This PDF file contains the front matter associated with SPIE Proceedings Volume 9593 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

An aliovalently calcium-doped cerium tribromide (CeBr3:Ca2+) crystal was prepared with a gamma-energy resolution (FWHM) of 3.2% at the ¹³⁷Cs 662 keV gamma energy. We completed a crystal assessment and calculated the predictive performance and physical characteristics using density functional theory (DFT) formalism. Detector performance, characteristics, calcium doping concentration, and crystal strength are reported. The structural, electronic, and optical properties of CeBr3 crystals were investigated using the DFT within generalized gradient approximation. Specifically, we see excellent linearity of photons per unit energy with the aliovalent CeBr₃:Ca²⁺ crystal. Proportionality of light yield is one area of performance in which Ce-doped and Ce-based lanthanide halides excel. Maintaining proportionality is the key to producing a strong, high-performance scintillator. Relative light yield proportionality was measured for both doped and undoped samples of CeBr₃ to ensure no loss in performance was incurred by doping. The light output and proportionality for doped CeBr₃, however, appears to be similar to that of undoped CeBr₃. The new crystal was subjected to additional testing and evaluation, including a benchmark spectroscopy assessment. Results, which present energy resolution as a function of energy, are summarized. Typical spectroscopy results using a ¹³⁷Cs radiation source are shown for our crystallites with diameters <1 cm. We obtain energy resolution of 3.2% before packing the crystallite in a sealed detector container and 4.5% after packing. Spectra were also obtained for ²⁴¹Am, ⁶⁰Co, ²²⁸Th, and background to illustrate the spectrosocopic quality of CeBr₃:Ca²⁺ over a broader energy range.

Neutron detectors are used for illicit material detection, neutron radiography, stellar investigations of chemical content including biological compounds in planetary terrain and to monitor nuclear power plant fuel products and radioactive waste. Li-containing chalcogenide materials are promising alternative thermal neutron detection materials due to the incorporation of the ⁶Li isotope at high density. ⁶LiInSe₂ is limited in its effective thermal neutron efficiency by ¹¹⁵In neutron capture which results in gamma decay rather than charge creation. This study includes investigations of mixed crystalline material ⁶LiIn_1-xGa_xSe₂ where the indium concentration is reduced by Ga substitution. The optical properties have been tuned by gallium substitution and radiation response has been observed.

The detection of thermal neutrons has traditionally been accomplished with ³He-tubes, but with the recent shortage of ³He, much research has gone into finding suitable replacements. Both relatively inefficient ¹⁰B- and ⁶LiF-coated silicon diodes and HgI₂ have been known for many years, and engineered structures in Si that have been filled with ¹⁰B and ⁶LiF have shown promise. These devices are intended to realize an optimal juxtaposition of neutron-sensitive material and semiconductor and thereby simulate a semiconductor containing B or Li. Such material has been realized for the first time in the form of ⁶LiInSe₂ in which collectable charge from the ⁶Li(n,t) reaction indicates a neutron event. In this paper we report neutron and gamma responses of ⁶LiInSe₂, we show pulse height spectra from pure gamma sources and from a thermal neutron source, and we derive the μτ product from the position of spectral features as a function of bias voltage. In addition, we demonstrate the observation of the beta decay of ^116mIn in samples exposed to thermal neutrons. This feature of the response serves as an additional confirmation of exposure to neutrons.

Thallium Bromide (TlBr) is a promising room-temperature radiation detector candidate with excellent charge transport properties. However, several critical issues are needed to be addressed before deployment of this material for long-term field applications. In this paper, the relevance and, scientific and technological progress made towards solving these challenges for TlBr have been discussed. The possible research pathways to mitigate the concerns related to this material have been analyzed and clearly established. Findings from novel experiments performed at CapeSym have revealed that the most significant factors for achieving long-term performance stability for TlBr devices involve physical and chemical conditions of the surface, residual stress, and choice of metal contacts. Palladium electrodes on TlBr devices resulted in a 20-fold improvement in the device lifetime when compared to its Br-etched Pt counterpart. Electron and hole contributions towards the spectroscopic response of the TlBr detector significantly depend on the interaction position of the incoming radiation and was clearly observed in this study. TlBr device fabrication techniques need significant improvement in order to attain reliable, repeatable, and stable, long-term performance.

In this article the results of experimental investigations of resistivity, charge collection efficiency, mobility lifetime μ _n·τ_n product and I-V curves dependencies on thermal treatment of Me-GaAs:Cr-Me X-ray sensors are presented. Experimental samples were the pad sensors with active area 0.1-0.25 cm2 and active layer thickness of 400-500 μm. The μ _n·τ_n product was estimated using charge collection efficiency dependency on bias measured with the use of gammarays of ²⁴¹Am source.

It was shown that thermal treatment in the temperature range of 200-500°C doesn’t lead to a sufficient degradation of sensor’s characteristics and can be used in array detectors processing.

The three most important desirable features in the search for room temperature semiconductor detector (RTSD) candidate as an alternative material to current commercially off-the-shelf (COTS) material for gamma and/or thermal neutron detection are: low cost, high performance and long term stability. This is especially important for pager form application in homeland security. Despite years of research, no RTSD candidate so far can satisfy the above 3 features simultaneously. In this work, we show that mercurous halide materials Hg₂X₂ (X= I, Cl, Br) is a new class of innovative compound semiconductors that is capable of delivering breakthrough advances to COTS radiation detector materials. These materials are much easier to grow thicker and larger volume crystals. They can detect gamma and potentially neutron radiation making it possible to detect two types of radiation with just one crystal material. The materials have wider bandgaps (compared to COTS) meaning higher resistivity and lower leakage current, making this new technology more compatible with available microelectronics. The materials also have higher atomic number and density leading to higher stopping power and better detector sensitivity/efficiency. They are not hazardous so there are no environmental and health concerns during manufacturing and are more stable making them more practical for commercial deployment. Focus will be on Hg₂I₂. Material characterization and detector performance will be presented and discussed. Initial results show that an energy resolution better than 2% @ 59.6 keV gamma from Am-241 and near 1% @ 662 keV from Cs-137 source can be achieved at room temperature.

Thermal neutron detection is of vital importance to many disciplines, including neutron scattering, workplace monitoring, and homeland protection. We survey recent results from our collaboration which couple low-pressure noble gas scintillation with novel approaches to neutron absorbing materials and geometries to achieve potentially advantageous detector concepts. Noble gas scintillators were used for neutron detection as early as the late 1950's. Modern use of noble gas scintillation includes liquid and solid forms of argon and xenon in the dark matter and neutron physics experiments and commercially available high pressure applications have achieved high resolution gamma ray spectroscopy. Little attention has been paid to the overlap between low pressure noble gas scintillation and thermal neutron detection, for which there are many potential benefits.

High barrier Schottky contact has been fabricated on 50 μm n-type 4H-SiC epitaxial layers grown on 350 μm thick substrate 8° off-cut towards the [11̅20] direction. The 4H-SiC epitaxial wafer was diced into 10 x 10 mm² samples. The metal-semiconductor junctions were fabricated by photolithography and dc sputtering with ruthenium (Ru). The junction properties were characterized through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Detectors were characterized by alpha spectroscopy measurements in terms of energy resolution and charge collection efficiency using a 0.1 μCi ²⁴¹Am radiation source. It was found that detectors fabricated from high work function rare transition metal Ru demonstrated very low leakage current and significant improvement of detector performance. Defect characterization of the epitaxial layers was conducted by deep level transient spectroscopy (DLTS) to thoroughly investigate the defect levels in the active region. The presence of a new defect level induced by this rare transition metal-semiconductor interface has been identified and characterized.

In the experiment SIGNAL, which is planned to take place on board spacecraft INTERHELIOPROBE, a xenon gammaray spectrometer is to be used. The gamma-ray spectrometer in question has been chosen because of its characteristics permitting detailed study of solar gamma-radiation under rough experimental conditions. The equipment is able to provide: high energy resolution (5-6-fold better than that of scintillation detectors), performance at high temperatures, steady operation under significant vibroacoustic load, and high radiation resistance of the working medium. The aforesaid properties of the xenon gamma-ray spectrometer meet goals and objectives of the experiment SIGNAL. The description of ballistics scenario and operation orbit of the INTERHELIOPROBE spacecraft (SC) are presented.

We report on the development of two new mechanically rugged, high light yield transparent ceramic scintillators: (1) Ce-doped Gd-garnet for gamma spectroscopy, and (2) Eu-doped Gd-Lu-bixbyite for radiography. GYGAG(Ce) garnet transparent ceramics offer ρ = 5.8g/cm³, Z_eff = 48, principal decay of <100 ns, and light yield of 50,000 Ph/MeV. Gdgarnet ceramic scintillators offer the best energy resolution of any oxide scintillator, as good as R(662 keV) = 3% (Si-PD readout) for small sizes and typically R(662 keV) < 5% for cubic inch sizes. For radiography, the bixbyite transparent ceramic scintillator, (Gd,Lu,Eu)₂O₃, or “GLO,” offers excellent x-ray stopping, with ρ = 9.1 g/cm³ and Z_eff = 68. Several 10” diameter by 0.1” thickness GLO scintillators have been fabricated. GLO outperforms scintillator glass for high energy radiography, due to higher light yield (55,000 Ph/MeV) and better stopping, while providing spatial resolution of >8 lp/mm.

The Remote Sensing Laboratory (RSL) is developing a tactical, networked radiation detection system that will be agile, reconfigurable, and capable of rapid threat assessment with high degree of fidelity and certainty. Our design is driven by the needs of users such as law enforcement personnel who must make decisions by evaluating threat signatures in urban settings. The most efficient tool available to identify the nature of the threat object is real-time gamma spectroscopic analysis, as it is fast and has a very low probability of producing false positive alarm conditions. Urban radiological searches are inherently challenged by the rapid and large spatial variation of background gamma radiation, the presence of benign radioactive materials in terms of the normally occurring radioactive materials (NORM), and shielded and/or masked threat sources. Multiple spectral anomaly detection algorithms have been developed by national laboratories and commercial vendors. For example, the Gamma Detector Response and Analysis Software (GADRAS) a one-dimensional deterministic radiation transport software capable of calculating gamma ray spectra using physics-based detector response functions was developed at Sandia National Laboratories. The nuisance-rejection spectral comparison ratio anomaly detection algorithm (or NSCRAD), developed at Pacific Northwest National Laboratory, uses spectral comparison ratios to detect deviation from benign medical and NORM radiation source and can work in spite of strong presence of NORM and or medical sources. RSL has developed its own wavelet-based gamma energy spectral anomaly detection algorithm called WAVRAD. Test results and relative merits of these different algorithms will be discussed and demonstrated.

Crystal detectors have been used widely in high energy and nuclear physics experiments, medical instruments and homeland security applications. Novel crystal detectors are continuously being discovered and developed in academia and in industry. In high energy and nuclear physics experiments, total absorption electromagnetic calorimeters (ECAL) made of inorganic crystals are known for their superb energy resolution and detection efficiency for photon and electron measurements. A crystal ECAL is thus the choice for those experiments where precision measurements of photons and electrons are crucial for their physics missions. For future HEP experiments at the energy and intensity frontiers, however, the crystal detectors used in the above mentioned ECALs are either not bright and fast enough, or not radiation hard enough. Crystal detectors have also been proposed to build a Homogeneous Hadron Calorimeter (HHCAL) to achieve unprecedented jet mass resolution by duel readout of both Cherenkov and scintillation light, where development of cost-effective crystal detectors is a crucial issue because of the huge crystal volume required. This paper discusses several R&D directions for the next generation of crystal detectors for future HEP experiments.

This paper presents a low-noise wide-dynamic-range pixel design for a high-energy particle detector in astronomical applications. A silicon on insulator (SOI) based detector is used for the detection of wide energy range of high energy particles (mainly for X-ray). The sensor has a thin layer of SOI CMOS readout circuitry and a thick layer of high-resistivity detector vertically stacked in a single chip. Pixel circuits are divided into two parts; signal sensing circuit and event detection circuit. The event detection circuit consisting of a comparator and logic circuits which detect the incidence of high energy particle categorizes the incident photon it into two energy groups using an appropriate energy threshold and generate a two-bit code for an event and energy level. The code for energy level is then used for selection of the gain of the in-pixel amplifier for the detected signal, providing a function of high-dynamic-range signal measurement. The two-bit code for the event and energy level is scanned in the event scanning block and the signals from the hit pixels only are read out. The variable-gain in-pixel amplifier uses a continuous integrator and integration-time control for the variable gain. The proposed design allows the small signal detection and wide dynamic range due to the adaptive gain technique and capability of correlated double sampling (CDS) technique of kTC noise canceling of the charge detector.

During the past decade, inorganic nanoparticles/polymer nanocomposites have been intensively studied to provide a low cost, high performance alternative for gamma scintillation. However, the aggregation of nanoparticles often occurs even at low nanoparticle concentrations and thus deteriorates the transparency and performance of these nanocomposite scintillators. Here we report an efficient fabrication protocol of transparent nanocomposite monoliths based on surface modified hafnium oxide nanoparticles. Using hafnium oxide nanoparticles with surface-grafted methacrylate groups, highly transparent bulk-size nanocomposite monoliths (2 mm thick, transmittance at 550 nm >75%) are fabricated with nanoparticle loadings up to 40 wt% (net hafnium wt% up to 28.5%). These nanocomposite monoliths of 1 cm diameter and 2 mm thickness are capable of producing a full energy photopeak for 662 keV gamma rays, with the best deconvoluted photopeak energy resolution reaching 8%.

A new neutron detector array (NEUANCE) is under development at the Los Alamos Neutron Science Center (LANSCE). After completion, NEUANCE will be installed in the central cavity of the 3.6π Υ-ray detector array DANCE located at the Lujan Center of LANSCE. The detector system, with simultaneous neutron and -ray detection capability, will be used to study neutron-induced capture and session reactions. The response of a EJ-309 scintillation detector to Υ-ray and neutron radiation was measured using the standard Υ-ray and ²⁵²Cf sources. The light from the detector was collected using a Hamamatsu photomultiplier tube or a Silicon photomultiplier GEANT4 was used to understand the light output and the optical photon transport in the scintillation. The detector geometry and optimum parameters for the data acquisition system were determined based on the test results and the simulations.

Investigation of prompt fission and neutron-capture Υ rays from fissile actinide samples at the Detector for Advanced Neutron Capture Experiments (DANCE) requires use of a fission-fragment detector to provide a trigger or a veto signal. A fission-fragment detector based on thin scintillating films and silicon photomultipliers has been built to serve as a trigger/veto detector in neutron-induced fission measurements at DANCE. The fissile material is surrounded by scintillating films providing a 4π detection of the fission fragments. The scintillations were registered with silicon photomultipliers. A measurement of the ²³⁵U(n,f) reaction with this detector at DANCE revealed a correct time-of-flight spectrum and provided an estimate for the efficiency of the prototype detector of 11.6(7)%. Design and test measurements with the detector are described. A neutron source with fast timing has been built to help with detector-response measurements. The source is based on the neutron emission from the spontaneous fission of ²⁵²Cf and the same type of scintillating films and silicon photomultipliers. Overall time resolution of the source is 0.3 ns. Design of the source and test measurements with it are described. An example application of the source for determining the neutron/gamma pulse-shape discrimination by a stilbene crystal is given.

A networked gamma radiation detection system with directional sensitivity and energy spectral data acquisition capability is being developed by the National Security Technologies, LLC, Remote Sensing Laboratory to support the close and intense tactical engagement of law enforcement who carry out counterterrorism missions. In the proposed design, three clusters of 2″ × 4″ × 16″ sodium iodide crystals (4 each) with digiBASE-E (for list mode data collection) would be placed on the passenger side of a minivan. To enhance localization and facilitate rapid identification of isotopes, advanced smart real-time localization and radioisotope identification algorithms like WAVRAD (wavelet-assisted variance reduction for anomaly detection) and NSCRAD (nuisance-rejection spectral comparison ratio anomaly detection) will be incorporated. We will test a collection of algorithms and analysis that centers on the problem of radiation detection with a distributed sensor network. We will study the basic characteristics of a radiation sensor network and focus on the trade-offs between false positive alarm rates, true positive alarm rates, and time to detect multiple radiation sources in a large area. Empirical and simulation analyses of critical system parameters, such as number of sensors, sensor placement, and sensor response functions, will be examined. This networked system will provide an integrated radiation detection architecture and framework with (i) a large nationally recognized search database equivalent that would help generate a common operational picture in a major radiological crisis; (ii) a robust reach back connectivity for search data to be evaluated by home teams; and, finally, (iii) a possibility of integrating search data from multi-agency responders.

One of the authors has proposed a simple-structure silicon X-ray detector (gated silicon drift detector: GSDD), whose structure is much simpler than commercial silicon drift detectors (SDDs). SDDs contain multiple built-in metal-oxide-semiconductor field-effect transistors (MOSFETs) or implanted resistors, whose fabrication processes lower the yield rate of detectors, and also require at least two high-voltage sources. On the other hand, GSDDs do not contain built-in MOSFETs or implanted resistors. Moreover, GSDDs require only one high-voltage source. Therefore, GSDDs greatly reduce the cost of the X-ray detection system. We fabricated prototype GSDDs that contained 0.625-mm-thick Si substrates with an active area of 18 mm², operated by Peltier cooling and a single voltage source. Its energy resolution at 5.9 keV from an ⁵⁵Fe source was 145 eV at -38°C and -90°V. Thicker Si substrates are required to enhance its absorption of X-rays. To detect X-ray photons with energies up to 77 keV for X-ray absorbance higher than 15%, we simulate the electric potential distribution in GSDDs with Si thicknesses from 0.625 to 3.0 mm. We obtain an adequate electric potential distribution in the thicknesses of up to 3.0 mm, and the capacitance of the GSDD remains small and its X-ray count rate remain high. The high reverse bias required in the 3-mm-thick GSDD was a third of that in a 3-mm-thick pin diode.

We investigated the response function of a planar Cd(Zn)Te detector designed for measurement of electron energy spectra and experimentally measured the response of Cd(Zn)Te detector to radiation of ⁹⁰Sr/⁹⁰Y reference radiation source. The obtained experimental spectra were compared with the spectra simulated by the Monte-Carlo method with Geant4 package. We managed to agree the simulated response with the experimental one using only two fitting parameters: products of mobility and average lifetime for electrons and holes. Thereby determined transport parameters of charge carriers were independently verified through the measurement of the positions of low energy ¹³³Ba photopeaks of a reference gamma-ray source.

We studied the electrical characteristics of Cd(Zn)Te detectors with rectifying contacts and varying thicknesses, and established that their geometrical dimensions affect the measured electrical properties. We found that the maximum value of the operating-bias voltage and the electric field in the detector for acceptable values of the dark current can be achieved when the crystal has an optimum thickness. This finding is due to the combined effect of generation-recombination in the space-charge region and space-charge limited currents (SCLC).

Using the differential thermal analysis we investigated parameters of melting and crystallization processes of the CdTe based phase in CdTe-Al system near the CdTe side (CdTe + 2 mol. % Al, CdTe + 4 mol. % Al and CdTe + 6 mol. % Al). Varying temperatures of the melts intermediate isothermal holding for 10-, 30- and 60 minutes during their heating up to 1423 K we determined conditions of the melts full homogenization. It was concluded about change of the CdTe phase melting mechanism with Al content rise.

The development of high-performance scintillation materials that emit light below 400 nm has prompted the development of improved solid-state UV photodetectors. While silicon provides a mature context for UV photodetectors, the high dark current due to its low band-gap (1.1 eV) limits the signal-to-noise performance when scaling the detector to large areas. Photodetectors fabricated in materials with a larger band-gap have the potential to surmount the performance limitations experienced by silicon. Al_xGa_1-xAs, is a material that provides a band gap from 1.55 eV to 2.13 eV, depending on the Al concentration. Using high Al concentration (0.7 < x < 1), Al_xGa_1-xAs to engineer a wider bandgap > 2eV is very desirable in terms of reducing dark noise. Due to its strong absorption of UV-light at the material surface, however, surface effects limit the quantum efficiency below 400 nm. Introducing surface layers that have a longer penetration depth for UV photons promises to boost the quantum efficiency in the UV while maintaining low dark current. This work describes the development of a photodiode fabricated in Al_xGa_1-xAs, x > 0.7, compared to an Al_xGa_1-xAs, x > 0.7 photodiode with an AlAs surface (x = 1). It presents the design of the photodiodes, simulations of their performance, the fabrication process, along with characterization data of fabricated photodiodes. We report on the surface effects of high aluminum concentration Al_xGa_1-xAs, x > 0.7, to provide a high quantum efficiency for photons below 400 nm, by examining the charge collection.

Rare earths-doped silica optical fibers have shown promising results for ionizing radiation monitoring, thanks to their radio-luminescence (RL) properties. However, the use of these systems for accurate and precise dosimetric measurements in radiation fields above the Cerenkov energy threshold, like those employed in radiation therapy, is still challenging, since a spurious luminescence, namely the “stem effect,” is also generated in the passive fiber portion exposed to radiation. The spurious signal mainly occurs in the UV-VIS region, therefore a dopant emitting in the near infrared may be suitable for an optical discrimination of the stem effect.

In this work, the RL and dosimetric properties of Yb-doped silica optical fibers, produced by sol-gel technique, are studied, together with the methods and instruments to achieve an efficient optical detection of the Yb³⁺ emission, characterized by a sharp line at about 975 nm.

The results demonstrate that the RL of Yb³⁺ is free from any spectral superposition with the spurious luminescence. This aspect, in addition with the suitable linearity, reproducibility, and sensitivity properties of the Yb-doped fibers, paves the way to their use in applications where an efficient stem effect removal is required.

Schottky barrier radiation detectors were fabricated on the Si-face of 50 μm thick detector grade n-type 4H-SiC epitaxial layers. The junction properties of the fabricated detectors were investigated by current-voltage (I-V) and capacitancevoltage (C-V) measurements. The radiation detector performances were evaluated by alpha pulse height spectroscopy using a 0.1 μCi ²⁴¹Am radiation source. Deep level transient spectroscopy (DLTS) measurements were carried out to identify and characterize the electrically active defect levels present in the epitaxial layers. The performance of the detector was found to be limited by the presence of electrically active defect centers in the epilayer. Deep level defects were reduced significantly by isochronal annealing. Surface passivation studies were conducted on n-type 4H-SiC epilayers for use on radiation detectors for the first time. Energy resolution of the detector was found to have improved after passivation and the life time killing defects that were responsible for preventing full charge collection were reduced significantly. Systematic and thorough C-DLTS studies were conducted prior and subsequent to isochronal annealing to observe evolution of the deep level defects.

Amorphous selenium (a-Se) alloy materials with arsenic, chlorine, boron, and lithium doping were synthesized for room temperature nuclear radiation detector applications using an optimized alloy composition for enhanced charge transport properties. A multi-step synthetic process has been implemented to first synthesize Se-As and Se-Cl master alloys from zone-refined Se (~ 7N), and then synthesized the final alloys for thermally evaporated large-area thin-film deposition on oxidized aluminum (Al/Al²O₃) and indium tin oxide (ITO) coated glass substrates. Material purity, morphology, and compositional characteristics of the alloy materials and films were examined using glow discharge mass spectroscopy (GDMS), inductively coupled plasma mass spectroscopy (ICP-MS), differential scanning calorimetry (DSC), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive analysis by x-rays (EDAX). Current-Voltage (I-V) measurements were carried out to confirm very high resistivity of the alloy thin-films. We have further investigated the junction properties of the alloy films with a wide variety of metals with different work functions (Au, Ni, W, Pd, Cu, Mo, In, and Sn). The aim was to investigate whether the choice of metal can improve the performance of fabricated detectors by minimizing the dark leakage current. For various metal contacts, we have found significant dependencies of metal work functions on current transients by applying voltages from -800 V to +1000 V.

Semi-insulating Cd_0.9Zn_0.1Te nuclear detector grade crystals were grown by a low temperature solution method from in-house zone refined (~7N) precursor materials. The processed crystals from the grown ingot were thoroughly characterized by using a non-destructive electron beam induced current (EBIC) contrast imaging method. The EBIC results were correlated with the infrared (IR) transmittance mapping, which confirms the variation of contrasts in EBIC is due to non-uniform distribution of tellurium inclusions in the grown CZT crystal. Electrical characteristics of defect regions in the fabricated detectors were further investigated by I-V measurements, and thermally stimulated current (TSC) measurements. Finally, to demonstrate the high quality of the grown CZT crystals, pulse height spectra (PHS) measurements were carried out using gamma radiation sources of ²⁴¹Am (59.6 keV) and ¹³⁷Cs (662 keV).