NOTE: End date changed to 12/31/2008 per NSBRI (5/2008)

The original aims of the project were to

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, 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 tissues 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 although only a short time was available for its design and development by the students.

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

We have 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 have 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. We were able to achieve in our benchtop system with a 10 um thick sensor 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 and could be packaged into a volume of less than 12 x 8 x 4 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 verify 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 National Space Biomedical Research Institute (NSBRI) grant.

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.

1. Based on our experience with the MIDN development, we designed and developed an advanced version of the instrument.

2. A prototype was developed that although did not include all of the specifications was able to achieve with a 10 um thick sensor a dE/dx ~ 3 keV/um in silicon that is equivalent to a lineal energy of ~1 keV/um in tissue.

BENCHTOP DEVELOPMENT SYSTEM

1. By designing and constructing a new Faraday cage that houses the sensor and preamplifier circuit, upgrading the signal transmission circuitry between the system and the data acquisition area, and designing a new data acquisition method, we were able to reduce the inherent noise level well below a keV/micron, allowing detection of the peak of the dose distributions for minimum ionizing protons, the most difficult particles to detect microdosimetrically.

2. In collaboration with the M. Sivertz and A. Rusek at BNL, we have developed a system that allows identification of incident particles, categorized them according to their mass-to-charge ratio and energy, and correlated them with individual events in the microdosimeter. Recall that our earlier work in this regard resulted in our identifying lighter ion contaminants in the beam and their contributions to the microdosimetric spectra, a fact that we subsequently learned was known to BNL personnel.

3. We measured the energy deposited in a microdosimeter with radiation beams of Carbon at 290 MeV/n and protons at 1 GeV/n, 600 MeV/n, 250 MeV/n, 100 MeV/n, and 50 MeV/n at the NSRL facility at the BNL and achieved a lower energy cutoff of < 1 keV/um in silicon equivalent to a lineal energy cutoff in tissue of < 0.3 keV/um.

ADVANCED SENSOR DEVELOPMENT

1. We now have prototypes of a new design of a solid-state microdosimeter with three dimension micron sized sensitive volumes, addressing some of the shortcomings identified earlier. This sensor was developed at the Centre for Medical Research Physics, and a new grant (Australian Research Council Discovery Project) was recently received by our collaborator to further support this project.

2. We have established collaborations with the EE (electrical engineering) departments at Johns Hopkins University (JHU) to explore the potential of developing alternative silicon sensors. These new sensors will be developed as part of our follow-on grant from the NSBRI.

3. With minimal support, JHU was able to supply us with two dies that have a variety of diodes for preliminary testing. A test fixture was developed to carry out tests, and measurements of alpha particles were successfully conducted.

RADIATION TRANSPORT CODES

1. We imported the radiation transport code GEANT4 and two corollary programs MULASSIS (multilayered shielding simulation software tool) and GEMAT. These Monte Carlo codes allow us to simulate the microdosimetry spectra in silicon devices.

2. We also have access to the MCNPX (Monte Carlo N-Particle eXtended) radiation transport code.

NOTE: End date changed to 12/31/2008 per NSBRI (5/2008)

1. Support the launch of the MIDN-MidSTAR instrument. We caution that this experiment is a student built instrument and is consequently a high risk opportunity.

2. Support data acquisition and reduction of the MIDN-MidSTAR data.

3. Carry out additional radiation beam tests at Brookhaven National Laboratory.

4. Reconcile the radiation beam tests with digital simulations

During the year as experimental data was becoming available with the present system we added an additional task of identifying and initiating development of improved solid-state microdosimeter sensors. We have satisfied several but not all of our second year objectives. Recall that the MIDN-I instrument for the MidSTAR-I mission consisted of three sensor systems (one exterior to the spacecraft, the second internal to the spacecraft, and the third internal to the spacecraft in a polyethylene absorber) connected to a custom multi-channel analyzer with storage and command capability. The instrument was integrated into the spacecraft on an accelerated schedule as the spacecraft was behind schedule. We originally requested a voltage of +/-9V which late in the program were told that the spacecraft could not supply and agreed to +/-6V. During initial integration of the instrument we found that only +5V was available. The spacecraft power system was changed to also provide +/- 5V which was marginal for our instrument. Integration with the spacecraft communication system was successful and sample pulser data was retrieved prior to and subsequent to spacecraft vibration and thermal tests. However, the noise on the +/-5 V power lines had a significant ripple well outside of specifications that directly affected the MIDN lower energy cutoff. Because of the delays that had already occurred in designing and producing the spacecraft, the team decided to proceed with the described problems. We added filters to suppress this noise but they were only marginally effective. Introducing a battery powered interface to the spacecraft power system was not an option because of the late date at which we has access to the spacecraft for testing. As a result, we were forced to set our lower energy threshold significantly higher than originally planned and much above the performance level of the system itself.

The spacecraft was launched on 9 March 2007 from the Cape Canaveral Air Force Station on an Atlas-V Centaur launch vehicle as part of a 6 military spacecraft mission. Midstar's orbit is an altitude of 492 km at an inclination of 46 degrees. MIDN has two modes of operation. The first mode is with the electronic pulser activated, to evaluate overall instrument performance. The second mode is with the pulser off so that observations can be made. The instrument with the pulser turned on is working as anticipated but the data collection mode has not obtained useful data due to the high energy cutoff.

Analysis subsequent to launch based upon the spectra measured at the Brookhaven facility and our transport codes indicates that given the anticipated low proton fluxes at the spacecraft altitude, the expectation of collecting data is remote. The instrument continues to operate and so far has accumulated 39 days of observations. On occasion in the pulser mode, the instrument does provide spurious data that we attribute to the magnitude of the minus voltage being less than 5 V. Improvements continue to be made to the bench-top engineering model used for developmental testing at the Naval Academy and at the NSRL facility at Brookhaven National Laboratory. Our runs at the NSRL in March 2007 were with carbon at 290 MeV/n and protons at 1 GeV/n. As a result of the improvement that we had made in our instrumentation, we reduced our low energy cutoff to < 1 keV/micron; a significant accomplishment. In addition, we compare favorably with that published data on tissue samples. We have prepared a presentation for the Navy and DoD Space Experiments Review Boards (SERB) proposing a MIDN-II instrument for a spacecraft or Space Station Mission that is planned to be presented in July and November 2007.

Our plan for next year is to:

1. Continue to support data collection of the MIDN-I instrument on the MidSTAR-I spacecraft.

2. Petition the NAVY and DOD SERB for a flight on the Space Station or a spacecraft of opportunity.

3. Evaluate the new potential microdosimeter sensors developed by the Electrical Engineering Department at Johns Hopkins.

4. Complete design a MIDN-II that is battery powered with characteristics such as the lower-energy threshold and amplifier gain that can be monitored and changed by remote command along with a total end-to-end calibration that includes a light-source that can be turned on or off to directly test the functionality of the entire instrument.

5. Carry out additional radiation beam tests at Brookhaven National Laboratory and reconcile data with the radiation transport code Geant-4.

1. The MIDN-I instrument was launched on the MidSTAR-I spacecraft.

2. Prior to launch and subsequent to vibration and thermal testing the instrument performed as anticipated but the low energy threshold had to be increased to reduce pile up due to noise much higher than we specified on the power lines from the spacecraft.

3. Filters added to reduce the noise were only partially effective.

4. Pulser data was consistent with pre-installation data and non-pulser data showed no counts.

5. In orbit the pulser, for the most part, has essentially reproduced the prelaunch observations but the non-pulser data has produced no observations.

6. We continue to collect observations and troubleshoot the performance of the instrument.

7. Ground based tests in which the ƒ{5 V supply has its magnitude reduced tend to reproduce many of the observed phenomenon.

8. Lessons learned: Need to insolate MIDN from the power supply by using batteries and utilize an LED in place of a pulser to obtain an end to end system test.

MIDN-II

1. Based on our experience with the MIDN-I development, we are designing an advanced version of the MIDN instrument called MIDN-II.

2. Requirements include: a. gain changed by remote command, b. lower-energy threshold changed by remote command, c. power supplied by dual sets of batteries with one charging while other provides power, d. exploring increased use of digital components to reduce low energy cutoff, e. use of a LED to excite the sensor to provide and end to end system test, and f. utilization of radiation hardened parts.

BENCHTOP DEVELOPMENT SYSTEM

1. By redesigning the Faraday cage that houses the sensor and preamplifier circuit and changing the cabling, we were able to reduce the inherent noise level.

2. We measured the energy deposited in a microdosimeter with radiation beams of Carbon at 290 MeV/n and degraded protons initially at 1 GeV/n at the NSRL facility at the BNL and achieved a lower energy cutoff of < 1keV/micron.

1. Complete qualification of the MIDN-MidSTAR instrument through vibration and thermal vacuum testing and integrate it into the MidSTAR spacecraft supporting efforts at the launch site, Cape Canaveral.

2. Continue development of the engineering and bench-top models to explore reductions in noise, power, and mass and increase sensitivity.

3. Develop the first version of the MIDN instrument to be used for beam tests in year 2.

4. Carry out preliminary testing at the Naval Academy with radiation sources and simulated pulses and carry out two trips to Brookhaven National Laboratory for additional beam tests.

5. Finalize implementation of the GEANT4 and MCNPX radiation transport codes and use the codes to help interpret the radiation test data.

We have satisfied our second year objectives.

The MIDN-MidSTAR instrument has been completed. Electrical tests, alpha source calibration tests have been carried out, the instrument has been vibration testes at the Naval Research Laboratory, and preliminary integration tests with the spacecraft are ongoing governed by the spacecraft development schedule. Integration with the spacecraft communication system was successful and sample pulser data was retrieved. The bench-top engineering model has been improved and used to carry out alpha calibration tests of the sensors at the Naval academy and in March at Brookhaven National Laboratory where we were assigned 24 hours of beam time of Fe. The bench-top system was also used in June for 32 hours of beam timeat Brookhaven of Fe, Ti, O, and protons. Because of schedule considerations and beam intensities of the Fe beam, we were did not test the MIDN-MidSTAR instrument at Brookhaven. Instead we carried out calibration tests with radioactive sources at the Academy and used the identical MIDN-MidSTAR sensor electronics during the beam tests at Brookhaven to assure its performance. The GEANT4 and MCNPX codes are operational and have been used to help interpret the experimental data. The space radiation transport code HZETRN was updated to more accurately represent the energy and charge spectra data measured by the Advanced Composition Explorer (ACE) for the last two solar cycles. The description of the energy and isotopic spectra of target fragment produced locally in a small detectors such as MIDN by high-energy protons and neutrons was improved using the quantum multiple scattering model of fragmentation (QMSFRG). QMSFRG has been extended to include a 190-isotopic grid and to add the contributions of nuclear coalescence for the production of 2-H, 3-H, 3-He, and 4-He fragments in nucleon and heavy ion induced reactions.

In addition we were selected for potential of a flight on the Space Station by the Department of Defense Space experiments Review Board. In addition, we may have a flight opportunity on another small satellite that the Academy may build.

Our plan for next year is to:

1. Support the launch of the MIDN-MidSTAR instrument. We caution that this experiment is a student built instrument and is consequently a high risk opportunity.

2. Support data acquisition and reduction of the MIDN-MidSTAR data.

3. Carry out additional radiation beam tests at Brookhaven National Laboratory.

4. Reconcile the radiation beam tests with digital simulations.

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 the situations and measurements described in the previous paragraph.

1. MIDN-MidSTAR.

a. Fabrication of the MIDN-MidSTAR instrument was completed.

b. Electrical testing was completed.

c. Calibration testing using an alpha source was completed.

d. Vibration testing of instrument at the Naval Research Laboratory was completed.

e. Initial integration of the instrument with the spacecraft communication system was initiated and will be carried out over the summer.

f. The spacecraft was able to turn the instrument on, initiate the built-in pulser, instruct the instrument to collect and store data, and then transmit the data to the spacecraft memory which was recovered and compared exactly with what was expected. These intial tests demonstrated that the spacecraft power system had a major problem, now being addressed through a redesign.

g. Continued testing will restart when the spacecraft power system is redesigned and the spacecraft flight harness completed.

h. MIDN requires 1.1 Watt of power.

2. The bench-top system was completed and issues of variations in noise from chip to chip were successfully addressed. Calibration of the sensors with the bench-top system established the calibration and dynamic range of the MIDN-MidSTAR instrument.

3. Two proposals were written to solicit radiation beam time at the Brookhaven NASA Space Radiation Laboratory. Awarded were 24 hours in the Spring 2006, 32 hours Summer 2006, and 32 hours Fall 2006; the total amount of time requested in the proposal.

4. The bench-top system was used in the radiation beam tests at Brookhaven National Laboratory that included Spring and Summer campaigns:

24 hours of Iron at 1 GeV/nucleon; 8 hours of Iron at 0.6 GeV/nucleon; 8 hours of Oxygen at 1 GeV/nucleon; 8 hours of Titanium at 1 GeV/nucleon; 8 hours of protons at 1 GeV/nucleon

4. Radiation Transport Codes.

Calculations were made using Geant4 and SRIM to compare the experimental data with the simulations. The Academy has been designated a beta site for the MCNPX radiation transport code. The space radiation transport code HZETRN was updated to accurately represent the energy and charge spectra data measured by the Advanced Composition Explorer (ACE) for the last two solar cycles. The description of the energy and isotopic spectra of target fragment produced locally in a small detectors such as MIDN by high-energy protons and neutrons was improved using the quantum multiple scattering model of fragmentation (QMSFRG).

5. The Navy and DoD Space Experiments Review Boards have each approved a version of the microdosimeter for a potential flight on the International Space Station for a shielding experiment. Funding must be secured.

6. Preliminary discussions have been held for inclusion of the MIDN instrument in a spacecraft called ParkinsonSat which the Academy may build. Again funds must be secured.