Mercury Module
The Mercury Module is a mercuric iodide radiation detector and preamplifier package.  A description of the advantages of mercuric iodide is contained below on this page.

Mercury-X
The Mercury-X x-ray detector has been discontinued as a product.

Detector Material
Constellation offers mercuric iodide detectors that can be incorporated with your electronics and packaging.

Mercuric Iodide Detectors

Mercuric Iodide is the most promising of all room temperature semiconductor materials currently under investigation for use as radiation detectors. It has excellent characteristics of wide dynamic range, good absorption vs. energy, high photoelectric effect vs. total absorption, low leakage current, and resistance to radiation damage. The combination of these desirable properties in a single material is unique.

Recent technological advances have made it possible to routinely fabricate individual detectors as large as 900 mm² x 5 mm thick (30 mm x 30 mm). Typical designs have a nominal operating range of 30 - 1300 keV with a spectral resolution of 3% or better at 662 keV. Detectors based on mercuric iodide operate satisfactorily at less than –20 to 60 degrees Celsius and have stable performance over a wide range of operating conditions over long time scales (> 6 months). The stability, resolution, efficiency, and radiation hardness of these detectors make them ideally suited for unattended monitoring systems either as radiation counters or spectrometers. Recent work in spectral peak fitting has shown that mercuric iodide detectors can be used for quantitative measurements.

The detectors are packaged together with a preamplifier in a small, lightweight and rugged "Mercury Module™" that can be used individually or can be incorporated into a larger monitoring system. Larger detector arrays are available as special designs.

Mercuric Iodide Properties

Mercuric iodide, in the single crystalline form suited for radiation detector applications, is a semiconductor with an electronic bandgap of 2.13 eV at room temperature. Because of this wide bandgap, the resistivity of the material approaches 10¹⁴ Ohm·cm. As a result, the leakage current of large area detectors is in the picoampere range, even at electric fields of 10⁴ V/cm. Other relevant properties of mercuric iodide as a radiation detector are:

• Electron mobility 60-80 cm² volt^-1 sec^-1

• Hole mobility 2-3 cm² volt^-1 sec^-1

• Electron Mu-tau product 2x10^-4 cm² volt^-1

• Hole Mu-tau product 5x10^-5 cm² volt^-1

Figure 1  Mercuric Iodide Crystal (500 grams total mass)

Mercuric iodide crystalline material has a density of 6.3 g/cm³, which gives it a high absorption coefficient for x-rays and gamma rays. The most distinguishing feature of the mercuric iodide, however, is the high atomic numbers of the constituent elements (80 and 53), which results in a very large photoelectric effect and a high full-energy peak efficiency, especially at higher gamma ray energies. This property of mercuric iodide is illustrated in Figure 2, where the excitation processes in different detector materials are displayed as a function of photon energies.¹

Figure 2  Radiation Absorption vs. Energy for Typical Detector Materials

Mercuric iodide’s high photoelectric effect exceeds that of common detector materials such as Cadmium Zinc Telluride and Germanium. In addition, the lower continuum and low leakage current help reduce the spectrum continuum, further increasing the peak-to-background ratios in spectra measured with mercuric iodide detectors.

Good absorption characteristics lead to good detection efficiencies for practical detectors. A recent study² compared the "intrinsic efficiency", defined as the ratio of photopeak counts to gamma rays striking the detectors, for several commercially available detectors; CZT coplanar grid, 100 mm² x 5 mm thick, mercuric iodide 100 mm² x 2.8 mm thick, NaI(Tl), 2000 mm² x 37.5 mm thick, and HPGe, 3100 mm² x 65 mm thick. Measurements of absolute detection efficiency for each detector show that mercuric iodide has excellent photopeak efficiency when data are normalized vs. detector area. This is shown in Figure 3, below. All the data shown in Figure 3 were obtained using actual detectors obtained from different manufacturers.

Figure 3  Photopeak Efficiency Comparisons for Mercuric Iodide and other Common Radiation Detectors

Detector Performance

Mercuric iodide gamma ray detectors are usually tested at three different energies: 662 keV (¹³⁷Cs), 122 keV (⁵⁷Co) and 59 keV (²⁴¹Am). Standard NIM electronics are used for power supply and processing of the pulses. The shaping times used with the Gaussian shaping amplifier are 24 microseconds for the higher energies and less for the ⁵⁷Co and ²⁴¹Am sources. Both the detector and the preamplifier are kept at room temperature. Figure 4 shows a representative spectrum of a ¹³⁷Cs point source, measured using a 25mm x 25 mm x 1.57 mm thick detector for 300 seconds live time. Note the good peak to Compton ratio of 14.5 and the full width at half maximum (FWHM) of 1.8%. This is typical of the best detectors.

Figure 4  Spectrum of 25mm x 25mm x 1.57mm Detector

Relative capabilities of mercuric iodide and CZT at detecting and characterizing ²³⁵U, a special nuclear material of interest in counter-terrorism efforts, are illustrated in Figure 5. A standard 100 mm² planar HgI₂ detector with a thickness of 2.8 mm was compared to a 100 mm² area x 5 mm thick CZT detector with a coplanar grid electrode. Both detectors were exposed in similar geometries to a weak ²³⁵U source. The same pulse-processing electronics were used for both detectors, although the settings were optimized for each independently. The intensity of the mercuric iodide spectrum is higher than that of the CZT detector. Due to its higher photoelectric efficiency, the 2.8 mm thick mercuric iodide detector exhibits a higher intrinsic efficiency than the 5 mm thick CZT detector.

Figure 5   ²³⁵U Spectra Measured with Mercuric Iodide and CZT Detectors

The resolution of this mercuric iodide detector was 5.7 keV FWHM at 186 keV. The gamma rays between 13 and 20 keV are partially resolved and clearly lie above the noise floor at 7 keV. In contrast, the CZT co-planar grid detector exhibits a FWHM of about 7.9 keV at 186 keV and a noise floor of about 12 keV. The high resolution of the mercuric iodide detector is expected to result in improved performance in monitoring and evaluating special nuclear materials such as ²³⁵U. This is especially true given the importance of resolving peaks in the 90 – 120 keV region in these applications.

Quantitative gamma spectroscopy using mercuric iodide.

Many spectroscopy applications require quantitative assay of radioactive materials. While some applications require the use of high resolution (HPGe) detectors, mercuric iodide detectors are more than adequate for many. Using a high quality, stable mercuric iodide detector and a recently developed peak shape function⁴, we are able to accurately determine photopeak areas. This is demonstrated in Figure 6, below:

Figure 6  Quantitative Photopeak Area Determination using Mercuric Iodide Detectors

The example shows the four gamma ray photopeaks of ¹³³Ba (276, 302, 356, and 383 keV). Also present are mercury escape peaks for each gamma ray peak. The peak fit function was determined for this detector by first measuring several well-isolated gamma ray lines. Once determined, peak fit parameters were applied to the overlapping ¹³³Ba gamma spectral lines. The summed fits are correct within 3-s , a common criterion for spectral fitting.

This example spectrum shows good peak-to-valley ratios. High ratios are essential for achieving the best possible minimum detectable activity. The combination of good peak-to-valley ratio’s and resolution make mercuric iodide a good replacement for physically larger NaI(Tl) scintillators.

Mercuric Iodide detector stability

Early mercuric iodide detectors often failed after days or weeks of operation. Detector "infant mortality" has been overcome with better quality control during detector manufacture. Detector longevity has been extensively studied⁵ over a six-month period. Detectors that performed well at the beginning of the study still were performing well at the end, based on the FWHM of several spectral lines, peak centroid location in the spectrum, and peak-to-valley ratios. Quantitative evidence indicates that mercuric iodide detectors will continue to operate for years.

The stability and performance of mercuric iodide detectors have also been tested while they were subjected to different types and intensities of beams of charged particles and neutrons. None of the experimental conditions used created observable changes in the properties of the detectors subjected to the particle beams.

Currently available mercuric iodide detectors

Constellation Technology currently offers mercuric iodide detectors in both standard and custom packaging. The Mercury Module^TM can be used as a hand-held mobile detector unit or be permanently installed on top of or to the side of a storage container. Shielding can be added to the sides and back of the module, so that it becomes a forward-looking unit with only the front face of the detector exposed to the radiation. A Mercury Moduleä is shown in Figure 7.

Figure 7  Mercury Moduleä