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THE USE OF MICROIXSIMETRIC TECHNIQUES I N RADIATION PROTECTION

Jing Chen, ESH-4

W.H. Casson, ESH-4 D.G. Vas i l ik , ESH-4

H.H. HSU, ESH-4

QCT 3 0 1996 O S T I

Health Physics of Radiation Generating Machines 30th Midyear Topical Meeting of t h e Health Physics Society, San Jose, CA, January 5-8, 1997

Los Alamos National Laboratory, an affirmative actionlequal opportunity empldyer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or rqrOduCe the published form of this contribution, or to allow others to do so, for US. Government purposes. The LoS Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Form No. 836 R5

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The Use of Micmdosimetric Techniques in Radiation Protection

Measurvmen ts

Jing Chen, Hsiao-Hua Hsu, William H. Casson, and Dennis G. Vasilik

MS G761

Los Alamos National Laboratory

Los Alamos, NM 87545

Abstract

A major objective of radiation protection is to determine the dose equivalent for routine

radiation protection applications. As microdosimetry has developed over approximately

three decades, its most important application has been in measuring radiation quality,

especially in radiation fields of unknown or inadequately known energy spectra. In these

radiation fields, determination of dose equivalent is not straightforward; however, the use

of microdosimetric principles and techniques could solve this problem. In this paper, we

discuss the measurement of lineal energy, a microscopic analog to linear energy transfer,

and demonstrate the development and implementation of the variance-covariance method,

a novel method in experimental microdosimetry. This method permits the determination

of dose mean lineal energy, an essential parameter of radiation quality, in a radiation field

of unknown spectrum, time-varying dose rate, and high dose rate. Real-time monitoring of

changes in radiation quality can also be achieved by using microdosimetric techniques.

Introduction

A major objective of radiation protection is to determine the dose equivalent for routine

radiation protection applications. Dose equivalent, H, is defined as a product of the

absorbed dose, D, and the radiation quality factor, Q, where Q takes into account the

biological effectiveness of different ionizing radiations. The quality factor is currently

defined as a function of linear energy transfer (LET), which, in a strict sense, is a

computational, not measurable quantity.

Microdosimetry is based on experimentally measurable quantities in small irradiated

volumes. Lineal energy, y, is a microscopic analog to LET. The measurement of lineal

energy can provide, in many cases, good approximations to determine quality factors and

thus the dose equivalent, as described in ICRU 40 (ICRU 1986).

It is well known that microdosimetric techniques are pertinent for measurements with low

doses and low dose rates. In this paper, we will present microdosimetric techniques that

can be equally well used in radiation fields of high dose rate.

In radiation fields of high dose rate, the determination of dose equivalent becomes difficult

because of the pile-up effect. This is especially the case in high-LET radiation fields in

which the quality factor is significantly larger than unity and also in radiation fields of

unknown or inadequately known energy spectra in which a conservative mean quality

factor should normally be used. Although modern digital data processing techniques can

help to some extent to correct for the pile-up effect numerically, many instruments are

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still limited to a given dose rate. The use of the variance-covariance method, a novel

microdosimetric technique, could help us to come out of this dilemma. To enable better

understanding of the variance-covariance method, we first discuss the variance method.

The Variance Method

Dose mean lineal energy, z, is defined as the dose-weighted average of the single-event

spectrum, f(y) (ICRU 1983). There is, however, a direct method that one can use to

determine dose mean lineal energy without having to know the single-event spectrum and

without the restriction of very low dose rates (as in the case of a single-event spectrum

measurement). This is the variance method devised by Bengtsson (Bengtsson 1970) and

successfully employed under laboratory conditions by Bengtsson and Lindborg

(Bengtsson and Lindborg 1974). Mathematically the variance method can be expressed as

where r is the frequency mean value of the multi-event spectrum of energy imparted E,

and V, is the relative variance of the multi-event spectrum.

In a constant radiation field, the measured signal variance results from the microdosirnetric

fluctuations of energy imparted in the detector. In a series of measurements of dose

- increments, the original signals are E ~ , c2, ..., I, and they are the basis for cal~ulations of

and V,. Correspondingly, the radiation dose and dose mean lineal energy are

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*

where m the mass and 7 the mean chord length of the detector volume.

The variance method was subject to the requirement of a constant radiation field, Le., the

dose increments per measurement interval had to remain exactly the same throughout a

series of measurements. Often, this condition cannot be met. The further development of

this method to meet more practical situations has resulted in the variance-covariance

method.

The Variance-Covariance Method

The limitation of the variance method was removed by the variance-covariance method

(Kellerer and Rossi 1984). Its mathematical form is

(3) - - E , = (v, - C r A B ) . E A *

The method uses two detectors (detector A and detector B) that determine energy

imparted in a series of synchronous measurement intervals @e., the two measuring

channels are triggered exactly at the same time). The basic measured signals are &Al, em, ...,

eAn for detector A, and cBl, eB2, ..., eBn, for detector B. In this method, the two detectors do

not need to be identical, but the ratio of doses in the two detectors should be constant.

The microdosimetric variance of energy imparted is obtained as the difference of two

terms. The first term, V,, is the relative variance of measured signals from one detector

(for instance, detector A), which includes microdosimetric and dose-rate fluctuations, Le.,

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the total fluctuation of detector signals. The second term, C,, is the relative covariance

between synchronous signals of the two detectors. It merely represents the dose-rate

fluctuation that effects the two detectors correlatively, while the microdosimetric

fluctuations in the two detectors are statistically independent. The difference of V,-C,

gives the microdosimetric fluctuation only. The microdosimetric fluctuation of energy

imparted in the detectors is radiation specific and reflects radiation quality. In a constant

radiation field, the observed covariance should be zero, and the variance-covariance

method reduces to the variance method.

The variance-covariance method has been successfully used in numerous micrudosimetric

investigations such as in clinical radiation fields (Honore et al. 1990; Chen et al. 1992;

Chen et al. 1994) and in space activities (Lindborg et al. 1995). Recently, the variance-

covariance method has been extended to the situation in which there are slow changes to

the ratio of dose rates in the two detectors (Kellerer 1996). This extension is important

for personal rem meters. In personal rem meters, the ratio of dose rate can change when

the person who carries the detectors moves in a radiation field.

The following is the new form of the variance-covariance method applied to variable dose

rates compounded by changes of the ratio of dose rates in the two detectors,

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It differs from equation 2 by a factor, k, and the ratio of mean energy imparted in the two

-- detectors in the covariance term. This factor, k - E , / E A , reflects the changes of the ratio

of dose rates in the two detectors. We can calculate k from successive measured signals:

* ‘ A i E A ( i + l ) k = (5)

The signal processing as given in equations 1 to 5 is easy to perform with digital signal

processing devices.

Conclusion

We have discussed the advantages of using microdosimetric parameters in operational

radiation protection and have presented well-established microdosimetric techniques. We

also have presented a new development that can help determine dose mean lineal energy

in various practical- radiation measurements. Although the exact relationship between the

definition of radiation quality factor in terms of LET and in terms of y is still under

discussion, we find that microdosimetric techniques should be employed, at least in

situations when conventional methods used in radiation protection become difficult or

even impossible. Two examples follow.

Example 1: Radiation field characteristics are neither known nor well known.

Because many conventional detection methods are developed for radiation fields of given

energy spectra, choosing a suitable rem meter in unknown radiation fields for the purpose

of radiation protection is difficult. The variance-covariance method is developed to

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determine the radiation dose and radiation quality, i.e., dose equivalent without

preknowledge of the radiation fields. The method is especially suitable for mixed radiation

fields, such as in the case of neutron dosimetry and dosimetry related to space activities.

Example 2: Radiation fields of high-dose rates. These fields occur when conventional

techniques fail due to the high pile-up effect and when the pile-up problem cannot be

solved even with an available flash analog digital converter and modern digital data-

processing techniques.

The variance-covariance method has been developed as a method suitable for a high dose

rate and for time-varying dose rate as well. Future activities in operational radiation will

definitely benefit from the use of microdosimetric techniques.

References :

Bengtsson, L. G. Assessment of dose equivalent from fluctuations of energy deposition.

In: Proceedings of the Second Symposium on Microdosimetry. EUR 4452 HG;

Ebert ed.; Euroatom, Brussels; 1970: 375-395

Bengtsson, L. G.; and Lindborg, L. Comparison of pulse height analysis and variance

measurement of the determination of dose mean specific energy. In: Proceedings

of the Fourth Symposium on Microdosimetry. EUR 5 122; J Booz ed.; Euroatom,

Luxembourg, 1974: 823-84 1

Chen, J.; Roos, H.; and Kellerer, A. M. Microdosimetry of diagnostic x-rays: applications

of the variance-covariance method. Radiat. Res. 132:271-276; 1992.

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Chen, J.; Hahn, K.; Roos, H.; and Kellerer, A. M. Microdosimetry of therapy electron

beams-measurements and Monte-Carlo simulations. Radiat. Prot. Dosim. 52:

435-438; 1994

Honore, H. B.; Jessen, K. A.; and Nielson, H. H. Variance-covariance measurement of the

dose mean lineal energy in beams for radiotherapy. Radiat. Prot. Dosim. 3 1 : 453-

455; 1990.

ICRU. The quality factor in radiation protection. Report 40; International Commission on

Radiation Units and Measurements; Bethesda, Maryland 1986.

ICRU. Microdosimetry. Report 36; International Commission on Radiation Units and

Measurements; Bethesda, Maryland 1983.

Kellerer, A. M.; and Rossi, H. H. On the determination of microdosimetric parameters in

time-varying radiation fields: the variance-covariance method. Radiat. Res. 97:

237-245; 1984.

Kellerer, A. M. Generalization of the variance-covariance method for microdosimetric

measurements 11. Formulae for varying dose-rate ratio in the detectors and

synopsis of results. Radiat. Environ. Biophys. 35: 1 17-1 19; 1996.

Lindborg, L.; Grindborg, J. E.; Gullberg, 0.; Nilsson, U.; Samuelson, G.; and Uotila, P.

TEPC measurements with the variance-covariance method on board aircraft.

Radiat. Prot. Dosim. 61:119-124; 1995.

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