Limits...
Compact Tissue-equivalent Proportional Counter for Deep Space Human Missions.

Straume T, Braby LA, Borak TB, Lusby T, Warner DW, Perez-Nunez D - Health Phys (2015)

Bottom Line: This was accomplished by assigning sequential sampling intervals as detectors “1” and “2” and requiring the intervals to be brief compared to the change in dose rate.Tests with g rays show that the prototype instrument maintains linear response over the wide dose-rate range expected in space with an accuracy of better than 5% for dose rates above 3 mGy h(-1).Limited tests with fission spectrum neutrons show absorbed dose-rate accuracy better than 15%.

View Article: PubMed Central - PubMed

Affiliation: *NASA Ames Research Center, Moffett Field, CA 94035; †Texas A&M University, College Station, TX 77843; ‡Colorado State University, Ft. Collins, CO 80523.

ABSTRACT
Effects on human health from the complex radiation environment in deep space have not been measured and can only be simulated here on Earth using experimental systems and beams of radiations produced by accelerators, usually one beam at a time. This makes it particularly important to develop instruments that can be used on deep-space missions to measure quantities that are known to be relatable to the biological effectiveness of space radiation. Tissue-equivalent proportional counters (TEPCs) are such instruments. Unfortunately, present TEPCs are too large and power intensive to be used beyond low Earth orbit (LEO). Here, the authors describe a prototype of a compact TEPC designed for deep space applications with the capability to detect both ambient galactic cosmic rays and intense solar particle event radiation. The device employs an approach that permits real-time determination of yD (and thus quality factor) using a single detector. This was accomplished by assigning sequential sampling intervals as detectors “1” and “2” and requiring the intervals to be brief compared to the change in dose rate. Tests with g rays show that the prototype instrument maintains linear response over the wide dose-rate range expected in space with an accuracy of better than 5% for dose rates above 3 mGy h(-1). Measurements of yD for 200 MeV n(-1) carbon ions were better than 10%. Limited tests with fission spectrum neutrons show absorbed dose-rate accuracy better than 15%.

Show MeSH
Block diagrams (a) of a conventional charge-sensitive preamplifier and (b) of the preamplifier modified here to integrate charge until the switch is closed by a signal from the microprocessor.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC4554228&req=5

Figure 2: Block diagrams (a) of a conventional charge-sensitive preamplifier and (b) of the preamplifier modified here to integrate charge until the switch is closed by a signal from the microprocessor.

Mentions: Application of the variance-covariance approach to microdosimetry requires measurement of the total charge produced by the detector in fixed time intervals. Many different approaches can be taken to determine the charge collected, but for a compact portable instrument, the preferred approach is the one that can be implemented in electronics with minimum mass and power consumption. Alternatives include measuring the current with an electrometer and integrating the charge for a fixed time using a voltage-to-frequency converter, recording the pulse height for individual events and summing for the desired time, or collecting charge on a capacitor for the specified time and reading the voltage on the capacitor. Of these basic approaches, measuring the charge on a capacitor appears to require the least electronics, since a relatively simple circuit based on a charge sensitive preamplifier and an analog-to-digital converter will achieve the needed results. The block diagram of a simple charge-sensitive preamplifier is shown in Fig. 2a. When charge from the detector is deposited at the negative input of the operational amplifier, its output voltage increases until the potential difference between the + and – inputs is 0. The output voltage then holds the charge on the feedback capacitor, C1. The feedback resistor, R1, which is usually 108 or 109 Ω, is used to bleed charge off of C1 to prevent the output voltage from reaching the amplifier’s maximum voltage when more charge is received from the detector. If R1 is replaced by a switch and a relatively small resistor, R1a (Fig. 2b), the output voltage will be proportional to the charge collected since the switch opened. The switch can be closed at the end of each charge measurement interval, but this is not necessary, since the voltage can be measured at the beginning and end of the interval and the difference will give the charge collected during the interval. Since the feedback resistor in Fig. 2a is a source of electronic noise, the approach illustrated in Fig. 2b has been used to reduce noise in pulse-height analysis systems.


Compact Tissue-equivalent Proportional Counter for Deep Space Human Missions.

Straume T, Braby LA, Borak TB, Lusby T, Warner DW, Perez-Nunez D - Health Phys (2015)

Block diagrams (a) of a conventional charge-sensitive preamplifier and (b) of the preamplifier modified here to integrate charge until the switch is closed by a signal from the microprocessor.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4554228&req=5

Figure 2: Block diagrams (a) of a conventional charge-sensitive preamplifier and (b) of the preamplifier modified here to integrate charge until the switch is closed by a signal from the microprocessor.
Mentions: Application of the variance-covariance approach to microdosimetry requires measurement of the total charge produced by the detector in fixed time intervals. Many different approaches can be taken to determine the charge collected, but for a compact portable instrument, the preferred approach is the one that can be implemented in electronics with minimum mass and power consumption. Alternatives include measuring the current with an electrometer and integrating the charge for a fixed time using a voltage-to-frequency converter, recording the pulse height for individual events and summing for the desired time, or collecting charge on a capacitor for the specified time and reading the voltage on the capacitor. Of these basic approaches, measuring the charge on a capacitor appears to require the least electronics, since a relatively simple circuit based on a charge sensitive preamplifier and an analog-to-digital converter will achieve the needed results. The block diagram of a simple charge-sensitive preamplifier is shown in Fig. 2a. When charge from the detector is deposited at the negative input of the operational amplifier, its output voltage increases until the potential difference between the + and – inputs is 0. The output voltage then holds the charge on the feedback capacitor, C1. The feedback resistor, R1, which is usually 108 or 109 Ω, is used to bleed charge off of C1 to prevent the output voltage from reaching the amplifier’s maximum voltage when more charge is received from the detector. If R1 is replaced by a switch and a relatively small resistor, R1a (Fig. 2b), the output voltage will be proportional to the charge collected since the switch opened. The switch can be closed at the end of each charge measurement interval, but this is not necessary, since the voltage can be measured at the beginning and end of the interval and the difference will give the charge collected during the interval. Since the feedback resistor in Fig. 2a is a source of electronic noise, the approach illustrated in Fig. 2b has been used to reduce noise in pulse-height analysis systems.

Bottom Line: This was accomplished by assigning sequential sampling intervals as detectors “1” and “2” and requiring the intervals to be brief compared to the change in dose rate.Tests with g rays show that the prototype instrument maintains linear response over the wide dose-rate range expected in space with an accuracy of better than 5% for dose rates above 3 mGy h(-1).Limited tests with fission spectrum neutrons show absorbed dose-rate accuracy better than 15%.

View Article: PubMed Central - PubMed

Affiliation: *NASA Ames Research Center, Moffett Field, CA 94035; †Texas A&M University, College Station, TX 77843; ‡Colorado State University, Ft. Collins, CO 80523.

ABSTRACT
Effects on human health from the complex radiation environment in deep space have not been measured and can only be simulated here on Earth using experimental systems and beams of radiations produced by accelerators, usually one beam at a time. This makes it particularly important to develop instruments that can be used on deep-space missions to measure quantities that are known to be relatable to the biological effectiveness of space radiation. Tissue-equivalent proportional counters (TEPCs) are such instruments. Unfortunately, present TEPCs are too large and power intensive to be used beyond low Earth orbit (LEO). Here, the authors describe a prototype of a compact TEPC designed for deep space applications with the capability to detect both ambient galactic cosmic rays and intense solar particle event radiation. The device employs an approach that permits real-time determination of yD (and thus quality factor) using a single detector. This was accomplished by assigning sequential sampling intervals as detectors “1” and “2” and requiring the intervals to be brief compared to the change in dose rate. Tests with g rays show that the prototype instrument maintains linear response over the wide dose-rate range expected in space with an accuracy of better than 5% for dose rates above 3 mGy h(-1). Measurements of yD for 200 MeV n(-1) carbon ions were better than 10%. Limited tests with fission spectrum neutrons show absorbed dose-rate accuracy better than 15%.

Show MeSH