WP3_TlBr_Temp_Dependence_FINAL.pdf

White Paper #3 on TlBr (External & Internal):

Temperature Dependence of TlBr Gamma‐Ray Crystals & Associated Electronics

Note: See separate White Paper on long‐term room‐temperature stability.

(Version: 210110)

This work presents a summary to date of the temperature dependence of TlBr when used to measure gamma ray spectra for applications such as Radio‐Isotope Identification Detectors (RIIDs) and represents a snapshot of our evolving information base.

The performance of TlBr detectors, as with all semiconductor detectors, depends on operating temperature. The following two factors affect the energy resolution performance of the TlBr: the generation of the signals at the electrodes, and the fluctuations (noise) introduced by leakage current.

The generation of the transient signals at the electrodes depends on geometric factors, including

“weighting potentials”, as well as the transport of the transient signal charge to the electrodes. The transport of the signal charge depends on the electric field and the charge mobility and lifetime. The temperature dependence of the leakage current depends on the temperature dependence of the concentration and mobility of charge carriers (including both electronic and ionic components).

The effect of temperature cycling on the spectral performance of a TlBr detector was measured for two temperature cycles using an Environmental Test Chamber (ETC) to control the temperature. Figure 1

(left) shows 137Cs pulse height spectra from the anode as a function of temperature for a 5mm × 5mm ×

12mm TlBr CFG* (Capacitive Frisch Grid) device as the temperature cycles through the following values for the first temperature cycle, cycle #1: 22, 10, 22, 33, and 22 °C. Figure 1 (right) shows measurements for a repeat of the same temperature cycle (cycle #2). The figure shows vertically offset spectra to allow easy comparison of their shapes.

Figure 1. 137Cs spectra recorded with a 5mm x 5mm x 12mm TlBr CFG detector as the temperature is cycled. First cycle (#1, left) and second cycle (#2, right). TPG refers to a reference signal from a Tail Pulse Generator of 18 mV injected through a 1pF capacitor.

After achieving each temperature setting, the device is held there for 15 minutes before the first spectrum reading is taken. The spectrum at that temperature setting is then measured 2 more times, at

15‐minute intervals. Between temperature settings, the temperature changes at a rate of 0.2 °C per minute until reaching the next temperature setting, at which time the same procedure is followed. The legend shows the nominal temperature (Tnom), which is the chamber temperature, the leakage current in nA, and the temperature from a thermistor in the aluminum box holding the CFG detector within the chamber. The bias voltage (1500 V) and shaping time (12 µs) were held constant for all these measurements. Both the detector and the readout electronics were inside the chamber, and the 137Cs source irradiated the detector from outside the chamber, a setup which produces a relatively large number of events at low energy (due to scattering) below channel number 300 in the MCA (Multi‐

Channel‐Analyzer) spectrum compared to the number of events in the 662‐keV photo‐peak.

Regarding results, one first sees that the measurements in Figure 1 demonstrate that the spectrum of the TlBr CFG device changes very little with the temperatures tested (note that the spectra are not depth corrected). Also, Figure 1 demonstrates that the spectra essentially “recover” from the small changes that do occur after changing the temperature upon returning to the original temperature, confirming the integrity of the contacts over this temperature range.

The position of the photo‐peak moves to slightly larger amplitudes as the temperature decreases, and the width of the photopeak narrows. These small changes in the position and resolution of the photo‐ peak might be attributed be an improved charge collection at the lower temperatures during the integration period.

0 500 1000 1500

C ou nt s in s

Pulse Amplitude (MCA Chan.)

Tnom, I(nA), T(°C) 22°C, 2.6, 20.7 22°C, 2.6, 20.8 22°C, 2.7, 20.9 33°C, 5.9, 32.2 33°C, 5.9, 32.3 33°C, 5.7, 32.2 22°C, 2.6, 20.7 22°C, 2.6, 20.7 22°C, 2.5, 20.8 10°C, 1.3, 9.3 10°C, 1.3, 9.3 10°C, 1.3, 9.4 22°C, 2.5, 20.7 22°C, 2.5, 20.8 22°C, 2.6, 20.7 keV

TlBr PS-CFG 171D2-5R

1.5 kV, 137-Cs Cycle #1TPG

18mV 1pF

0 500 1000 1500

C ou nt s in s

Pulse Amplitude (MCA Chan.)

Tnom, I(nA), T(°C) 22°C, 2.8, 20.7 22°C, 2.8, 20.7 22°C, 2.9, 20.9 33°C, 6.0, 32.2 33°C, 6.0, 32.3 33°C, 5.8, 32.2 22°C, 2.9, 20.8 22°C, 2.9, 20.7 22°C, 2.9, 20.8 10°C, 2.0, 9.4 10°C, 2.0, 9.3 10°C, 2.0, 9.5 22°C, 2.9, 20.7 22°C, 2.9, 20.7 22°C, 2.9, 20.8 keV

TlBr PS-CFG 171D2-5R

1.5 kV, 137-Cs Cycle #2TPG

18mV 1pF

The change in the leakage current with temperature reflects the effect of temperature on the material resistivity (the inverse of conductivity). The conductivity depends on the mobility, which includes contributions from lattice scattering, impurity scattering, ionic conduction, and carrier concentrations

(intrinsic and dopants). Ionic conduction increases the conductivity (decreases the resistivity) with temperature and thus increases the leakage current. Also, the effect of impurity scattering is reduced at increased temperatures, which increases the mobility and leakage current. The effect of lattice scattering, however, exhibits the opposite effect with temperature.

The tail‐pulse generator reference, labeled “TPG” in the plots, demonstrates constancy of the preamplifier gain with temperature and the relatively small contribution of leakage current and readout noise to the energy resolution of the spectra. The 23 °C temperature range of the experiment produces a negligible change, ~0.07%, in the 1‐pF value of the coupling capacitor for the TPG reference, which is located in the environmental chamber with the pre‐amplifiers as its temperature coefficient is

±30 ppm/°C. Processing the data for 3D correction factors would help correct for geometric effects, thus improving the energy resolution and possibly reducing the variations seen in the spectra at different temperatures, but 3D correction wasn’t applied to the spectra shown above. Note that the 2.7 eV bandgap of TlBr makes the intrinsic carrier concentration and its contribution to the leakage current negligible at the measured temperatures.

Note: There are several ways, both passive and active, to minimize changes in TlBr detector performance with varying operating temperature. These methods could either reduce the magnitude of changes in temperature or maintain a constant (e.g., room‐temperature) temperature of the detector, much as is done with CZT detectors. Temperature control would also reduce effects from the

56 ppm/°C thermal expansion coefficient of TlBr. Another technique to mitigate temperature effects is to lower the bias when not in use, which helps mitigate long‐term effects of parasitic electrochemistry at the electrodes. Again, in the experiments described herein, no passive or active temperature controls were implemented to best extract the explicit temperature dependencies of the TlBr crystals and associated electronics were those temperatures to be experienced.

* The devices were actually configured as PS‐CFG (Position Sensitive – Capacitive Frisch Grid) detectors but the position‐sensitive functionality was not utilized; hence, the fact that 3D corrections were not performed on this data.