ICNTS conference, ID 191

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ICNTS conference, ID 191

• Alpha-track radon measurements at very low and
  high exposures
• by
• Tryggve Rönnqvist, PhD
• Gammadata Mätteknik AB, Uppsala, Sweden

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Gammadata
Gammadata was founded 1986 by scientists from
Uppsala University in order to measure the
radioactive fall-out from the reactor at
Chernobyl.
In 1988 Gammadata expanded its measurement
services to include radon measurements in
dwellings and workplaces. A development of
electronic instrumentation for radon detection in
air and soil was also initiated.
Today Gammadata consists of three major parts
with about 180 employees in total. Gammadata
Mätteknik AB handles the measurement services and
consists of 15 employees.
To date approximately 450000 cesium tests and
more than 1000000 passive radon detectors have
been measured in this accredited laboratory.
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Measurement of radon in airClosed alpha-track
film with filter

• Function
• Radon gas enters the detector through the small
  gap between the bottom and top of the detector
  holder. No radon decay products can enter the
  detector from the outside (filter).
• In the detector an alpha-track film (CR-39) is
  located which is sensitive to alpha decay. The
  measured area of the film is about 1,4 cm2.
• The detector holder is manufactured in
  electrically conducting plastic in order to avoid
  problems with static electricity.

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Measurement of radon in airClosed alpha-track
film with filter

• Long-term measurement
• Measurement period at least 2 months (3 months
  recommended).
• Ageing effects have little influence. No major
  problems to store the detectors (recommended
  storage time, max. 1,5 years).
• Short-term measurements
• Larger detector holder (RAPIDOS) provides better
  counting statistics and a better signal to noise
  relation.
• Measurement period at least 7 days (usually 10
  days).
• The detectors should not be stored for a longer
  time (max. 8 months).

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Measurement uncertainties Long-term radon
measurements
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Measurement uncertaintiesShort-term measurements
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Measurement of radon in airHigh exposures

• Counting method at high exposures
• Gammadata has developed and calibrated an
  accredited counting method for radon exposures up
  to 50000 kBqh/m3.
• The method is based on counting the total
  radiated area of the alpha-track film with
  compensations made for the size of individual
  tracks on the film.
• The method is used when individual tracks overlap
  to much, typically in the exposure range
  7000-50000 kBqh/m3.
• Measurement uncertainties at high exposures
• The uncertainty of the method due to e.g.
  different track sizes and exposures is usually
  about 10 .
• The counting method requires clean CR-39
  plastic with a low background since no background
  is removed with blob analysis.

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Measurement uncertaintiesMain contributions

• Long-term measurements (about 90 days)
• Exposure (kBqh/m3) Main uncertainty
  contributions
• 0-250 (0-120 Bq/m3) Background
• 250-450 (120-200 Bq/m3) Calibration background
• 450-6000 (200-2800 Bq/m3) Calibration
• gt 6000 (gt 2800 Bq/m3) High exposure method
• Short-term measurements (about 10 days)
• Exposure (kBqh/m3) Main uncertainty
  contributions
• 0-40 (0-170 Bq/m3) Background transport
• 40-60 (170-250 Bq/m3) Background number of
  tracks
• 60-90 (250-380 Bq/m3) Calibration, background
  tracks
• 90-3000 (380-12500 Bq/m3) Calibration
• gt 3000 (gt 12500 Bq/m3) High exposure method
• (A calibration uncertainy of 5 is assumed.
  Transport exposures can give contributions up to
  5 kBqh/m3.)

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Measurement of radon in airQuality control of
CR-39 plastic

• Quality control of CR-39 plastic
• Gammadata has the following requirements on the
  suppliers for each sheet of CR-39 plastic
• Quality samples are etched and the sum of the
  background level and background deviation should
  be less than about 70 tracks/cm2.
• The median value for the number of counts with
  the high exposure counting method should be less
  than about 200 counts/cm2.
• In this quality control procedure some sheets are
  rejected but the main part of the sheets (more
  than 70) have a background level and a
  background deviation less than 10 tracks/cm2
  (recent productions from Intercast Europe).
• Before producing the detectors, a new quality
  control procedure is made where 4-5 of the films
  are etched and measured.
• Sheets with a very low background and with a
  background deviation less than 10 tracks/cm2 are
  approved for possible short-term measurements.

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Measurement of radon in airBackground calculation

• Considerations for background calculations
• The background level is mainly sheet dependent.
  Therefore, the background level should be
  calculated for each sheet of CR-39 plastic.
• Enough films from each sheet should be used for
  the background calculation in order to get good
  statistics. The background films should be taken
  from different parts of the sheet.
• Due to aging effects and alpha-decay from the
  detector holders, the background levels increase
  with time. The background level should be
  measured just before the exposed detectors will
  be returned.
• Gammadata use about 6 of the films for the
  background calculation. Together with the quality
  control more than 10 of the films are used for
  quality control and background calculation.
• Since the background increase with time,
  detectors which should be used for short-term
  measurements should have a short storage time.

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Summary

• Alpha-track radon measurements at low exposures
• In order to make radon measurements at low
  exposures (out-door and short-term measurements)
  with high quality, it is important to
• Use CR-39 plastic with a low background since
  the background level is the main uncertainty.
• Use new detectors since the background
  increase with time.
• Determine the background level just before the
  exposed detectors are returned.
• Use enough films to get good statistics in the
  quality control and background calculation.
• Alpha-track radon measurements at high exposures
• In order to make radon measurements at high
  exposures (gt 7000 kBqh/m3) with high quality, it
  is important to
• Use very clean and transparent CR-39 plastic.
• Make careful calibrations and tests of the
  counting method.