The REPTile instrument has undergone extensive testing to confirm detector operability and measure inherent system noise over temperature, energy deposition in the detectors, and incident particle flux. We have also verified instrument functionality in completely integrated spacecraft tests, measuring count rates from cosmic ray muons and a radioactive source.
Cosmic Ray Muon Testing
The first fully integrated spacecraft test involved measuring muon counts at the surface of the Earth. When a very high-energy (>100MeV) galactic cosmic ray (GCR) interacts with the atmosphere, it produces a shower of particles that dissipate its energy into the atmosphere. Muons are one of the secondary particles produced. They have a very low interaction cross-section and a mean lifetime of 2.2μs—long enough for the relativistic particles to reach the Earth’s surface. At the surface of the Earth they typically have energies of 4GeV, which is enough energy to pass through buildings and organic tissue, as well as all four of REPTile’s detectors. 4GeV muons at the Earth’s surface deposit ∼0.6MeV in the silicon detectors when perpendicularly incident, enough to trip the first reference voltage and be measured by the instrument.
To measure muon flux, the integrated spacecraft was positioned with the look direction of the instrument oriented vertically. The spacecraft was then commanded into science mode, in which the REPTile instrument begins to store count rate data. For this specific test, the CubeSat took muon data for 30 minutes, and then the data was requested and downloaded through the radio frequency (RF) link. Since the muon flux is proportional to the receiving area of the detector, we expect to have 4x more counts on the 40mm detectors than the 20mm. As expected, the larger detectors (detectors 2, 3, and 4) see ∼4x higher counts. This test confirms the basic functionality of all four detectors.
Radioactive Source (Strontium 90) Testing
The fully assembled spacecraft was also tested with a strontium-90 radiation source, fastened to the outside of the REPTile collimator. Strontium-90 has a half-life of 28 years and decays into yttrium-90, emitting an electron with maximum energy of 0.546 MeV. Yttrium-90 has a half-life of 2.7 days and decays into Zirconium-90, emitting an electron with maximum energy of 2.28 MeV. Both isotopes emit electrons in a continuous kinetic energy spectrum from zero to the maximum. An independent empirical measurement of the strontium 90 spectrum was made and fit to a power law. Using the fit, the theoretical count rate for each of REPTile’s differential energy channels was calculated. Data collected from the fully integrated strontium-90 test agreed with the theoretical count rate within expectation, confirming functionality of the instrument despite the design challenges presented by an energetic particle telescope for a CubeSat platform.