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Nuclear & Non-Radon Apps

 Alpha Surface Contamination
 Thoron in Air
 Tritium in Air, Water and on Surfaces
 Environmental Gamma Radiation, Low energy X rays
 Beta Dosimetry Applications
 Ion Concentration in Air
 Measurement of Fast Neutrons

 

 


Alpha Surface Contamination

The standard electret ion chamber can be modified to include  a thin window that allows alpha particles to enter the sensitive volume, thus becoming an alpha monitor. One standard version has a depth of 2.8 cm, diameter of 8 cm with window made of aluminized Mylar of thickness 0.8 mg/cm2. Detailed work was carried out on the topic of "Utilization of Electret Ion Chambers for Characterization of Gross Alpha Emission from Indoor Surfaces" under a CRADA (Cooperative Research and Development Agreement) program between Rad Elec Inc. and Oak Ridge National laboratory. This has been published in 1993 USDOE Methods Compendium Protocol and is available on request. It is usable for routine alpha survey of alpha contaminated surfaces such as concrete, wood, vinyl floors or painted surfaces with sensitivity requirement of DOE clearance limit. Versions with larger volume are also available. These can be used with or without window. Following publications are available for further details:

  • Ken Kasper "Passive Electret Ion Chambers-Technology Monitor Column" Health Physics 76:475-476 (1999)
  • S.K.Dua, S.K.Biswas, P.Szerszen, J.Boudreax, and M.A. Ebadian: "Measurement of Surface Alpha Contamination Using Electret Ion Chambers" Health Physics 76:664-674 (1999)
  • S.K.Dua, P.S.Jawalikar, M.V.Reddy, P.Szersen, R.W.Rose, M.A.Ebadian and P.Kotrappa : Ceramic Tiles as Inexpensive Large Area test-Beds for Comparative Assessment of Electret Ion Chambers and Other instruments used for Measuring Alpha-Particle Emission Rates
  • Radioactivity & radiochemistry Volume 11 No. (July 2000)
  • S.K.Dua, M.V.Reddy, P.Szerszen, R.W. Rose, D.Roelant, and M.A.Ebadian "Electret Ion Chambers for Estimating Alpha Particle Energy" Health Physics 79:703-711 (2000)
  • S.K.Dua, P.Kotrappa et al Measurement of Alpha particle Energy using windowless electret ion chambers Health Physics 83:549-552 (2002)
  • Thomas G.Hobbs "Radioactivity Measurements on Glazed Ceramic Tiles (using EIC Alpha Monitors)Journal of the National Institute of Standards and Technology 105:275 (2000)
  • Meyer, K.E., Gammage, R.B., Dudney, C.S., Reed-Walker, S., Kotrappa, P., Wheeler, R.V., Salasky, M., "Field Measurements of Plutonium And Americium In Soils At The Nevada Test Site Using Passive Alpha Detectors" Radioactivity And Radiochemistry 5: 26-41 (1994) 

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Thoron in Air thorons (156 x 256).jpg

Thoron is an isotope of radon, which originates from thorium and has a very short half life of about 1 minute. Measuring Thoron is important in some areas rich in thorium or in areas where thorium is handled. Thoron can also be an interference in measuring radon. The standard E-PERMs are designed to minimize the response to 220Rn (thoron) by restricting the diffusion entry time. This is achieved by having a very small area filter for the passive diffusion. Because of short half life (about one minute); it decays before entering the active area of the E-PERM. The chamber of such unit was modified deliberately by increasing the filtered diffusion area from 0.3 cm2 to 30 cm2 to allow thoron to diffuse into the chamber with very little delay time. This modified unit is called the Radon-Thoron E-PERM or RT E-PERM because it responds to both radon and thoron.  

The figure shows a schematic view of the RT E-PERM.  A series of holes were drilled in the body of the chamber, and the holes subsequently covered with an electrically conducting filter paper (carbon coated Tyvek paper of 0.06 mm thickness). The area of the opening was about 30 cm2. These were calibrated by exposing to known thoron atmosphere in a thoron test facility of CANMET, Elliot Lake Laboratory, Mining Research Laboratory, and Elliot Lake, CANADA. It is necessary to expose a set consisting of one radon and one radon-thorn E-PERM simultaneously in the test atmosphere. Results are used to calculate both radon and thoron concentrations in air.

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Tritium in Air and Water and on Surfaces

Electret ion chambers with large filtered openings provide access to tritium vapors. Being an ionizing vapor, it is detected. Incorporating such detectors inside a sealed jar containing tritium contaminated water provides a means of determining tritium concentration in water. A windowless device is used for assessing tritium on surfaces.

Additional information is available from Rad Elec Inc. Following publications are available for more details:

  • Surette, R.A., and Wood, M.J., "Evaluation of Electret Ion Chambers for Tritium Measurements" Health Physics 65:418-421(1993)
  • Kotrappa, P., Hobbs, T and Brown, D., "Electret Ion Chambers For Passive Measurement of Airborne Tritium: Theory and Practice" Radioactivity and Radiochemistry 6:50-58(1995)
  • Brock, J., and Meyer, K., "Report On Field Measurements of Surface Tritium Contamination in Two Buildings at the Savannah River Site" ORNL (DOE) Internal Report (1995)

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Environmental Gamma Radiation, Low energy X rays

Electret ion chambers become gamma monitors when sealed in a radon leak tight enclosures. Usually sealable Mylar bags or any other enclosures are used. These provide integrated gamma exposure over a time, in units of mR or nGy. Dividing such exposure by exposure period leads to the average exposure rate.

Chambers being made of electrically conducting plastic, these provide energy independent response. These have been used in the place of TLDs. certain areas where low energy gamma emitters such as Americium-241, EICs provide superior performance over TLDs. For very low energy applications the EICs with thin windows, such as 7 mg/cm2 thick carbon coated Tyvek, are used. With a choice of different chamber volumes and electrets, a wide variety of sensitivities and ranges are possible. For more information, please contact REI for details. Following publications provide additional information:

  • Hobbs, T., Kotrappa, P., Tracy, J., and Biss, B., "Response Comparison of Electret Ion Chambers, Lif TLD, And HPIC" Radiation Protection Dosimeter: 63: 181-188 (1996)
  • Fjeld, R.A., Montague, K.J., Haapala, M.H.,and Kotrappa, P., " Field Test Of Electret Ion Chambers For Environmental Monitoring" Health Physics 66:147-154(1994)
  •  Stieff, L.R., Kotrappa, P., Harley, N.," Thin Window Electret Ion Chambers for Calibrating the Sieman's Dermopan 2 Dermatology Therapy Unit" Paper MPM-A.7 HPS 1997 Meeting Health Physics 72:S3 (1997)
  • David H. Kraig and W.Allen Treadaway  "Field comparison of Electret Ion Chambers and Thermoluminscent Dosimeters at Los Alamos National Laboratory"  LANL report LA-UR-99-5186 (Jan 2000).

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Beta Dosimetry Applications

Electret ion chambers with a conducting Tyvek window have been used for measuring beta dose rates. Passive beta particle dosimeters are needed for several applications including monitoring the environment around nuclear reactors and the contaminated atmospheres found in many nuclear facilities.  An electret ion chamber with window area of 50 cm2, a volume of 144 cm2, a window (carbon coated Tyvek) thickness of 7 mg cm-2 and a depth of 2.88 cm is built and calibrated to meet the requirement. These find use in characterizing beta sources used in medical applications. Please see the brochure (page 6 of this manual) for further information.

As pointed out earlier, 90Sr-Y is used for calibration. Limited experiments carried out indicated that the response was within 15 % of the correct value for different standard beta radiation sources (204Tl with average beta energy of 0.244 MeV and 147Pm with average beta energy of 0.062 MeV) used for calibration purposes

Please contact Rad Elec for additional information.

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Ion Concentration in Air

Positive and negative ions are produced in air, mainly due to radon and terrestrial/cosmic radiation sources. Measuring ion concentration in air indirectly provides a measure of these sources. Electrets (electrically charged piece of Teflon) when exposed in the environment collect ions of opposite sign, leading to a measurable decrease in charge, depending upon the exposure time and ion concentration. This work describes a method of correlating electret discharge rate to the ion concentration as measured by a calibrated ion density meter. Once calibrated, electrets can then be used to measure ion concentration of either sign. Air ion concentration in ambient air was measured to be about 200-ion ml-1, measured over several hours. Both positive and negative ion concentrations were similar. In a typical room, negative ion concentration was about 3500 ml-1 and surprisingly, there were no positive ions at all in that room. Being an integrating passive device, method provides a unique possibility of measuring low or high concentrations of positive or negative ions over extended periods, which is difficult to do with other ion concentration measuring instruments.

  • P.Kotrappa "Electrets to measure ion concentration in air" Health Physics 89:164-167 (2005)

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Measurement of Fast Neutrons

Electret ion chambers (EIC) are passive integrating ionization chambers consisting of a stable electret mounted inside a small chamber made of electrically conducting polypropylene.  The electret, a charged Teflon(R) disk, serves both as a source of the electrostatic field and as a sensor.  Ions produced by radiation inside the chamber are collected on the electret, causing a reduction of its surface charge.  The reduction in charge is measured using a portable electret surface charge reader.  Such a reduction is a function of the total ionization during the period of measurement. With appropriate calibration, the measured reduction in charge is converted into appropriate units of radiation.  EICs are widely used for indoor radon measurements and for environmental gamma radiation.   The 200 ml EIC responds to both fast neutrons (caused by the interaction of hydrogen in polypropylene and the release of ionizing protons into the sensitive volume) and gamma radiation. Such chambers are called GFN-EICs.  The chambers lined with 1.5 mm thick carbon coated Teflon (Teflon does not have hydrogen and therefore does not respond to fast neutrons) respond only to gamma radiation. These chambers are called G-EICs.  When the G-EICs are coated with boron-carbide, they become sensitive to thermal neutrons(thermal neutrons interact with the boron releasing ionizing alpha radiation into the sensitive volume).  These chambers are called GTN-EICs.  The G, GFN and GTN are deployed simultaneously in mixed radiation fields to resolve and measure gamma, thermal neutrons and fast neutron dose equivalents.  These EICs were calibrated using the NIST (National Institute of Standards and Technology) Standard NS-100 252Cf as a fast neutron source and NS-130 252Cf D2O moderated  as a thermal neutron source.  The NIST 137Cs gamma standard was used to calibrate G-EICs.   The lower limit of detection (defined as the dose equivalents measurable with an error of ±25%) is 40 mrem for fast neutrons, 10 mrem for thermal neutrons and 3 mrem for gamma radiation.  The EICs exhibited very little directional dependence (less than 10%) and dose rate dependence over the two orders of magnitudes covered by the different NS standards.  Being passive integrators, these can be left in the field for extended period to measure very low dose equivalent rates

  • J. Clark, P.Kotrappa and F.Stieff "Electret Ion Chambers (EIC) For Measuring Dose Equivalents in Mixed Fields Of Thermal Neutrons, Fast Neutrons And Gamma Radiation"

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