During long-term exploration missions, space radiation will present a risk to astronaut health and could affect the performance of onboard equipment. It will be important to have accurate and reliable detectors that measure radiation dose in real time and indicate the onset of intense radiation from solar particle events.
Dr. Thomas Borak and colleagues are developing dosimeters small enough to fit in a spacesuit or within a backpack during a space walk, also called extravehicular activity (EVA). The device will be sensitive to a large range of charge particles and measure radiation dose and dose rate during an EVA or inside the spacecraft. As the radiation intensity increases during solar particle events, the dosimeter can issue a warning directly to astronauts as well as mission control. This will be a signal for the astronauts to implement measures that include termination of the EVA and travel to a safe haven that contains protective shielding.
Thomas B. Borak, Ph.D.
Colorado State University
Project Tasks Task 1: Design, Fabrication and Testing Mod 1 Prototype Detector
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 extravehicular activity (EVA) 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 18 mm, and the wall thickness is 3 mm for a total diameter of 24 mm (~ 1 inch). To maintain electrical conductivity, aluminum vacuum chambers with a shell thickness of 0.5 mm were designed and gold plated. 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 which simplifies the solution of these very challenging problems. 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 computer-aided dispatch vendors. More importantly, the geometry can be changed parametrically to optimize a design for robustness, weight, size and cost.
We have fabricated four versions of the TEPC and vacuum chambers. Two versions are made up of a spherical detector and a single-wire, anode-operated, with the wall at high voltage and the anode at ground. One version was made with a spherical detector and a 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 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)
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 the Mod 3 system based on the results of the experimental investigations with the Mod 2. A new vacuum chamber has been successfully machined using aluminum with a wall thickness of 0.5 mm. Improvements to the insulation materials for all detectors have been implemented, and a second version of the multi-wire 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 solar particle events (SPE) on the lunar surface. Computations using the Monte Carlo Code Particle and Heavy Ion Transport Code System 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 spacesuit 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 estimations 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 BFOs, 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 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.
Thomas B. Borak, Ph.D.
Colorado State University
This type of dosimeter has additional applications for first responders to nuclear accidents or terrorist attacks. It can also provide real-time dosimetry during commercial spaceflight, diagnostic and therapeutic medical procedures such as proton and carbon ion radiation therapy, and surveillance activities associated with U.S. homeland security and nuclear non-proliferation. It can also serve as an area monitor with live-time capabilities that provide dose rate as well as estimates of quality factors for radiation workers as well as to the general public.