TIGRE

Tracking and Imaging Gamma Ray Experiment

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Introduction

Low energy X-rays (0.1-5 keV) can be made to reflect from certain metals provided they are incident at very shallow angles. In this energy range, one can construct a grazing incidence telescope, which uses a series of nested hyperbolic mirrors to focus low energy X-rays onto an X-ray detector. Hard X-rays (5-100 keV) do not reflect at all. However, they can be blocked out by certain dense materials. A popular type of hard X-ray telescope uses a coded mask, where a special pattern of holes is machined in some dense material. Hard X-rays pass through the holes only and strike an X-ray detector below. By processing the pattern seen by the X-ray detector, a picture of the hard X-ray sky can be reconstructed. OSSE, an instrument aboard the Compton Gamma-Ray Observatory (CGRO) uses a titanium collimator to block out all but a small portion of the sky. The collimator can be moved to examine different parts of the sky, or may scan across regions.

In the gamma rays (above a few hundred keV), photons have so much energy that they begin to penetrate even the densest materials, generally creating secondary background radiation in the process. Up to about 10 MeV, Compton scattering is the dominant interaction, and Compton telescopes such as COMPTEL aboard CGRO and the UCR Compton Double Scatter Telescope make use of this interaction to observe photons in this energy range. Above about 10 MeV, the Compton scattering cross section drops, and pair production becomes the dominant interaction. EGRET, the highest energy instrument on CGRO, detects electron-positron pairs produced by high energy gamma rays incident on the detector.

The Tracking and Imaging Gamma Ray Experiment, or TIGRE, uses state-of-the-art solid state strip detectors to act simultaneously as a Compton telescope and a low energy pair detector. As such, TIGRE will observe with significant sensitivity from 0.3-100 MeV. Below, we will describe the basic design and operation of TIGRE, as well as it's scientific objectives. Click here for reprints of papers pertaining to TIGRE.

Instrument Description

TIGRE is designed to act simultaneously as a Compton telescope and a pair detector. The standard Compton telescope basically consists of two layers of scintillator, separated by some distance. Each layer is divided into several components to give some degree of spatial information about the location of the event. In the case of COMPTEL, these are circular modules, while for the UCR instrument, they were long bars. Ideally, an incoming gamma ray Compton scatters in the top layer (D1), with the energy of the recoil electron being measured there. The gamma ray is then absorbed in the bottom layer (D2). The Compton scatter formula can then be used to calculated the scatter angle of the gamma ray. The locations of the interactions in D1 and D2 are found by timing and pulse height analysis, and can be used to give the direction of the scattered photon. The time of flight of the photon from D1 to D2 can also be measured to discriminate downward from upward (background) events.

In the standard Compton telescope, no information is recorded about the direction of the scattered electron in D1. Neglecting polarization effects, Compton scattering is azimuthally symmetric, and thus for an individual event, it is only possible to narrow down the potential direction of the incident photon to a circle on the sky. This circle is centered about the scattered photon direction, and it's radius is the Compton scatter angle. Further, there is some spread perpendicular to the circle due to the finite energy resolutions of D1 and D2. Ideally, one collects many photons from a source, giving a reasonably accurate location and flux for that source. This is complicated by the existence of background and other sources, as the circles from various parts of the sky tend to overlap significantly.

Despite these complications, Compton telescopes have been used successfully for medium energy gamma ray astronomy. However, an improved instrument is required to place significant constraints on the origin of MeV gamma rays from a variety of objects (see Science section below). TIGRE is designed to overcome many of the limitations of Compton telescopes . First and foremost, the use of SSDs gives the potential to track the Compton recoil electron. With the knowledge of the scattered electron's energy and direction, the possible directions of the incident gamma ray describe a small arc on the sky, rather than a complete ring, greatly reducing potential source confusion and increasing sensitivity. The electron tracking also allows kinematic rejection of various backgrounds, such as events which first interact in D2, as well as events which are not completely absorbed by the detector. This background rejection capability leads to enhanced sensitivity and resolving power. A second significant advance in the design of TIGRE is the use of the CZT strip detectors. The fine pitch (< 1 mm) of the CZT strip detectors allows high spatial resolution to be attained without a large (> 1 m) separation between D1 and D2. This not only results in a more compact instrument, but also allows us to extend D2 up the sides of the detector, to form a sort of open-ended box surrounding D1. Such a configuration allows for many more coincidences between D1 and D2, improving efficiency in the Compton regime by a factor of 5-10 over previous Compton telescope designs. In addition, surrounding D1 in this way allows events with large scatter angles to be detected. As the dependence of the Klein-Nishina formula on photon polarization is most pronounced for large scatter angles, TIGRE will also be a highly effective gamma ray polarimeter.

Pair detectors, such as EGRET, typically consist of a charged particle tracking system followed by a calorimeter. In the case of EGRET, the particle tracking system is a spark chamber, conisisting of fine metal meshes separated by gaps which are filled with xenon gas, while the calorimeter is an array of large sodium iodide crystals. The plates of the spark chamber serve as converter material, in which high energy gamma rays produce electron-positron pairs which are subsequently tracked through the spark chamber. TIGRE will also act as a low energy pair detector. The SSDs will serve both as converter and tracker, while the CZT arrays will act as the calorimeter. Due to the relatively low density of silicon, TIGRE is an effective pair detector in the energy range 10-100 MeV, allowing low energy pairs to propagate through several layers of the detector. The use of TIGRE as both a Compton telescope and pair detector gives the instrument a relatively constant efficiency of about 10% over the entire energy range 0.3-100 MeV. Combining this with a wide field of view (+/- 60 degrees) and the background rejection capability afforded by electron tracking, TIGRE is well-suited for a variety of gamma-ray astronomical observations, which we shall describe below.

Scientific Objectives

Gamma Ray Pulsars

Accreting Sources

Diffuse Line emission

Diffuse Continuum Emission

Active Galactic Nuclei

Supernova lines

Gamma Ray Bursts

Solar Flare Spectroscopy

Polarization Measurements

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