The purpose of this task was to design, build, and assemble a prototype Tissue Equivalent Proportional Counters (TEPC) that would satisfy the basic specifications outlined by NASA for a dosimeter for astronauts during lunar EVAs (extravehicular activities) and as area monitors in space craft and habitats. The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18 mm and the wall thickness is 3 mm for a total diameter of 24 mm (~1 inch). Aluminum vacuum chambers with a shell thickness of 0.5 mm were designed and gold plated to maintain electrical conductivity. A system using a high sensitivity mass spectrometer was assembled to measure vacuum leaks for the assembled detectors with high special specificity.

We have been using a version of the software package LORENTZ 3D to model the electric field inside a spherical detector with a linear collector. This uses special modeling techniques based on the Boundary Element Method to make the solution of these very challenging problems a simple matter. The geometry of the problem can be created with the geometric modeler built into the electric field solvers or can be imported from any of the major CAD (computer-aided design) vendors. More importantly, the geometry can be changed parametrically to optimize a design for robustness, weight, size and, of course, cost.

We have fabricated seven versions of the TEPC and Vacuum Chamber. Three versions with spherical detector and single wire anode operated with the wall at high voltage and the anode at ground were delivered for electronics development and testing. Two versions with a spherical detector and single wire anode operated with the wall at ground and the anode at high voltage have been used for TEPC development and comparative analysis. Two versions of a detector using a new hybrid design with a parallel wire grid surrounding the anode we designed and built. The objective of this design was to form a virtual cylindrical geometry around the anode with that would improve the spatial resolution of the TEPC without distorting the signals required for microdosimetry applications. The single and multi wire detectors with grounded anodes were exposed to Neutrons from a PuBe source at Colorado State University and high energy charged particle beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC) synchrotron in Chiba Japan and the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory. This included the following ions and energies: 56Fe (380 MeV/amu), 18Ar (300 MeV/amu), 12C (200 MeV/amu), and 1H (230 MeV/amu). Measurements were taken at several angles of incidence to determine the angular response of the detector. These results were compared with similar measurements using a commercial TEPC with a Rossi design that has a helical grid surrounding the anode to provide a uniform angular response. Analysis of detector response using digital signal processing was initiated. This system replaced a preamplifier and shaping amplifier with a programmable logic device (PLD) that captures the output signal from the TEPC and digitizes the amplitude in 10 nsec intervals.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during solar particle events (SPE) on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. These data were compared with the dose that would be delivered to the skin beneath a space suit with an areal density of 0.4 g/cm2. It is clear that a stainless steel vacuum chamber in Mod 1 needs to be replaced with lighter and thinner materials. These results will be important in determining what additional modifications will be necessary to achieve the design goal for real time measurements to the skin and blood forming organs (BFO).

Task 3: Modeling the Variance-Covariance Method

The original proposal for the EVA dosimeter was based on the concept of having two independent proportional counters that would be used to obtain estimates of dose, D, and a quality factor, Q, based on estimating using the variance-covariance method. It was recognized that because of size limitations, the proportional counters would have to be located too close to one another to satisfy the condition that a single particle could not intercept both detectors. The additional constraint that one of the detectors must measure the dose at the skin surface and the other at a depth corresponding to the blood forming organs, makes the original variance-covariance method with paired detectors impractical. We have developed a method using a single detector in a variance-covariance scheme. The concepts are based on collecting the charge, q(i), in a single TEPC for n successive time intervals, i. The method proposed by Borak at Colorado State University (CSU) separates the data set into two groups of n/2 entries of values for q(i) based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance and each of the two sets of n/2 values (odd or even) to estimate a variance. Monte Carlo codes have been written to test the algorithmic using microdosimetric spectra obtained from measurements in Task 1. The tests indicated that if the change in dose rate between successive intervals was less than 1%, the single detector scheme provided reliable estimated dose rate and dose averaged lineal energy for estimating quality factors. The analysis also indicated that the estimate of dose mean lineal energy for high energy heavy ions (HZE) and recoil protons from PuBe neutrons converged to the correct value when the number of intervals exceeded 100 and the width of each interval was selected such that the mean number of events in each time interval (i) was less than 30.

Three Detector systems have been delivered to Texas A&M University (TAMU) for installation of the preamplifier and on to NASA Ames Research Center for data acquisition and analysis hardware. The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18 mm and the wall thickness is 3 mm for a total diameter of 24 mm (~1 inch). A gold plated aluminum vacuum chamber designed to accommodate the TEPC and pre amplifier has been fabricated and leak tested. The hemispherical dome and vacuum chamber wall surrounding the TEPC has a thickness of 0.5 mm. The units were assembled and leak tested using a He vacuum leak test system.

Task 2: Determine the response of the TEPC to low energy protons expected during a SPE

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. These data were compared with the dose that would be delivered to the skin beneath a space suit with an areal density of 0.4 g/cm2. These results will be important in determining what modifications will be necessary to achieve the design goal that the EVA dosimeter provides real time measurements to the skin and BFO.

Task 3: Development and testing of the Variance/Covariance algorithm

The original variance/covariance method requires two independent detectors to measure dose and dose averaged lineal energy that is used to obtain a radiation quality factor for the incident particles. We have been developing a method using a single detector in a variance-covariance scheme The concepts are based on collecting the charge, q(i), in a single TEPC for n successive intervals of (i) with the condition that the change in dose rate is very small between (i) and (i+1). The method simulates two detectors by separating the data set into two groups of n/2 entries of values for q(i) based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance between odd and even measurements and each of the two sets of n/2 values (odd or even) to estimate a variance. Monte Carlo codes have been developed to test the algorithms in terms of the width of each interval of (i) and the number of intervals, n, that need to be collected. The estimate of dose mean lineal energy converged to the correct value when n was greater than 100. The width of the interval, specified by the mean number of events in each interval (i), yielded consistent results from a mean number of events from 1 to 40 for all three distributions. Tests have been made to determine the effectiveness of dose rate changes between charge collection intervals.

The purpose of this task was to design, build, and assemble a prototype Tissue Equivalent Proportional Counters (TEPC) that would satisfy the basic specifications outlined by NASA for a dosimeter for astronauts during lunar EVAs. The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18mm and the wall thickness is 3mm for a total diameter of 24mm (~ 1 inch). Aluminum vacuum chambers with a shell thickness of 0.5mm were designed and gold plated to maintain electrical conductivity. A system using a high sensitivity mass spectrometer was assembled to measure vacuum leaks for the assembled detectors with high special specificity.

We have been using a version of the software package. LORENTZ 3D™ to model the electric field inside a spherical detector with a linear collector. This uses special modeling techniques based on the Boundary Element Method make the solution of these very challenging problems a simple matter. The geometry of the problem can be created with the geometric modeler built into the electric field solvers or can be imported from any of the major CAD vendors. More importantly, the geometry can be changed parametrically to optimize a design for robustness, weight, size and, of course, cost.

We have fabricated four versions of the TEPC and Vacuum Chamber. Two versions were with spherical detector and single wire anode operated with the wall at high voltage and the anode at ground. One version was with spherical detector and single wire anode operated with the wall at ground and the anode at high voltage. Another detector was a new hybrid design with a parallel wire grid surrounding the anode. The objective of this design was to form a virtual cylindrical geometry around the anode with that would improve the spatial resolution of the TEPC without distorting the signals required for microdosimetry applications. The detectors were exposed to high energy charged particle beams at the HIMAC synchrotron in Chiba Japan. This included the following ions and energies: 56Fe (380 MeV/amu), 18Ar (300 MeV/amu), 12C (200 MeV/amu), and 1H (230 MeV/amu). Measurements were taken at several angles of incidence to determine the angular response of the detector. These results were compared with similar measurements using a commercial TEPC with a Rossi design that has a helical grid surrounding the anode to provide a uniform angular response.

We have begun the design of Mod 3 system based on the results of the experimental investigations with Mod 2. A new vacuum chamber has been successfully machined using Al with a wall thickness of 0.5mm. Improvements to the insulation materials for all detectors have been implemented and a second version of the multiwire grid has been designed and is being fabricated.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. These data were compared with the dose that would be delivered to the skin beneath a space suit with an areal density of 0.4 g/cm2. It is clear that a stainless steel vacuum chamber in Mod 1 needs to be replaced with lighter and thinner materials. These results will be important in determining what additional modifications will be necessary to achieve the design goal for real time measurements to the skin and BFO.

Task 3: Modeling the Variance-Covariance Method

The original proposal for the EVA dosimeter was based on the concept of having two independent proportional counters that would be used to obtain estimates of dose, D, and a quality factor, Q, based on estimating using the variance-covariance method. It was recognized that because of size limitations, the proportional counters would have to be located too close to one another to satisfy the condition that a single particle could not intercept both detectors. The additional constraint that one of the detectors must measure the dose at the skin surface and the other at a depth corresponding to the blood forming organs, makes the original variance-covariance method with paired detectors impractical.

We are developing a method based on using one detector in a variance-covariance scheme. The concepts are based on collecting the charge, zi, in a single TEPC for n successive time intervals. The method proposed by Borak at CSU separates the data set into two groups of n/2 entries of values for zi based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance and each of the two sets of n/2 values (odd or even) to estimate a variance. Monte Carlo codes have been written to test the algorithmic using microdosimetric spectra obtained from measurements in Task 1. The results will be used to optimize the design of the electronics for the variance-covariance method to obtain radiation quality factors.

We have fabricated four versions of the TEPC and Vacuum Chamber. Two versions were with spherical detector and single wire anode operated with the wall at high voltage and the anode at ground. One version was with spherical detector and single wire anode operated with the wall at ground and the anode at high voltage. Another detector was a new hybrid design with a parallel wire grid surrounding the anode. The objective of this design was to form a virtual cylindrical geometry around the anode with that would improve the spatial resolution of the TEPC without distorting the signals required for microdosimetry applications. Measurements were taken at several angles of incidence to determine the angular response of the detector to high energy charged particle beams at the HIMAC synchrotron in Chiba Japan. These results were compared with similar measurements using a commercial TEPC with a Rossi design that has a helical grid surrounding the anode to provide a uniform angular response.

*The single wire detectors operating with the anode a ground potential could detect protons above background noise.

*The single wire detectors operating with the anode a ground potential underestimated the frequency mean lineal energy (Dose) by about 50%. This may be due to extracameral effects generated from particles passing through the vacuum chamber but not through the TEPC.

*A detector with parallel-wire grid correctly measured the frequency mean lineal energy (Dose) and dose meal lineal energy (Quality factor) for the C, Ar, and Fe beams.

*The single wire detector operating with the anode at high voltage had an angular dependence that varied by more than 50%.

*The multiwire detector showed an angular dependence that varied less than 10%.

Task 2: Modeling Detector Response

Computations using the Monte Carlo Code PHITS have been made to determine the energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. * The TEPC should be capable of measuring the dose and dose rate from protons with energies sufficient to penetrate either the space suit for EVA applications.

Task 3: Modeling the Variance-Covariance Method

We are developing a method based on using one detector in a Variance/Covariance scheme proposed by Dr. Borak. Monte Carlo codes have been written to test the algorithmic using microdosimetric data from measurements in Task 1. *More than 100 sequential time intervals are necessary for convergence of the dose mean lineal energy used for estimated of quality factors.

*Timing intervals that have a significant number of intervals without an energy deposition event should be registered as zero energy deposition. But because of noise, they could be assigned as events with very low LET. This would not influence dose but could cause an underestimate of Quality Factor.

The purpose of this task was to design, build and assemble a prototype Tissue Equivalent Proportional Counter (TEPC) that would satisfy the basic specifications outlined by NASA for a dosimeter for astronauts during lunar EVAs.

The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18mm and the wall thickness is 3mm for a total diameter of 24mm (~ 1 inch). A stainless steel vacuum chamber designed to accommodate the TEPC and pre amplifier has been fabricated and leak tested. The hemispherical dome surrounding the TEPC has a wall thickness of 1 mm. This is welded to a cylindrical sleeve with a vacuum tight flange that can be easily removed whenever modifications to the components are necessary.

The unit has been assembled and leak tested using a He vacuum leak test system.

Initial signals were observed using neutrons from a PuBe source. The vacuum was stable for periods of several days. The gain of the proportional counter was reproducible after many repeated gas filling operations.

The detector was exposed to high energy charged particle beams at the HIMAC synchrotron in Chiba Japan. This included the following ions and energies: 14N (180 MeV/amu), 20Ne (180 MeV/amu), 28Si (600 MeV/amu), and 56Fe (500 MeV/amu). Measurements were taken at several angles of incidence to determine the angular response of the detector. These results were compared with similar measurements using a commercial TEPC that has a helical grid surrounding the anode to provide a uniform angular response. Analysis is continuing to evaluate how to reduce variations in response as a function of incidence angle.

We have begun the design of Mod 2 system based on the results of the experimental investigations with Mod 1. A new vacuum chamber has been successfully machined using Al with a wall thickness of 0.5mm. Mechanical parts for Mod 2 are being fabricated and assembly of the system is in progress.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. These data were compared with the dose that would be delivered to the skin beneath a space suit with an areal density of 0.4 g/cm2. It is clear that a stainless steel vacuum chamber in Mod 1 needs to be replaced with lighter and thinner materials. These results will be important in determining what additional modifications will be necessary to achieve the design goal for real time measurements to the skin and BFO.

Task 3: Modeling the Variance-Covariance Method

The original proposal for the EVA dosimeter was based on the concept of having two independent proportional counters that would be used to obtain estimates of dose, D, and a quality factor, Q, based on estimating using the variance-covariance method. It was recognized that because of size limitations, the proportional counters would have to be located too close to one another to satisfy the condition that a single particle could not intercept both detectors. The additional constraint that one of the detectors must measure the dose at the skin surface and the other at a depth corresponding to the blood forming organs, makes the original variance-covariance method with paired detectors impractical.

We are developing a method based on using one detector in a variance-covariance scheme. The concepts are based on collecting the charge, zi, in a single TEPC for n successive time intervals. The method proposed by Borak at CSU separates the data set into two groups of n/2 entries of values for zi based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance and each of the two sets of n/2 values (odd or even) to estimate a variance. Monte Carlo codes have been written to test the algorithms and determine if there are any limitations to this process.

Modifications to this dosimeter could be applied for in-situ dosimetry of patients undergoing heavy ion radiation therapy or measuring stray radiation near high energy particle accelerators.

The purpose of this task was to design, build and assemble a prototype Tissue Equivalent Proportional Counter (TEPC) that would satisfy the basic specifications outlined by NASA for a dosimeter for astronauts during lunar EVAs.

Mod 1 of the spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The unit was built, assembled and leak tested using a He vacuum leak test system. Initial signals were observed using neutrons from a PuBe source. The vacuum was stable for periods of several days. The gain of the proportional counter was reproducible after many repeated gas filling operations.

The detector was exposed to high energy charged particle beams at the HIMAC synchrotron in Chiba Japan. This included the following ions and energies: 14N (180 MeV/amu), 20Ne (180 MeV/amu), 28Si (600 MeV/amu), and 56Fe (500 MeV/amu). Measurements were taken at several angles of incidence to determine the angular response of the detector. These results were compared with similar measurements using a commercial TEPC that has a helical grid surrounding the anode to provide a uniform angular response. Analysis is continuing to evaluate how to reduce variations in response as a function of incidence angle.

We have begun the design of Mod 2 system based on the results of the experimental investigations with Mod 1. A new vacuum chamber has been successfully machined using Al with a wall thickness of 0.5mm. Mechanical parts for Mod 2 are being fabricated and assembly of the system is in progress.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy deposition in the TEPC using protons with an energy spectrum from a SPE in October 2003. These data were compared with the dose that would be delivered to the skin beneath a space suit. It is clear that a stainless steel vacuum chamber in Mod 1 needs to be replaced with lighter and thinner materials. These results will be important in determining what additional modifications will be necessary to achieve the design goal for real time measurements to the skin and BFO.

Task 3: Modeling the Variance-Covariance Method

We are developing a method using a single detector in a variance-covariance scheme to obtain dose, D, and quality factor, Q. The method proposed by Borak at CSU separates the data from successive time intervals into two groups of entries based on odd and even indices. The odd-even pairs of data are used to obtain the covariance and either of the two sets of odd or even data are used to estimate a variance. Monte Carlo codes have been written to test the algorithms to insure that they reproduce the correct values of D and Q.

The purpose of this task was to design, build and assemble a prototype Tissue Equivalent Proportional Counter (TEPC) that would satisfy the basic specifications outlined by NASA for an EVA dosimeter for astronauts during lunar EVAs. This Mod1 design would include the proportional counter, and first stage preamplifier that are contained in a small vacuum chamber suitable for testing with PuBe neutron sources and charged particle beams.

We have used Solid Works to facilitate the design of Mod1. The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18mm and the wall thickness is 3mm for a total diameter of 24mm (~ 1 inch).

The preamplifier board has been designed and bench tested for noise at TAMU. The stainless steel vacuum chamber designed to accommodate the TEPC and pre amplifier has been fabricated and leak tested. The hemispherical dome surrounding the TEPC has a wall thickness of 1 mm. This is welded to a cylindrical sleeve with a vacuum tight flange that can be easily removed whenever modifications to the components are necessary.

The final printed circuit boards have been designed and are being fabricated. This will serve as the connection between the pre amplifier and the base plate of the vacuum chamber that also contains the voltage and signal feed-throughs.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy spectrum of protons entering the gas cavity for mono-energetic protons that are uniformly incident upon the detector. The purpose of this is to establish the low energy cutoff for protons that do not penetrate the vacuum chamber and plastic wall as well as the attenuation of energy for protons that do reach the gas cavity.

These data show that protons incident at 100 MeV have their energy attenuated by about 10%, whereas protons entering at 50 MeV have an energy attenuation of about 40%. Protons less than 30 MeV do not reach the gas cavity and therefore are not detected by the TEPC.

Computations are underway to model these effects using the spectrum of incident protons from various SPE events. This will be important in determining the dose and dose rate response of the EVA dosimeter during high intensity SPE episodes.

Task 3 Modeling the Variance-Covariance Method

The original proposal for the EVA dosimeter was based on the concept of having two independent proportional counters that would be used to obtain estimates of dose, D, and a quality factor, Q, based on estimating using the variance-covariance method. It was recognized that because of size limitations, the proportional counters would have to be located too close to one another to satisfy the condition that a single particle could not intercept both detectors. The additional constraint that one of the detectors must measure the dose at the skin surface and the other at a depth corresponding to the blood forming organs, makes the original variance-covariance method with paired detectors impractical.

It was suggested that it might be possible to use one detector in a variance-covariance scheme. The concepts are based on collecting the charge, zi, in a single TEPC for n successive intervals.

One method proposed by Borak at CSU separated the data set into two groups of n/2 entries of values for zi based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance and each of the two sets of n/2 values (odd or even) to estimate a variance.

John Lakness, from NASA Ames Research Center, used a method based on the derivative of the discrete data set by taking the difference between successive measurements. There are clear similarities between the two methods. Both processes rely on summing values of z, z2 and pairs zi,zj.

The Borak process separates the data set into two parts to estimate a covariance for the paired data and then combines them in the end to estimate by taking the average of the variance minus covariance obtained from the two parts.

The Lakness process in effect gets the covariance by pairing successive values of z and sums up z2 twice.

Several data sets were simulated using a constant value of D for each interval and a variable value of D to evaluate dose rate effects on the estimate of which is used to determine quality factor.

The results indicated that both methods converged toward the true value of for the simulated data sets. We plan to continue these investigations to evaluate both methods at very high dose rates and very low dose rates where there may be no events in a given timing interval.

The purpose of this task was to design, build and assemble a prototype Tissue Equivalent Proportional Counter (TEPC) that would satisfy the basic specifications outlined by NASA for an EVA dosimeter for astronauts during lunar EVAs. This Mod1 design would include the proportional counter, and first stage preamplifier that are contained in a small vacuum chamber suitable for testing with PuBe neutron sources and charged particle beams.

We have used Solid Works to facilitate the design of Mod1. The spherical TEPC is based on a single-wire anode with recessed guard ring insulators to shape the electric field near the poles. The diameter of the gas cavity is 18mm and the wall thickness is 3mm for a total diameter of 24mm (~ 1 inch).

The preamplifier board has been designed and bench tested for noise at TAMU. The stainless steel vacuum chamber designed to accommodate the TEPC and pre amplifier has been fabricated and leak tested. The hemispherical dome surrounding the TEPC has a wall thickness of 1 mm. This is welded to a cylindrical sleeve with a vacuum tight flange that can be easily removed whenever modifications to the components are necessary.

The final printed circuit boards have been designed and are being fabricated. This will serve as the connection between the pre amplifier and the base plate of the vacuum chamber that also contains the voltage and signal feed-throughs.

Task 2: Modeling Detector Response

The objective of this task is to determine the response of the TEPC under ambient conditions and during SPE events on the lunar surface. Computations using the Monte Carlo Code PHITS have been made to determine the energy spectrum of protons entering the gas cavity for mono-energetic protons that are uniformly incident upon the detector. The purpose of this is to establish the low energy cutoff for protons that do not penetrate the vacuum chamber and plastic wall as well as the attenuation of energy for protons that do reach the gas cavity.

These data show that protons incident at 100 MeV have their energy attenuated by about 10%, whereas protons entering at 50 MeV have an energy attenuation of about 40%. Protons less than 30 MeV do not reach the gas cavity and therefore are not detected by the TEPC.

Computations are underway to model these effects using the spectrum of incident protons from various SPE events. This will be important in determining the dose and dose rate response of the EVA dosimeter during high intensity SPE episodes.

Task 3 Modeling the Variance-Covariance Method

The original proposal for the EVA dosimeter was based on the concept of having two independent proportional counters that would be used to obtain estimates of dose, D, and a quality factor, Q, based on estimating using the variance-covariance method. It was recognized that because of size limitations, the proportional counters would have to be located too close to one another to satisfy the condition that a single particle could not intercept both detectors. The additional constraint that one of the detectors must measure the dose at the skin surface and the other at a depth corresponding to the blood forming organs, makes the original variavariance-covariance method with paired detectors impractical.

It was suggested that it might be possible to use one detector in a variance-covariance scheme. The concepts are based on collecting the charge, zi, in a single TEPC for n successive intervals.

One method proposed by Borak at CSU separated the data set into two groups of n/2 entries of values for zi based on odd and even indices. The n/2 pairs of data (odd and even) are used to obtain the covariance and each of the two sets of n/2 values (odd or even) to estimate a variance.

John Lakness, from NASA Ames Research Center, used a method based on the derivative of the discrete data set by taking the difference between successive measurements.

There are clear similarities between the two methods. Both processes rely on summing values of z, z2 and pairs zi,zj.

The Borak process separates the data set into two parts to estimate a covariance for the paired data and then combines them in the end to estimate by taking the average of the variance minus covariance obtained from the two parts.

The Lakness process in effect gets the covariance by pairing successive values of z and sums up z2 twice.

Several data sets were simulated using a constant value of D for each interval and a variable value of D to evaluate dose rate effects on the estimate of which is used to determine quality factor.

The results indicated that both methods converged toward the true value of for the simulated data sets. We plan to continue these investigations to evaluate both methods at very high dose rates and very low dose rates where there may be no events in a given timing interval.

This project will develop a new generation of dosimeters to satisfy requirements while astronauts are on the surface of the moon. Specifically, this dosimeter will measure the dose and dose rate during normal lunar conditions as well as during the harsh radiation environments associated with solar particle events (SPE).

The design is based on a tissue equivalent proportional counter (TEPC). It will be configured as a compact module that can be placed in an extravehicular activity (EVA) spacesuit or backpack. This module will record and display dose rate and activate an alarm when the particle intensity increases at the onset of an SPE. It will be sensitive to the large dynamic range of charged particles as well as to neutrons emerging from the lunar surface. There will also be a second detector incorporated in the dosimetry module to serve as a safety backup during an SPE. This will consist of a plastic scintillator read out by a Geiger Avalanche Photodiode. The same module can be connected to a Remote Control Unit located on a lunar rover or nearby tool during EVA. This will extend the capabilities to include pulse-height analysis of events that can be used to estimate equivalent dose using radiation weighting factors derived from the distribution of lineal energy recorded by the TEPC.

While the concepts outlined here are specifically designed for a lunar EVA, the advanced design could become the basis for the next generation of active space radiation dosimeters.