Calibration and simulation of a silicon dosemeter for ambient dose equivalent in low-earth orbit space.

A particle dosemeter (PD) is a payload of NEXTSat-2 in the low-earth orbit (LEO). The absorbed dose in LEO needs to be converted into the ambient dose equivalent (H*(10)). Due to a mixed field in LEO, the calibration factors (klow and khigh) should be determined for the low-and high-linear energy transfers (LET) (below and above 1.5 keV/µm), respectively. The PD was irradiated with a 137Cs source at the Korea Radiation Solution facility to obtain H*(10) and absorbed doses. However due to the lack of sources for the high-LET calibration, H*(10) and an absorbed dose were calculated by simulating PD for the high-energy neutron field at CERN-EU high-energy Reference Field. The measured klow of PD had a difference of 5.1% and 9.5% from the calculated value of PD and the measured value of Liulin detectors, respectively. However, a difference in khigh between PD and Liulin was explained by the contribution of non-neutron components to Liulin in the measurements.

A Study on the Performance of a Silicon Photodiode Sensor for a Particle Dosimeter and Spectrometer.

A lunar vehicle radiation dosimeter (LVRAD) has been proposed for studying the radiation environment on the lunar surface and evaluating its impact on human health. The LVRAD payload comprises four systems: a particle dosimeter and spectrometer (PDS), a tissue-equivalent dosimeter, a fast neutron spectrometer, and an epithermal neutron spectrometer. A silicon photodiode sensor with compact readout electronics was proposed for the PDS. The PDS system aims to measure protons with 10-100 MeV of energy and assess dose in the lunar space environment. The manufactured silicon photodiode sensor has an effective area of 20 mm × 20 mm and thickness of 650 µm; the electronics consist of an amplifier, analog pulse processor, and a 12-bit analog-to-digital converter for signal readout. We studied the responses of silicon sensors which were manufactured with self-made electronics to gamma rays with a wide range of energies and proton beams.

Characterization of a new tissue equivalent proportional counter for dosimetry of neutron and photon fields: comparison of measurements and Monte Carlo simulations.

The tissue equivalent proportional counter (TEPC) is widely recognized as an important dosimetric technique particularly for complex radiation fields. The Korea Astronomy and Space Science Institute (KASI) has recently developed a new spherical TEPC to monitor the space radiation environment in the low earth orbit. The purpose of this study is to examine the performance of the TEPC against standard photon (137Cs) and neutron (252Cf) sources through ground-based measurements and Monte Carlo simulations prior to its actual implementation. Lineal energy distributions, microdosimetric spectra and dosimetric quantities for a 2 µm simulated site in pure propane gas were determined for both sources. Both the measured and calculated 137Cs spectra were shown to occur below 11 keV µm-1 that is the typical range covered by photon sources. Complete coincidence of their electron edge regions was also observed. Meanwhile, the proton edge from the measured 252Cf spectra was found to be in good agreement with those from the simulated ones and the literature. The gamma, recoil proton and heavy ions peaks expected for neutron sources were well defined, albeit deviations in the gamma region. The absorbed dose and dose equivalent for both irradiation conditions were also successfully obtained. The dose equivalent for 252Cf was found to be ten times the absorbed dose whereas it remained the same for 137Cs. The discrepancies observed in the low lineal energy region for both irradiation conditions were caused by intrinsic limitations on the experimental set-up and simulation configurations. This mainly contributed to a difference in the measured and calculated dose mean lineal energies of about 4.1% and 8.7% for the photon and neutron cases, respectively. Nevertheless, fair consistency with published data suggested that our TEPC could adequately reproduce the expected microdosimetric distributions for complex radiation fields.

On the origin of near-infrared extragalactic background light anisotropy.

Extragalactic background light (EBL) anisotropy traces variations in the total production of photons over cosmic history and may contain faint, extended components missed in galaxy point-source surveys. Infrared EBL fluctuations have been attributed to primordial galaxies and black holes at the epoch of reionization (EOR) or, alternately, intrahalo light (IHL) from stars tidally stripped from their parent galaxies at low redshift. We report new EBL anisotropy measurements from a specialized sounding rocket experiment at 1.1 and 1.6 micrometers. The observed fluctuations exceed the amplitude from known galaxy populations, are inconsistent with EOR galaxies and black holes, and are largely explained by IHL emission. The measured fluctuations are associated with an EBL intensity that is comparable to the background from known galaxies measured through number counts and therefore a substantial contribution to the energy contained in photons in the cosmos.