner radiation belt is expected to decrease rapidly when the satellite
leaves the belt, whereas the Ge activity should accumulate on much longer time periods.
We also see in Fig. 4 that when the detectors are shielded from the low-energy proton flux –
the assumed C layer of 1.3 g cm-2 stops protons of less than about 35 MeV – the passage
7
See http://www.nndc.bnl.gov/nudat2/
through the Earth’s radiation belts has no significant effect on the detector activation as long
as Zp > 10 000 km. The predicted effective radiation activities outside the belts are about 5 to
10 times higher than those calculated for detectors on an equatorial LEO, depending on the
solar activity. However, our results for the LEO probably underestimate the true activation,
because we have not considered the albedo neutrons in our calculations.
4. Conclusions
Material activation is significantly lower on an equatorial LEO than on an HEO or a highEarth orbit. However, the sensitivity (and the field-of-view) of a gamma-ray instrument on a
LEO is also limited by the high flux of atmospheric photons.
The passage of a spacecraft on an HEO through the Earth’s radiation belts has no significant
effect on the detector activation compared to the one produced outside the belts, as long as the
perigee altitude Zp > 10 000 km. However, the orbital parameters of an HEO can significantly
evolve with time, as a result of the Earth's oblateness and the luni-solar gravitational
perturbations. Thus, launched into an HEO with an initial perigee altitude of about 9000 km,
INTEGRAL reached a minimum altitude of less than 3000 km about five years later.
Whatever the orbit, the activation of Si detectors is significantly less than that of Ge detectors.
Moreover, the radioactive nuclei produced in Si have much shorter lifetimes than the main
radioisotopes produced in Ge. As a result, the activity of Si detectors on an HEO passing
through the inner radiation belt should rapidly decrease when the satellite leaves the belt,
which is not the case for Ge.