The complexity of spaceflight design requires reliable, fault-tolerant equipment capable of providing real-time dosimetry during a mission, which is not feasible with existing thermoluminescent dosimeter (TLD) technology. Real-time monitoring is important for low-Earth-orbiting spacecraft and interplanetary spaceflight to alert the crew when solar particle events (SPE) increase the particle flux of the spacecraft environment. In this innovation, the personal dosimeter is comprised of a tissue-equivalent scintillator coupled to a solid-state photomultiplier.
To make scintillation materials useful for dosimetry, a better optical detector is needed. The SSPM (solid-state photomultiplier) solves the optical detector problem. The solution involves attaching the tissue-equivalent scintillation material to an integrated detector-on-a-chip using an SSPM fabricated with complementary metal-oxide-semiconductor (CMOS) technology. This provides the means to manufacture a high-sensitivity digital dosimeter integrated with the readout and support electronics.
An array of CMOS Geiger photodiode (GPD) pixels, which are avalanche photodiodes operated in Geiger mode, comprises the SSPM. In Geiger mode, a single optical photon triggers the pixel to produce a digital Geiger pulse for each optical photon detected. The array of GPD pixels is monolithically integrated to the readout and data storage and transfer electronics. The plastic scintillator is coated with a thin material that reflects optical photons, thus scintillation photons reflect inside the crystal until absorbed by the SSPM.
The prototype device easily rests in the palm of a hand, and is powered using a compact lithium-ion battery, or is powered externally through a USB cable. The radiation-sensitive component of the dosimeter is coupled to analog signal processing components and a microprocessor that can maintain processing fidelity up to 5×105 events per second. The dynamic range of the dosimeter has been verified from 1-GeV protons (0.22 keV/mm in H2O) to 420 MeV/n Fe (201.1 keV/mm in H2O). The dosimeter confirmed doses within ≈3% of the expected dose for 1-GeV protons over 6 to 36 mSv of dose, illustrating a functional instrument for deep space missions.
This work was done by James Christian, Christopher Stapels, and Erik B. Johnson of Radiation Monitoring Devices, Inc.; and Eric Benton of Oklahoma State University for Johnson Space Center. MSC-24394-1/5254-1