A microdosimeter is perhaps the only active detector capable of directly determining the mean radiation quality of a mixed or unknown radiation field. Therefore, the dose equivalent and effective dose can be developed, from which the radiation risk can be assessed in real time. Objectives of this research project were to develop a rugged, portable, low-power, low-mass, solid-state microdosimeter suitable for an area sensor, a spacecraft, habitat or as a personnel monitor, such as a spacesuit. Objectives also included verification of its performance through radiation source and beam tests and comparison of experimental results with radiation transport codes. The original objectives were expanded to include a student-developed instrument for the MidSTAR-I spacecraft launched in March 2007.

. Original Aims

.
1. Demonstrate that a small, compact and portable flight-qualifiable, solid-state microdosimeter can be developed to measure quantitative information on the dose and dose distribution of energy deposited in silicon cells of tissue size and by inference in tissue.

2. Analyze data from radiation beam experiments and compare with radiation transport codes to provide quantitative information on the radiation environment, potential risk and the accuracy of the codes to correctly calculate energy deposition spectra.

3. With data from radiation beam experiments correlated with radiation transport codes, to determine the effectiveness of selected materials to minimize the total risk from primary and secondary radiation.

The specific objectives of the MIcroDosimeter iNstrument (MIDN) instrument are to:

1. Make real-time measurements of the radiation environment to assess risk (dose equivalent).

2. Actively warn crew during onset of enhanced radiation events.

3. Allow crew to determine safe locations during enhanced radiation events.

4. Provide observations to validate and improve space radiation environment models.

5. Provide observations to validate and improve radiation transport theories for shield materials and different tissue types.

While not part of this proposal, a student design effort developed an early version of a MIDN instrument that was launched on the MidSTAR-I spacecraft in 2007 with only a short time available for its design and development by the students.

We have satisfied, most but not all, of our aims and instrument development objectives.

We successfully evolved two sets of instrumentation: a bench-top system to evaluate instrument components without regard for power or size and two prototype flight instruments. Each instrument consists essentially of a sensor, sensor electronics, amplifiers, analog-to-digital conversion and a multichannel analyzer under computer or microprocessor control. We tested these instruments at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). Ions examined include iron, oxygen, silicon, hydrogen, carbon and titanium. In our benchtop system with a 10 um thick sensor, we were able to achieve a dE/dx < 1keV/um in silicon that is equivalent to a lineal energy of approximately 0.4 keV/um in tissue. In our flight prototype instrument with a 10 um thick sensor, we were able to achieve a dE/dx ~ 3 keV/um in silicon that is equivalent to a lineal energy of ~ 1 keV/um in tissue. The anticipated flight system will require a power of approximately one watt, could be packaged into a volume of less than 1284 cm, and should have a dE/dx < 1keV/um in silicon that is equivalent to a lineal energy of approximately 0.4 keV/um in tissue. These are significant accomplishments that satisfy the primary objectives of the research and verify our original hypotheses that silicon microdosimetery appears to be a viable alternative to assess a mixed and unknown time-varying radiation field to estimate regulatory risk.

This is the final year of this NSBRI grant.

Earth-based Applications of Research Project
Microdosimetric techniques are perhaps the only experimental methods for actively determining the radiation quality of mixed or unknown radiation fields and their dose equivalent. Likewise, the compact nature of a solid-state microdosimeter along with its low-voltage and low-power consumption and remote telemetry makes such a device ideal for in-suit personnel monitors as well as area monitors. The radiation quality and the corresponding dose equivalent and/or effective doses form the basis of regulatory dose limits both in the U.S. and internationally as the basis for the evaluation of potential overexposures. Generally, in radiation fields with average quality factors greater than one, those radiation components with the highest quality may represent a component of the dose comparable to the dose uncertainty. For example, as the energy of x-ray therapy machines increases to accommodate intensity modulated radiotherapy and other new techniques, the contributions of secondary neutrons produced in the shielding materials to the whole-body exposure of the clinical personnel as well as the patients themselves increase. With a quality factor as high as twenty, a one or two percent neutron component can contribute as much as twenty to thirty percent of the dose equivalent. Likewise, in radiation storage and clean-up, it is the dose equivalent or effective dose, not the physical absorbed dose, that determines the need and level of clean up, yet it is the physical dose that is usually measured because of the difficulty in measuring dose equivalent in the field by personnel who are not experts in microdosimetry. Finally, the detection of radiation emitted by nuclear materials that may be used in terrorist activities requires cheap, reliable and rugged microdosimeters that can determine small changes in the radiation environment and issue reliable alerts in real time.

The use of prior methods is limited in part because of the complexity, sensitivity and lack of reliability of the most commonly used instruments, gas proportional counters. The compact system that we have developed for space applications would likewise be applicable for these situations and measurements described in the previous paragraph.

We have established for the first time in a solid-state microdosimeter a lowered energy cutoff of dE/dx < 1 keV/um in silicon that is equivalent to a lineal energy cutoff of < 0.4 keV/um in tissue. Thus we have an instrument that can be used in space and terrestrially to directly assess regulatory risk.

Project Description
NASA Task Book Entry