R.S. Miller 1,* , R. Freelove 1 , E. Layden 1 1 University of Alabama in Huntsville *Corresponding Author: [email protected] Inorganic scintillators are appropriate for nuclear γ-ray spectroscopy due to their high stopping power and light yields, both of which contribute directly into excellent detection efficiency. Maximizing science return, however, depends not only on g-ray detection but also on spectroscopic performance. Our goal is to optimize these factors to produce a device capable of meeting science-based spectroscopic performance requirements relevant for astrophysics and planetary science, while simultaneously minimizing size, weight, and power resources (SWaP). Silicon photomultipliers (SPM) are a viable opto-electronic alternative to traditional scintillator readout schemes. Integrated into a high-resolution spectroscopy system they represent an enabling technology, providing a number of key implementation benefits such as: ruggedness, compactness, low mass, insensitivity to magnetic fields, and low bias voltage (~30V) operation. While identified originally to address power challenges, SPMs facilitate the use of low-cost scintillating materials, achieve excellent spectroscopic performance, mitigates implementation complexity, and reduce instrument mass significantly - key benefits that in turn may reduce cost. High-Resolution Gamma-Ray Spectroscopy with Silicon Photomultipliers Abstract Silicon Photomultiplier (SPM) SPM Parameters Spectroscopic Optimization Spectroscopic Performance Ongoing Development The fundamental limit to obtain the required energy resolution is Poisson statistics - the intrinsic statistical knowledge in the number of scintillation photons detected by a suitable sensor. In general, the maximum obtainable energy resolution (FWHM) can be parameterized as where N phe is the photoelectron yield of the sensor used to detect scintillation photons, and v(M) is the variance in sensor gain [11]. This fundamental limit is shown in Figure 1, along with experimentally observed resolutions for inorganic scintillators using traditional photomultiplier tubes as readout sensors. Maximizing spectroscopic resolution requires optimization of parameters: Maximize N phe : • Materials with high scintillation light yield • Optimize match optical emission spectrum & detection QE • Maximize Photon Detection Efficiency (PDE) Minimize v(M): • Reduce sensor gain variations • Reduce scintillator non-proportionality • Mitigate scintillator crystal inhomogeneities R ≡ ∆E E =2.354 1+ v (M ) N phe Spectral characteristics. (top) The wavelength dependent SPM PDE for previous- (dark) and latest-generation (light) SPM fabrication; (middle) emission spectrum of CsI(Tl); and (bottom) effective PDE for SPM coupled to CsI (Tl). Also shown (thin middle line) is the QE for an extended-red multialkali PMT photocathode. Figure 7. Breakdown & optical current uniformity. (Top) The distribution of breakdown voltages; (bottom) the distribution of optical currents, a proxy for response uniformity. (left panels) first generation SPM production, (right panels) the latest production results (right). Optical current variations <±10% are obtainable. Measured energy resolution of scintillators for 662 keV γ-rays as a function of the scintillation yield, expressed as the number of photoelectrons observed with a photomultiplier tube (v(M)=0.1).The solid curve is the theoretical lower limit governed by counting statistics. Prototype SPM Array. The array incorporates a 3×3 matrix of SPM modules for a total of 144 pixels. Radiation Tolerance Total Irradiation Does (TID) Test Source: Energy: Dose: 60 Co ≤ 1.2 MeV 150 krad total (30 krad steps) • I dark vs. Voltage measured after irradiation • No failure of any component • 5% optical responsivity drop after 150 krad • Linear increase in dark current during irradiation (<50%) Displacement Damage (DD) Test Proton Fluence (p/cm 2 ): 10 10 , 10 11 , 10 12 Energy: 63 MeV • I dark vs. Voltage measured after irradiation • 10/13/60% optical responsivity drop at specified fluences • Increased dark current mitigated by cooling (thermionic) Uniformity High Uniformity Simplifies Implementation & Minimizes Systematics SPM detectors are manufactured using standard CMOS technology which results in highly uniform microcell breakdown characteristics, typically within ±0.06V. Such a small breakdown range is significant since it simplifies the electronics requirements for biasing large numbers of detectors. Response uniformity is also good, variations are less than ±10% max/min, a 4-fold improvement in uniformity over the previous generation of devices. High uniformity makes it possible to discriminate precise numbers of photoelectrons (i.e. photon counting) detected as distinct, discrete levels upon readout, with dynamic range limited by the number of microcells. Optical Performance High-Sensitivity Optical Photon Collection Improvements in fabrication techniques have led directly to increases in SPM optical responsivity. Characterized by photon detection efficiency (PDE) - the product of the device ʼ s wavelength dependent quantum efficiency (QE) and the fill factor of the photosensitive area - it can be directly compared to the QE of traditional photomultiplier photocathodes. The current generation of SPMs have peak performance at wavelengths >400 nm and improvements in fabrication techniques have led to a >2-fold improvement in SPM optical responsivity over devices produced only a year ago. Spectral Response Responsivity Match to Bright Scintillators Thalium-doped cesium iodide, CsI(Tl), is an excellent spectral match to SPM PDE. • Yield: 54,000 photons/MeV • Peak Emission: 540 nm • Hygroscopic: slightly • Primary Decay Time: 1000 ns • Non-Proportionality: <5% above 0.1 MeV Plateauing above 1 MeV The SPM is a novel, high gain, single photon sensitive sensor based on a summed parallel array of identical and independent Geiger-mode avalanche photodiodes and quenching resistor combined into elements called microcells. SPM detectors are manufactured using standard CMOS technology which results in highly uniform breakdown characteristics. Each microcell is: • Structured as a p-n diode • Provides low-noise amplification of single photoelectrons (~10 6 gain) • Biased above the breakdown voltage with no current flow • Photon initiates avalanche breakdown Microcells are pseudo-binary single photon detectors Schematics of single microcell (left) and subsection of an SPM array ʻpixelʼ (center), and a photo of a similar subsection of an actual SPM (right) Measured SPM Pulse-height (pC) Proxy for Number of Firing Microcells Proxy for Number of Incident Photons Proxy for Incident γ -ray Energy Single-pixel photoelectron histograms are highly discrete, owing to the SPMʼs: • Inherent high gain • Low multiplication noise • Uniform response within a pixel These features enable SPM pulse-height measurements to serve as a proxy for microcell counting and as a proxy for the number of photons incident on SPM. Prototype Array Based on Senslʼs SPMArray4 • 3×3 mm 2 pixels, 144 pixels per array • Signal readout for each pixel • Summed for spectroscopy applications • Integrated digital temperature sensor for gain compensation Laboratory measurements of prototype SPM-based spectrometer module utilizing CsI(Tl). Data shown were obtained at room temperature (~23˚C) and 10˚C using laboratory radiological standards at a bias voltage 2V above breakdown. Resolution is anticipated to improve by ~20-25% at a bias voltage 4V above breakdown, with a corresponding increase in noise - impact to spectroscopic resolution under study. Room Temp 10˚ C 1st Generation SPMArray4: 2nd Generation SPMArray4: SPM Array Prototype 7.5% 4.7% − 2.6% TBD TBD • FWHM @ 662 keV • Dark Count: ~8MHz @ 23˚C • Dark Count: ~0.5kHz @ 10˚C All performance results validated/ duplicated with analytic model of SPM functionality • Bias Voltage: 30V • Temperature: 23.7˚C • Front-End Electronics - Leverage Proven Space-Qualified FEE Implementation Approaches • Module Design, Assembly, and Thermal Modeling - Evaluation of Passive & Active Cooling Approaches, Inform Assembly Design • Additional Radiation Tolerance Testing - Derive Impact to Spectroscopic Performance • Instrument Performance & Simulation - GEANT-based Spectrometer Module Simulation, Incorporate Into Full Instrument Model(s) Acknowledgement Supported in part by a LUNAR NASA Lunar Science Institute research grant References for presented work available upon request Optimization, combined with next-generation opto-electronic readout devices provides high-resolution, cost-effective gamma-ray spectroscopy solutions