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Radiation-Resistant and Temperature-Insensitive Solid State Photomultipliers


TECHNOLOGY AREA(S): Materials, Electronics, Nuclear, Sensors

OBJECTIVE: To improve or develop silicon-based photomultipliers that are radiation resistant and insensitive to variations in environmental temperature, suitable to be used in equipment for warfighting missions under nuclear battlefield environments. The proposed new Silicon-based Photomultiplier (SiPM) shall demonstrate orders of magnitude higher radiation tolerance for both below and above the breakdown voltage over the commercial state-of-the-art SiPMs available today.The new SiPM shall also demonstrate orders of magnitude reduction in gain sensitivity to the environmental temperature, with the objective to achieve gamma-ray isotope identification without external temperature compensation.While silicon-based photomultipliers remain to be preferred development path due to the high level of technology maturity and industry support, this topic does not rule out the use of other wide band gap semiconductor materials [2], referred to as solid state photomultiplier, or SSPM, as long as the performance requirements described herein can be met.

DESCRIPTION: Silicon photomultipliers have many advantages over the traditional vacuum-based photomultiplier tubes (PMTs), including single photon sensitivity, higher photon detection efficiency, lower bias voltage/power consumption, compactness, matching wavelength with the emerging high-performance scintillators, negligible aging effects, immunity to magnetic fields, and can be mass produced. The main drawbacks of SiPMs, compared to PMTs, include high dark count rate, limited effective detection area, cross-talk between neighboring cells, after pulsing,TAB A and they are subject to radiation damages. These drawbacks has severely limited its applications in programs intended for the nuclear battlefield, such as the NBCRV Merlin/Viper and CSIRP programs. In recent years, these drawbacks have been dramatically reduced thanks to cooperative developmental effort of the private industry and government. Despite this, progresses in reducing SiPM dark current dependency on radiation and temperature are still lacking to fully qualify the field deployment of SiPMs in radiation detectors operating under nuclear battlefield environments.

The effect of radiation in silicon detectors [3] below the breakdown voltage at which avalanche multiplication becomes significant is very well studied and documented. Radiation damage to silicon photomultipliers primarily include x-ray induced surface radiation damage and high-energetic electrons, photons and hadrons (such as neutrons) induced bulk radiation damage. Factors limiting SiPM [4] operation in high-radiation environment including significantly increased dark current during operation below and above the breakdown voltage due to deep level defects produced by radiation in the silicon bulk, and loss of single photon counting resolution due to the increased noise. As an example, a typical SiPM can increase its dark current by about three orders of magnitude after irradiation with neutrons at 1-MeV-equivalent energy up to fluence of 1014-1015 cm-2 at -30 °C below voltage, whereas above breakdown voltage the increase can be more than six orders of magnitude. A considerable degradation in SiPM performance is already evident even at much lower neutron fluence of 108-1010 cm-2 [5]. Consequently, SiPMs lose single photon counting resolution at relatively low neutron fluence, typically around ~1010 cm-2 at room temperature. Other limitations under high-radiation environments include reduction in SiPM gain and photo detection efficiency due to high generation-recombination rate, damage in dielectric-silicon interface and charge trapping in non-depleted entrance layer; increased power consumption due to p-n junction temperature; and increased breakdown voltage due to induced changes in doping concentration. It is well-known that changing temperature can have a significant impact on SiPM operations, most notably fluctuates the breakdown voltage and the dark current. As a result, all SiPMs presently integrated in radiation sensors require external temperature compensation for gain stabilization, e.g. the bias voltage is adjusted according to the environmental temperature in order to keep the overvoltage fixed.

For these reasons, even though SiPMs have many advantages over PMTs working with modern scintillators, their nuclear survivability is limited and thus are largely excluded from deployment where nuclear survivability is a key requirement, such as for the CBRNE sensor suite upgrade for the Stryker Nuclear Biological Chemical Reconnaissance Vehicle (NBCRV). In order to take the advantage of new families of high-performing scintillators, the defense community has an urgent need to improve the radiation tolerance and temperature insensitivity of the SiPMs. This topic seeks solutions to overcome the operational limitations of the SiPMs in high-radiation environment and to minimize the temperature sensitivity of the SiPM on its gain stability while maintaining many of the competitive edges of the SiPMs over PMTs. Potential directions for such SiPMs development include but are not limited to:

  • Reducing the dark noise generation in SiPMs by avoiding surface current reaches the multiplication region, reducing diffusion from the non-depleted bulk, and optimizing the field in the depletion region;
  • Reducing the cell occupancy by reducing the cell active volume and cell recovery time;
  • Limiting breakdown voltage increases by reducing the thickness of the depletion region;
  • Reducing damage in SiPM entrance window.
  • Reducing the temperature coefficient of the quenching resistors.

At the end of the Phase I and II development, the goal of this SBIR is to demonstrate pathways and develop near commercial-grade prototype SiPMs capable of operating under nuclear environments, respectively. If the photomultiplier(s) is developed based on other wide band gap semiconductor substrate alternative to silicon, the SSPM shall be capable of achieving the same objective.

Specific requirements for each development phases are:

PHASE I: Develop detailed realistic simulation model of SiPMs/SSPMs in order to optimize the SiPM/SSPM designs for sensor applications in nuclear battlefields. Conduct experiments to systematically quantify the radiation fluence dependence of the individual parameters, such as mobility and multiplication coefficient as function of doping, field-enhanced generation of traps, microscopic measurements on defects through defect engineering, etc. The outcome of the optimization will depend on the targeted wavelength and the expected operation temperature for the SiPM/SSPM. In addition, model the effect of annealing to recover single photo-electron resolution to better define limits on the operational range. At the end of Phase I, identify proof-of-concept and potential pathways to develop radiation-resistant and temperature-insensitive SiPMs/SSPMs that can meet the Phase II threshold/objective requirements for prototyping in Phase II.

PHASE II: Further optimizations for the candidate pathways to achieve highly radiation-tolerant and temperature-insensitive SiPMs/SSPMs are conducted. Develop at least two copies of SiPMs/SSPMs prototypes for each design variation. Demonstrate the prototype survivability on nuclear battlefield environment by meeting the following threshold {objective} performance requirements [6][7]:Graceful degradation that still allows the SiPM to be used for dose rate measurements {retain the original performance} after irradiation under 108 cGy/sec of gamma-rays at 667 keV (equivalent to 2.5x1017 cm-2/s gamma-ray flux), up to a total gamma-ray dose limit of 1000 cGy.Graceful degradation that still allows the SiPM to be used for dose rate measurements {retain the original performance} after irradiation under 4x1010 cGy/sec of neutrons at 1-MeV-equivalent energy (equivalent to 1018 cm-2/sec neutron flux), up to a total neutron dose limit of 1000 cGyGain variation is less than 5% for 667 keV gamma-rays over a temperature range of -20 °C to +50 °C with minimal temperature compensation {less than 1% without temperature compensationTest the prototype SiPMs/SSPMs in a relevant environment at government designated test facility, such as the pulse reactor at White Sands Missile Range (WSMR). Evaluate the results to determine the ability of the proposed solution to satisfy requirements for military use in the nuclear battlefield.

PHASE III: Following a successful Phase II development and demonstration, Phase III will further improve SiPM/SSPM design, engineering, ruggedization, scalability, manufacturability, and maturation to fully meet nuclear survivability requirements, including the development of a plan to enable successful technology transition at the end of this phase. Develop dual commercial/military use system(s) to integrate SiPMs/SSPMs into radiation sensors, enhancing the tool set of Warfighters while minimizing the exposure to risks.

KEYWORDS: radiation detection, neutron/gamma radiation detection, photo detector, single photon detection, radiation damage


[1] K. Shah, “New scintillator detectors”, 19th International Conference on Crystal Growth and Epitaxy (ICCGE-19), Keystone, CO, Jul 2019. [2] G. Lutz, “Semiconductor radiation detectors - device physics”, Springer Verlag Berlin/Heidelberg. [3] G. Lindstrom, “Radiation damage in silicon detectors”, Nucl. Instr. & Meth. A, 512 (1) (2003) 30 – 43. [4] E. Garutti, et. al, “Radiation damage of SiPMs”, Nucl. Instr. & Meth. A, Vol. 296, 11 May 2019, Page 69-84. [5] M. Calvi, et. al, “Single photon detection with SiPMs irradiated up to 1011 cm-2 1-MeV-equivalent neutron fluence”, arXiv:1805.07154v2 [physics.ins-det] 15 Jan 2019. [6] ICRP Publication 74, Conversion coefficient for use in radiological protection against external radiation. [7] Seog-Guen, Kwon, et. al, “Calculation of Neutron and Gamma-Ray Flux-to-Dose-Rate Conversion Factors”.

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