Do your ionizing radiation readings give you what you expect?

Elaborated text following a brief poster presentation at the HPS meeting in Seattle, 1996

Please read the preceeding paper here on Internet for this content integrated to experimental data with calibration and selection criteria for portable and personal meters: Étalonnage, vérification et choix...

 

Radioprotection J.-M. Légaré

 

Main Page Jean-Marc Légaré

Professional Radiation Protection Services

Updated July 7, 2009

 

Contents

 

1. Expectations

2. Means for reaching our goals

2.1 Proper choice (check list) of equipment

2.2 Calibration and testing of physical quantities and units for various quantity specifications

2.3 Taking advantage of simple accurate radiation therapy technology

2.4 Use of backscatter (or peak scatter) factors for real persons

2.5 Use of exposure dose, absorbed dose and kerma (air), and rates if for X and gamma rays

2.6 Use of the Sievert (Sv) only for well identified biophysical quantities as related to the energy spectrum and original hypotheses

2.7 Knowing the terminology, hypotheses and corresponding limitations used

2.8 Use of conversion factors to go from basic quantities to biophysical concepts

2.9 Knowing the meaning of programmed meters (characteristics, hypotheses, limitations, etc.)

2.10 Use of published data

3. Conclusions

4. References

 

 

1.Expectations

 

We expect readings from measurements of separate types of external radiation in proper quantities and units with possibility of conversion to specific biophysical concepts as applied to real persons over the planet.

This paper deals with simple survey meters and direct reading and electronic personal dosimeters. Radiation situations being considered here are:

Contact alpha and beta sources,

Distant beta, electrons, X and gamma rays and neutrons.

 

2. Means for reaching our goals

 

2.1 Proper choice (check list) of equipment

A good variety of survey meters and personal dosimeters or monitors corresponding to our needs are sometimes found on the market. However, it is important to add or replace certain items related to experience and specific needs (administrative, technical, financial, servicing, etc.). See the selection criteria before purchasing (See Tables I, II and III of my work on selection and calibration, entitled Étalonnage , vérification et choix de compteurs..., here on Internet).

 

2.2 Calibration and testing of physical quantities and units for various quantity specifications

  1. Precalibration checks: batteries, loose parts, zero position, etc.
  2. Calibration: see published protocols from organisations below
  3. NCRP-122, 1995 is particularly useful.
  4. Contact sources:
    1. Bq, Bq/surf., cps (1 side) for the source seen.
    2. Beta dose and dose rates (mGy, mGy/h) to skin and other media specified
  5. Distant sources (all except alpha):
    1. Absorbed dose (µGy) or rate (µGy/h) for a given medium; ex. air, water, muscle
    2. Kerma (µGy) or kerma rate (µGy/h) in free air for X and gamma rays
    3. Exposure in milliRöntgen (mR) or exposure rate (mR/h)
    4. Fluence (No. photons/surf. or particles/surf.)
    5. Biophysical dose or rate, according to a well identified biophysical concept, as related to energy distribution when it is known abd to physical and biological factors

Radiation quality is usually given as energy spectrum, maximum energy, average and/or effective energy; first and second HVL (specify if small or large field). There is a link between hard beam HVL and energy. See figures of the book by Légaré and Aubé as given in part here on Internet.

 

2.3 Taking advantage of simple accurate radiation therapy technology

One can take advantage of clinical dosimetry e. g. calibration, BSF, % Depth dose data, tissue (water) air ratios, absorbed dose (Rad or Gy), isodose curves, etc., especially with representative water phantoms and specified tissues. Corrections are known for lungs, bones, etc. adjustable to individual cases (CT scan, etc.). Take into account all known patient and irradiation factors. Specified soft tissue dose can be directly measured or evaluated at depths of 0, 0,07 mm (skin), 3 mm (eyes), 10 mm or more... These can be used in radioprotection.

See the Québec Clinical Dosimetry System (Technique québécoise de dosimétrie clinique). It is described in Strahlentherapie. It is reproduced here on Internet with the kindness and generosity of the Editor Urban & Vogel whom I wish to thank very sincerely. This technique gives central axis exposure dose distributions of just about all situations of single and opposing fields used in radiotherapy and radiobiology (water slabs of all thicknesses between 0 and 26 cm) for X and gamma rays up to 1250 keV (Co 60). The central axis depth dose data can be applied to individual field isodose curves.

 

2.4 Use of backscatter (or peak scatter) factors for real persons

In radiotherapy, BSF are usually given up to 20 cm x 20 cm fields and for whole body cobalt 60 irradiation. See Brit J.. Radiol., Suppl. 17, 1983. Other pertinent data are needed experimentally for whole body irradiation. See graph of BSF vs energy and field size (up to 50 cm x 50 cm = 60 x 46 cm2) here on Internet from the book of Légaré and Aubé.

 

2.5 Use of exposure dose, absorbed dose and kerma (air) and rates if for X and gamma rays

The 20 year old finding of Kerma (air) is useful to replace the R which is undesired in certain organizations. The R exposure dose unit and rate have the merit to be directly measurable from electrical charges and electrical current due to ionization.

 

For X and gamma rays, the kerma (air) conversion factors are:

0,876 cGy = 1,0 R for 10-1500 keV; 1 Gray (SI) = 1 Joule/kg = 100 Rad

0,883 cGy = 1,0 R for 2000 KeV

0,885 cGy = 1,0 R for 3000 KeV

or simplify to

0,88 cGy = 1,0 R (10-3000 KeV)

8,8 µGy = 1,0 mR (10-3000 KeV)

See ICRU 47, 1992, p. 23 for quoted kerma (air) conversion factors vs energy. For the absorbed dose in air, the practical conversion factors are usually the same for X and gamma radiation. For other media, they can be found vs energy in the literature.

 

I think it is still useful and worthwhile to measure in mR and mR/h, at least until meters give proper X and gamma absorbed dose or kerma values in µGy and µGy/h.

 

2.6 Use of the Sievert (Sv) only for well identified biophysical quantities as related to the energy spectrum and original hypotheses

For X, gammas, electrons, and betas, it is assumed that the relative biological effect is 1,00. This is not always so. For neutrons, there was not always agreement on the values. RBE has been replaced by the Quality Factor (ICRP- 26, 1977) and then modified to Radiat. Weighting Factor (ICRP-60, 1991). Note the hypotheses assumed on Sv/Gy and Sv/R ratios used on commercial meters.

 

Some manufacturers assume that for X and gamma rays 10 µSv/h = 1 mR/h. This is usually wrong, especially when the biophysical concept used is not even indicated nor the energy distribution known and taken into account.

 

2.7 Knowing the terminology, hypotheses and corresponding limitations

A multitude of terms, concepts have come into existence since ICRP-26, 1977 and ICRP-60, 1991.

The words, or the inversion of words e. g. Dose Equivalent and Equivalent Dose have different meanings! The definitions were given specific uncommon meaning, usually different from those of dictionaries!

Specific hypotheses are not necessarily met in practice and over the years. Limitations should also be known and taken into account.

Little or no anatomical considerations of people of different continents and origins have been given to phantoms so far. The above representative persons and individual ones should be taken into account to reach the personalized whole body biophysical "Effective Dose", "Biophysical Tissue and Organ Doses", etc. See also 2.8.

 

2.8 Use of conversion factors to go from basic quantities to biophysical concepts

A multitude of conversion factors or conversion coefficients have come out in the literature to link, one at a time, some of the quantities!

It is best to avoid going from one concept to another concept, e. g. even when wishing to go from replacing Effective Dose Equivalent (ICRP-26, 1977) to Effective Dose (lCRP-60, 1991) with the continuing change in Tissue Weighting Factors related to Effective Dose. Also note that body ED values are smaller than EDE and are both different for smaller and larger persons for the same X and gamma irradiation. The former EDE and the replacing ED values as well as the conceptual Tissue and Organ Doses differ also with age, weight, shape and sex of the person, and source distance!

 

2.9 Knowing the meaning of programmed meters (characteristics, hypotheses, limitations, etc.)

"Smart" meters exist already. Neutron-Rem rate meters have been existing for a long time on the basis of older RBE values at different neutron energies.

For programmed meters, mostly to come, quantities, characteristics and limitations, must be well identified and the meter should offer the possibility of being easily reprogrammable for specific functions, modified conception parameters, etc.

Energy distribution can be taken into account e. g. by means of proper filters as it is already done in certain personal (individual) dosimeters or better.

 

2.10 Use published data

IAEA, ICRP, ICRU, NCRP, ANSI, ISO, PTB, CEC, OCDE, Radiat. Prot. Dosim., Health Physics, Radioprotection, Journal of Radiological Prot., Suppl. no 17, 1983 of Brit. J. Radiol., Strahlenschutz und Praxis, Atompraxis, Nordic Assoc. of Clinical Physics (NACP), Am. Assoc. Phys. Med. (AAPM), Med. Physics in Med. & Biol., and other national and transnational documents.

 

3. Conclusions

 

Choose equipment to your needs with pertinent characteristics, price, calibration, testing and servicing

Take advantage of radiotherapy technology and its published data (calibration, absorbed dose, BSF, % DDD, tissue/air ratios, isodose curves, etc.)

Use medium absorbed dose, kerma (air) and medium absorbed dose rates or air kerma rates for X and gamma rays where we wish to replace the R unit: 8,8 µGy (air) = 1,0 mR (10-3000 keV) in air.

Use Sv units only with proper biophysical quantities: Effective Dose Equivalent, Effective Dose,... as related to the energy spectrum, usually unknown, and the various hypotheses (see 2.7 and 2.8)

Use programmed meters whose characteristics, hypotheses and limitations are proper and written, and which can be reprogrammable.

 

4. References

 

IAEA

Selected publications

 

ICRP

Recommendations of the International Commission on Radiological Protection

ICRP Publication 26, 53 p. 1977

Pergamon Press, Oxford ISBN O 08 021511 4

 

ICRP

1990 Recommendations of the International Commission on Radiological Protection ICRP Publication 60, 201 p. 1991

Pergamon Press, Oxford ISBN O 08 041144 4

 

ICRU

Measurement of dose equivalents from external photon and electron radiations.

ICRU Report 47, 40 p, April 15, 1992

 

ICRU

Quantities and units in radiation protection dosimetry

ICRU Report 51, 19 p, September 2, 1993

 

ISO

Exposure meters and dosimeters - General methods for testing

ISO - 4071, 29 p. 1978

 

LÉGARÉ, JEAN-MARC

Étalonnage économique entre 28 et 1250 keV pour les appareils de radioprotection utilisés dans la surveillance de l'environnement (Reproduced here on Internet)

Gouvernement du Québec, 22 p., 1990, ISBN 2 - 550 - 20797 - 1 (Reproduced here on Internet)

 

LÉGARÉ, JEAN-MARC

Étalonnage, vérification et choix de compteurs portatifs pour alpha, bêta, gamma, rayons X et neutrons, de stylodosimètres à lecture directe, de signaleurs de poche et de dosimètres électroniques individuels; dosimétrie et concepts biophysiques, (Reproduced here on Internet).

 

LÉGARÉ, JEAN-MARC and AUBÉ, P.-B.

Spécifications de blindage pour les installations de radiographie industrielle et concepts dosimétriques (Partly reproduced here on Internet)

Gouvernement du Québec, 160 p., 1995 ISBN 2 - 550 - 24310 - 2

 

NCRP

Dosimetry of X-ray and gamma-ray beams for radiation therapy in the energy range 10 keV to 50 MeV.

NCRP Report No 69, 110 p., 1981

 

NCRP

Calibration of survey instruments used in radiation protection for the assessment of ionizing radiation fields and radioactive surface contamination.

NCRP Report No 112, 213 p., December 31, 1991

 

NCRP

Use of personal monitors to estimate effective dose equivalent and effective dose to workers for external exposure to low-LET radiation.

NCRP Report No 122, 64 p., December 27, 1995

 

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Do your ionizing radiation readings give you what you expect?