ICNTS conference, ID 191

Alpha-track radon measurements at very low and high exposures

by

Tryggve Rönnqvist, PhD

Gammadata Mätteknik AB, Uppsala, Sweden

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 450’000 cesium tests and more than 1’000’000 passive radon detectors have been measured in this accredited laboratory.

Gammadata

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.

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).

Measurement uncertainties Long-term radon measurements

Relative uncertainties - 90 days long-term measurements

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Radon concentration (Bq/m3)

SQRT(tracks)/tracks

Background uncertainty = 10tracks/cm2

Background uncertainty = 30tracks/cm2

Background uncertainty = 20tracks/cm2

Measurement uncertaintiesShort-term measurements

Relative uncertainties, 10-days short-term measurements

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Radon conc. (Bq/m3)

SQRT(tracks)/tracks

Background uncertainty = 10tracks/cm2

Background uncertainty = 30tracks/cm2

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.

Measurement uncertaintiesMain contributions

Long-term measurements (about 90 days)Exposure (kBqh/m3) Main uncertainty contributions0-250 (0-120 Bq/m3) Background250-450 (120-200 Bq/m3) Calibration & background450-6000 (200-2800 Bq/m3) Calibration> 6000 (> 2800 Bq/m3) High exposure method

Short-term measurements (about 10 days)Exposure (kBqh/m3) Main uncertainty contributions0-40 (0-170 Bq/m3) Background & transport40-60 (170-250 Bq/m3) Background & number of tracks60-90 (250-380 Bq/m3) Calibration, background & tracks90-3000 (380-12500 Bq/m3) Calibration> 3000 (> 12500 Bq/m3) High exposure method

(A calibration uncertainy of 5 % is assumed. Transport exposures can give contributions up to 5 kBqh/m3.)

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.

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.

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 (> 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.