Pair Production Detectors for Gamma-ray Astrophysics David J. Thompsonand Alexander A. Moiseev

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Pair Production Detectors for Gamma-ray

Astrophysics

David J. Thompson ∗and Alexander A. Moiseev

Abstract

Electron-positron pair production is the essential process for high-energy γ-ray

astrophysical observations. Following the pioneering OSO-3 counter telescope, the

ﬁeld evolved into use of particle tracking instruments, largely derived from high-

energy physics detectors. Although many of the techniques were developed on

balloon-borne γ-ray telescopes, the need to escape the high background in the at-

mosphere meant that the breakthrough discoveries came from the SAS-2 and COS-

Bsatellites. The next major pair production success was EGRET on the Comp-

ton Gamma Ray Observatory, which provided the ﬁrst all-sky map at energies

above 100 MeV and found a variety of γ-ray sources, many of which were vari-

able. The current generation of pair production telescopes, AGILE and Fermi LAT,

have broadened high-energy γ-ray astrophysics with particular emphasis on multi-

wavelength and multimessenger studies. A variety of options remain open for future

missions based on pair production with improved instrumental performance.

Keywords

Gamma rays; High-energy astrophysics; Active galactic nuclei; Pulsars; Gamma-

ray bursts; Gamma-ray telescopes; Pair production

David J. Thompson

NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA, e-mail: david.j.

thompson@nasa.gov

Alexander A. Moiseev

University of Maryland, College Park, MD, USA e-mail: amoiseev@umd.edu

∗corresponding author

arXiv:2210.14121v2 [astro-ph.HE] 31 Oct 2022

2 David J. Thompson and Alexander A. Moiseev

1 Introduction

For energies above a few tens of MeV, photons interact with matter, other photons,

or magnetic ﬁelds primarily by electron-positron pair production (γ→e−+e+).

Pair production is an explicit illustration of Einstein’s E=mc2, where the energy E

of the photon is converted into two particles with mass m, with the speed of light

c being the conversion factor. This physical process has important implications for

detection of astrophysical γrays:

1. High-energy (greater than about 50 MeV) γrays cannot be reﬂected or refracted;

there are no mirrors or lenses for these photons.

2. Gamma rays coming from space interact in Earth’s upper atmosphere; therefore

direct detection can only be done from space or from the edge of the atmosphere.

3. Properties of high-energy γrays can only be derived from measurements of the

electron and positron resulting from pair production. Gamma-ray instrumenta-

tion at these energies consists of charged particle detectors.

These considerations drive the most important factor in design of high-energy γ-

ray telescopes. The real challenge is not in detecting the electron-positron pair.

The critical issue is separating the cosmic pair-production events from back-

ground. Thanks largely to developments in particle physics, various types of highly

efﬁcient particle detectors are available, and many of these have been applied to

astrophysical γ-ray instruments. Background comes in two forms:

1. Space is ﬁlled with charged particles. Cosmic rays, solar energetic particles, and

trapped particles in Earth’s magnetic ﬁeld outnumber high-energy γrays by or-

ders of magnitude. These charged particles are highly penetrating for most ener-

gies in the pair production regime, so shielding is impractical. Such particles can

masquerade as γ-ray interaction products.

2. Many of the charged particles in space have enough energy to undergo inelastic

nuclear interactions, producing secondaries such as neutral pions, which decay

very quickly into γrays in the same energy range as seen by pair-production de-

tectors. These secondary γrays are indistinguishable from cosmic γrays. The

target material for such interactions can be local, as part of the detector or sup-

porting structure, or it can be more diffuse, such as Earth’s atmosphere.

Building detectors to deal with these issues was initially stimulated by the recog-

nition that charged-particle cosmic rays in the Galaxy must interact with the inter-

stellar gas to produce γrays in the MeV to GeV energy range [37,56]. Since that

time, detector technologies and access to space have undergone dramatic changes,

and pair production telescopes have revealed a broad array of high-energy γ-ray

sources in addition to the diffuse Galactic radiation that provided the original im-

petus for the ﬁeld. In this chapter, we review various approaches that have been

taken or proposed as ways to use pair production to conduct astrophysical research.

Section 2describes the early counter instruments; sections 3and 4present the early

imaging γ-ray telescopes; section 5outlines the current state of instrumentation; and

Pair Production Detectors for Gamma-ray Astrophysics 3

section 6covers some aspects of the future of pair-production telescopes. Additional

information can be found in earlier review articles[44,66].

4 David J. Thompson and Alexander A. Moiseev

Fig. 1 Simpliﬁed schematic diagram of the OSO-3 γ-ray instrument, adapted from [49].

2 Counter Detectors

The 1960s saw the emergence of several types of pair-production telescopes, many

of which were carried on high-altitude research balloons. The greatest success from

this decade was the use of scintillation counters on satellites. The ﬁrst hints of cos-

mic γ-ray detection using this method came from Explorer XI [48], and the real

breakthrough came from a counter γ-ray instrument on the OSO-3 satellite [49].

Figure 1is a scale drawing of the OSO-3 γ-ray telescope, showing the variety of

instrumentation used to detect the photons and reject the background. The plastic

scintillator read out by photomultipliers and surrounding the detectors provides a

ﬁrst-level rejection of charged particles. Plastic scintillator is highly efﬁcient in par-

ticle detection while offering minimal absorption of γrays. A γray entering through

the top of the instrument undergoes pair production in a Cesium Iodide (CsI) or plas-

tic scintillator layer. The electron-positron pair produce a signal in the directional

Lucite Cerenkov counter. The directionality discriminates against upward-moving

particles. Finally, the particles deposit energy in the layers of tungsten and Sodium

Iodide (NaI), allowing a measurement of the original γray energy. An experimental

calibration of the instrument showed an effective energy threshold of about 50 MeV

and a peak effective area of about 9 cm2, with an angular response having a Full

Width Half Maximum of about 24◦. No arrival direction information for individual

photons within that wide opening angle was possible.

Pair Production Detectors for Gamma-ray Astrophysics 5

In an era when rocket reliability was uncertain, three copies of this telescope

were made. The ﬁrst was lost when its satellite failed to reach orbit. The second

was the one on OSO-3, launched into a low-Earth orbit on a Delta rocket in 1967

March. The third was used for calibration. The OSO satellites were spin stabilized,

with the γ-ray instrument located in the rotating part of the satellite. As the satellite

precessed to follow the Sun (its primary mission), the rotation allowed the γ-ray

telescope to sweep out the entire sky over the 16 months of operation, ending when

the onboard tape recorders failed.

During the mission, 621 γrays from the sky and a much larger number of atmo-

spheric γrays were detected. Three important results emerged:

• As expected, atmospheric γrays vastly outnumber cosmic photons in the energy

range above 50 MeV. The Earth limb is particularly bright, and the east-west ef-

fect from geomagnetic screening of the positively charged cosmic rays is visible.

• The sky events show a clear peak toward the Galactic equator and a concentration

toward the central region of the Milky Way, conﬁrming the idea of high-energy

γrays being produced by cosmic-ray interactions with interstellar gas.

• An apparently isotropic emission is visible, with γrays arriving from all direc-

tions in the sky.

The OSO-3 results represented a milestone. These were the ﬁrst very-high-

conﬁdence observations in high-energy γ-ray astrophysics. Although the statistics

were limited and the angular response quite broad, the careful design of the in-

strument demonstrated that the challenge of measuring cosmic γrays in a high-

background environment could be met.

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PairProductionDetectorsforGamma-rayAstrophysicsDavidJ.ThompsonandAlexanderA.MoiseevAbstractElectron-positronpairproductionistheessentialprocessforhigh-energyg-rayastrophysicalobservations.FollowingthepioneeringOSO-3countertelescope,theeldevolvedintouseofparticletrackinginstruments,largelyderivedfr...

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Pair Production Detectors for Gamma-ray Astrophysics David J. Thompsonand Alexander A. Moiseev

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