Performance of electret ionization chambers in magnetic field.

Electret ionization chambers are widely used for measuring radon and radiation. The radiation measured includes alpha, beta, and gamma radiation. These detectors do not have any electronics and as such can be introduced into magnetic field regions. It is of interest to study the effect of magnetic fields on the performance of these detectors. Relative responses are measured with and without magnetic fields present. Quantitative responses are measured as the magnetic field is varied from 8 kA/m to 716 kA/m (100 to 9,000 gauss). No significant effect is observed for measuring alpha radiation and gamma radiation. However, a significant systematic effect is observed while measuring beta radiation from a 90Sr-Y source. Depending upon the field orientation, the relative response increased from 1.0 to 2.7 (vertical position) and decreased from 1.0 to 0.60 (horizontal position). This is explained as due to the setting up of a circular motion for the electrons by the magnetic field, which may increase or decrease the path length in air depending upon the experimental configuration. It is concluded that these ionization chambers can be used for measuring alpha (and hence radon) and gamma radiation in the range of magnetic fields studied. However, caution must be exercised if measuring beta radiation.

Radon monitor calibration using NIST radon emanation standards: steady flow method.

The National Institute of Standards and Technology (NIST) polyethylene-encapsulated 226Ra/222Rn emanation (PERE) standards (old SRM 4968 and new SRMs 4971, 4972, and 4973) provide precise radon emanation rate, certified to a high degree of accuracy (approximately to 2%). Two new SRM 4973 standards containing totally 1036 Bq (0.028 microCi) of 226Ra, emanate 0.114 Bq (3.08 pCi) of 222Rn per min. Air passing over such sources at a flow rate of 1 l min(-1) will have a radon concentration of 114 Bq m(-3) (3.08 pCi l(-1)). This paper describes a practical calibration system and the actual calibration verification data obtained at different flow rates, for E-PERM passive radon monitors, Femto-Tech and Alpha Guard Continuous Radon Monitors. The use of such an affordable and easy to use system by the manufacturers and users of radon measurement devices will bring uniform standards with traceability to a NIST standard source and is considered an important step in standardising radon measurement methods.

Measurement of alpha particle energy using windowless electret ion chambers.

Electret ion chambers are inexpensive, lightweight, robust, commercially available, passive, charge-integrating devices for accurate measurement of different ionizing radiations. In an earlier work a chamber of dimensions larger than the range of alpha particles having aluminized Mylar windows of different thickness was used for measurement of alpha radiation. Correlation between electret mid-point voltage, alpha particle energy, and response was developed and it was shown that this chamber could be used for estimating the effective energy of an unknown alpha source. In the present study, the electret ion chamber is used in the windowless mode so that the alpha particles dissipate their entire energy inside the volume, and the alpha particle energy is determined from the first principles. This requires that alpha disintegration rate be accurately known or measured by an alternate method. The measured energies were within 1 to 4% of the true values for different sources (230Th, 237Np, 239Pu, 241Am, and 224Cm). This method finds application in quantitative determination of alpha energy absorbed in thin membrane and, hence, the absorbed dose.

Elevation correction factors for E-PERM radon monitors.

E-PERM radon monitors are based on the principle of electret ion chambers and are usually calibrated in a standard radon chamber located at sea level. Corrections are needed if the monitors are used at elevations other than sea level. These were experimentally determined for three models of commercially available electret ion chambers (E-PERM) as functions of elevation above sea level. These corrections are minor and should be applied for obtaining more accurate results.

A practical E-PERM (electret passive environmental radon monitor) system for indoor 222Rn measurement.

The technical and scientific basis for the measurement of indoor 222Rn concentration using an E-PERM (Electret passive environmental radon monitor) has been described in our earlier work. The purpose of this paper is to describe further development of a practical and convenient system that can be used routinely for indoor 222Rn measurement. The ion chamber is now made of electrically conducting plastic to minimize the response from natural gamma radiation. A spring-loaded shutter method is used to cover and uncover the electret from outside the chamber. The electret voltage reader has been modified to improve the accuracy and the ease in operation. The calibration, performance, error analysis, and lower limits of detection for these standardized versions of E-PERMs are also described.

An electret passive environmental 222Rn monitor based on ionization measurement.

The electret passive environmental 222Rn monitor (E-PERM) is an extension of electret dosimeters used for measurement of x and gamma radiation. An E-PERM consists of a small cup or canister, having an electret at the bottom, and a filtered inlet at the top. The 222Rn gas entering through the filter and the decay products formed inside the cup generate ions which are collected by the electret. The reduction of charge (or surface potential) on the electret is a measure of time integrated 222Rn exposure. An E-PERM of 220-mL volume with an electret of 0.23 cm thickness gave a surface potential drop of 2.5 V for 37 Bq m-3 d (1 pCi L-1 d). The electret voltage was measured with a specially built surface potential voltmeter. This sensitivity was found adequate for a 1-wk measurement of 222Rn in homes. For longer term measurements, an E-PERM of 40-mL volume and an electret of 51-micron thickness was developed which gave a surface potential drop of 2.6 V for 37 Bq m-3 y (1 pCi L-1 y). Other combinations of chamber volume and electret thicknesses gave responses between these two values. The surface potential of electrets made from Teflon FEP were shown to stay stable even under extreme conditions of relative humidity. The ion collection process in E-PERMs was also shown to be independent of humidity down to an electret surface potential of 100 V.