|
Photon Detector Categories
Germanium (IGLET, IGLET-X, GLP) and Si(Li) Planar,
and LO-AX
Coaxial Low-Energy Photon Spectrometers
Detectors of choice for high-resolution low photon energies are Si(Li)
(SLP Series), Planar (GLP Series), LO-AX, IGLET, and IGLET-X detectors. For each of these
detectors the following information is provided: active diameter, active depth, and
resolution at 5.9 keV measured with optimal time constants. For GLP and LO-AX detectors an
additional specification (energy resolution at 122 keV) is provided. For IGLET and IGLET-X
detectors a high count rate specification 5.9 keV resolution at 100 kcps at a 0.5 µs time
constant, is given.
Intrinsic Efficiency
Intrinsic (full-energy) efficiency is the probability that a photon of a
given energy e, impinging on the front of the detector will be
completely absorbed by the detector element. Although the intrinsic efficiency is not a
standard specification for SLP, GLP, IGLET, and IGLET-X detectors, it is a parameter of
interest for GLP detectors from 3 to 100 keV, for IGLET detectors from 3 to 50 keV and
down to very low energies for SLP and IGLET-X detectors.The low energy portion of the
intrinsic efficiency curves for SLP and IGLET-X detectors is
dominated by the beryllium
window thickness. The curves in Fig. 17 show the intrinsic efficiency for SLP detectors;
those in Fig. 18 show intrinsic efficiency values for GLP and IGLET detectors, and Fig. 19
the intrinsic efficiency for IGLET-X detectors in which the beryllium window thickness
dominates the low energy efficiency.Typically, SLP series detectors are "black" (total absorption)
for energies up to 20 keV, while GLP series detectors are "black" for energies
up to 120 keV.
Maximizing IGLET-X Efficiency for Ultra-Low-Energy X RaysIn a windowless mode of operation, IGLET-X detectors are capable of
detecting photons of energy <0.5 keV. A windowless detector is supplied with a gate
valve which can be opened when the detector is placed in a vacuum common to the source of
the x rays. Windowless detectors are supplied with a beryllium window on the gate valve to
allow a general test of the detector performance upon arrival.
At energies below 3 keV the Intrinsic Full Energy Efficiency of SLP and
IGLET-X detectors is greatly reduced by x-ray absorption in the endcap window. Figure 17
displays the results obtained with Bewindows of different thickness.Timing at Low Energies with Planar Germanium Detectors
For timing measurements at energies below 150 keV planar HPGe (GLP
series) detectors are the best choice. Table 7 shows results obtained with GLP detectors
of 10 cm2. Note: LO-AX, IGLET, or similar quasi-planar detectors are unsuitable
for such measurements.
|
Table 7. Timing at Low Energies with 10 cm2 Active
Area Planar Detectors*. |
|
Source |
Energy (keV) |
Time Resolution(ns) |
|
22Na |
20 ±10
100 ±10
511 ±5 |
20 ±2
8.5 ±1
4.5 ±0.2 |
|
133Ba |
31 ±3
81 ±3
85 ±5
356 ±5 |
19 ±2
Isomer
11 ±1
6.0 ±0.5 |
|
152Eu |
41 ±3
122 ±5
125 ±5
344 ±5
779 ±5 |
15 ±1
Isomer
6.5 ±0.5
5.0 ±0.2
3.8 ±0.3 |
|
*Data courtesy of Dr. Kim Lister, Argonne National
Laboratory. |
|
