MICROCOPY RESOIUTION TEST CHART NBS 1010r; (ANSI and ISO TEST CHART No. 2)

MICROCOPY RESOIUTION TEST CHART NBS • 1010r;

(ANSI and ISO TEST CHART No. 2)

PHOTOORAPHIC SCIENCES CORPORATION 770 B«KET ROAD

fl O. BOX 338 i n WEBST-.rt, NEW YORK U580

rr^p<Dor

Dipartimento Ambiente

X AND GAMMA RAYS IRRADIATON TESTS

FOR EVALUATING THE PERFORMANCES

OF ITALIAN DOSIMETRY SERVICES

A. CAVALLINI

Centro Ricerche "E*!; Clementel" di Bologna S. KAFTAL, V. KLAMERT

CESNEF, Politecnico di Milano

„n r, 7, RT/AMB/94/02

ENTE PER LE NUOVE TECNOLOGIE. L'ENERGIA E LAMBENTE

Dipartimento Ambiente

X AND GAMMA RAYS IRRADIATON TESTS

FOR EVALUATING THE PERFORMANCES

OF ITALIAN DOSIMETRY SERVICES

Centro Ricerche "Ezio Clementel" di Bologna S. KAFTAL. V. KLAMERT

CESNEF, Politecnico di Milano

Tasto parvanuto rwi marzo 1994

I contanuti tacnico-sciantifici dai rapporti tacnici dall'ENEA

Abstract

X ANO GAMMA RAYS «RADIATION TESTS FOR EVALUATING THE PERFORMANCES OF ITAUAN OCSMETCVf SERVICES"

The ENEA-EDP Group (Personal Dosimetry «Experts) has already evaluated the - reKabWy of more than 50 % of the 70 Italian persomi dosimetry Services, which agreed lo

test their dosimeters through X and gamma insolation in air.

Film. TL and both film and TL are used as detector.'». The X a r t gamma rays energy ranges from 30 keV to 1250 i.eV (beam defined by ISO 1 X 7 ) . Exposures range from 5.2 10-6 C/kg io 1.3 10"3 C*g.

Son* dosimeters have been irradiated with a single energy level and others with two energy levels.

For each one of the nearly 4.000 dosimeters already tested, the rat» R has been calculated:

R - & R x . where: Xv is the exposure evaluated by the Service

Xj» is the actual exposure.

The R distributions have been analysed tor the dosimeters using firm or TL as detectors, as a function of irradiation energy and as a function of exposure values.

The i «suits obtained by all tested Services are commented.

Separate comments deal also with possible reasons of the failures lo pass the tests.

Riassunto

PROVE DI IRRAGGIAMENTO PER LA VALUTAZIONE DELL'AFFIDABILrrA DEI SERVIZI ITALIANI DI DOSIMETRIA.

Il Gruppo ENEA-EDP (Esperti in Dosimetria Personale) ha già verificalo .'affidabilità di più dei 50% dei 70 Servizi di dosimetria personale operanti in Italia, che hanno vo;uto sottoporre i loro dosimetri alla prova d> irraggeremo in aria con radiazioni X e gamma.

Cerne rivelatori vengono usati i film, i TL e congiuntamente "im e TL Llntervalo di energia delie radiazioni x e gamma va da 30 keV a 1.250 keV (i tassi sono definiti ntriSO 4037: L'intervallo delle esposizioni va da 5,2 10'6 C/kg a 1,3 10'3 C/kg.

Una pane dei dosimetri viene irraggiata con una sole energia, mentre gli altri vengono irraggiati contemporaneamente con due diverse energie.

Per ciascuno dei quasi 4.000 dosimetri irraggiati, * stato calcolato M rapporto:

R . & Xa

dove: Xv * l'esposizione valutata dal Servizio Xa * l'esposizione convenzionalmente vera.

La distribuitone degli R è stata analizzata per I film e per i TL in funzione dell'energia di irraggiamento e in funzione dell'esposizione.

Vengono commentati i risultati ottetti dai Servizi e vengono ricercate le cause per qualche caso di non superamento della prova.

X AND GAMMA RAYS IRRADIATION TESTS FOR EVALUATING THE PERFORMANCES OF ITALIAN DOSIMETRY SERVICES '3)

A.Cavallini: S.Kaftal". V.KIamert** •ENEA - PAS/FIBI/DOSI Bologna, Italy "CESNEF, Poli'ecnico di Milano, Italy

In Italy today more than 70 personal dosimetry Services process about 2,000,000 whole body dosimeters per year. These are mainly "protection category" for low energy, high energy and mixed low and high energy photons.

Italian radiation-protection laws do not require any authorization or licence in order to open and run a dosimetry Service, nor it is required that the Service's capability and reliability be tested. On the other hand, Italian radiation protection regulations require that the choice of a reliable Service be made by a so-called "Qualified Expert".

While waiting for an expected change in the current regulations, it was deemed necessary to organize a test program, in order to help the Qualified Expert make his choice and give the Services the means to check themselves. Such a test program could also help identify systematic errors or wrong methods and suggest improvements.

Several years ago, acting privately, a small group of experts in personal dosimetry, who were working with the major and oldest Italian Services, decided to study and then carry out the above test program, for photon radiation only. In 1984 this group became an official working group of ENEA (National Agency for the Development of Nuclear and Alternative Energy) 3S ENEA-EDP.

Today, Services may ask on a voluntary basis to undergo the tests, and those that pass them receive a reliability recognition (accreditation). It should be noted, however, that this accreditation has no legal value.

The accreditation may be requested for one or more of three groups, identified by their energy range (Tablel).

Table 1 - Dosimeters group classification

GROUP 1 ' 2 3 ENERGY (MeV) 0.250 - 3.0 0.020 - 0.250 0.020 - 3.0

( ' Speach delivered at the Second Conference on "Radiation Protection and Dosimetry", Orlando, Florida, 31 October - 3 November 1988

-3-The accreditation relates only to the group for which the Service has successfully undergone the tests. Passing the tests separately for group 1 and group 2 is not equivalent to passing them for group 3.

The' test program is made up of two steps. First, the Service's methods, facilities, instruments, organization r.r.d so on are investigated by means of a detailed questionnaire and an extensive interview conducted by at least two members of ENEA-EDP. (It is interesting to point out that many Services do not have a clear idea of what a serious personal dosimetry really entails).

Once a Service has passed the first step, it is allcved to undergo the second one, which is a fairly severe proficiency test, but restricted to 5 . 2 - 1 06 - 1 . 3 - 1 03 C/kg (0.020 - 5 R) exposure range.

In Table 2 and Table 3 are shown the 5 energy levels and the 5 exposure ranges used in the test.

Table 2 - Energy levels

LEVEL 1 2 3 4 5 ENERGY (keV) 2 0 - 4 0 > 4 0 - 8 0 > 80 - 130 > 1 3 0 - 2 5 0 > 2 5 0 - 3 0 0 0

Table 3 - Exposure ranges

RANGE 1 2 3 4 5 EXPOSURE (C/kg) 5 . 2 1 0 "6 - 1.610-5 > 1.6 I O "5 - 3 . 9 1 0 - 5 > 3 . 9 1 0 - 5 - 1.3-10'4 > 1 . 3 1 0 "4 - 5.2 I O '4 > 5 . 2 1 0 "4 - 1 . 3 1 0 -3 ( R ) 0.020 - 0.060 > 0.060 - 0.150 > 0.150 - 0.500 > 0.500 - 2.000 > 2.000 - 5.000

For each energy level included in the requested accreditation group, one photon beam is selected and a couple of dosimeters are irradiated at one exposure value for each range.

-Further, couples of dosimeters are irradiated with two beams, each of a differe.it

energy level, in ail possible level combinations but at exposure values chosen only from the 3. 4 and 5 range.

This" means that *z: group 3, 110 dosimeters are irradiated under 55 different energy and exposure values.

Heavily filtered X-rays (B series ISO 4037) and Cs-137 and Co-60 gamma rays are used, and the dosimeters are irradiated at normal incidence, free in air.

The exposure evaluated by the Service (Xv) and the actual exposure supplied by the

irradiation Laboratory (Xa) must meet the following condition:

/ 2XQ \ X„ / Xn \

where X0 is the lower limit of the exposure range, 5.2-10"6 C/kg (20 mR) (Figure 1). In

what follows, the ratio X, /Xa will be called Ft.

0 300 600 900 1200 Exposure (uC/kgj

Fiacre 1 - Acceptance boundaries

No more than 10% of all the Service's tested dosimeters, but no more than 2 of those that have been exposed to any of the radiation qualities, may exceed the limits established by the formula. Alternatively, no more than 1 dosimeter for every radiation quality may exceed the above limits.

Since 1986, 40 Services have been tested in four sessions: 16 using films, 21 using TL and 3 using both detectors.

The results are summarized in Table 4 and Table 5.

-5-Table 4 - Results of tested Services

Services failed at the first step

Services failed at the irradiation test Accredited Services Total Film 3 8 5 16 TL 2 3 16 21 Film + TL -3 3

Table 5 - Number of dosimeters per year processed by accredited, failed and under-test Services

Detector TL r i l m Film + TL Total (A) 260,000 125,000 35,000 420,000 ( B ) 90,000 280,000 20,000 390,000 Total (A) + (B) 350,000 405,000 55,000 810,000 (C) 14,000 360,000 374,000 (A) No. of dosimeters, accredited Services

(B) No. of dosimeters, failed Services (C) No. of dosimeters, under-test Services

As the tables clearly show, the percentage of accredited film Services is much lower than that of the TL Services.

Although nearly 4,000 dosimeters have been treated, the figures shown in the following do not allow to draw general conclucions such as: What type of dosimeter is better? Which dosimetric method is more reliable? It is possible only to judge each single Service and try to deduce some trend.

For instance, a detailed analysis of the results oLtaincd for all tested dosimeters shows a trend towards underestimation of the exposures at certain energies or energy combinations, main!/ for film and film + TL dosimeters irradiated at low + high energies (Figuro 2). However, no such differences splitting the results along an exposure axis have been noticed {Figure 3).

-• FIU' D TL 0 F1LM-TL O.C • ^ CM O O — Csi O o CM C U Ì-CM o Cv: ENERGY (keV) C\! U CM «• O *— o If) CVi . •v o — o m _C\i _ CM o CVi

Figure 2 - Distribution of the mean values of R (all tested Services)

5 2 !5S 15 3 - 3 8 7 3 8 7 - 1 2 9 1 2 6 - 2 5 8 2 5 8 - 1 2 9 0

Exposure range (.uC/kg)

Figure 3 - Distribution of the mean values of R (all tested Services)

From an observation of the accredited Services' results alone, the R values for TL and film dosimeters approach satisfactory a gaussian distribution (continuous line), with a mean value very close to 1 (Figure 4 ). The dotted line is reiated only to values included into the acceptance boundaries.

2 5 0 n 200 150-3" 100-o c a

S"

TL

s:

of accredited Services n: No. of R values

o: No. of R values exceeding the acceptance boundaries s: standard deviation

-This observation, however, has no actual weight because of the still low number of samples gathered for both types of tested Services. A far more useful approach is to consider the R values and the related standard deviations for each Service. The next table (Table 6) shows the R and standard deviation values calculated for accredited Services, separately for LiF, BeO and film dosimeters.

Table 6 - R and standard deviation for different detectors (Accredited Services) Detector LiF BeO Film Service 1 2 3 4 5 6 7 8 9 1 2 3 4 1 2 3 4 5 N.of dosimeters 10 110 110 110 76

n o

The comparison between the R and standard deviation values provides some interesting information. For example, BeO Service n.1 (group 3) underestimates the exposures, but the standard-deviation is low: this is probably the consequence of a systematic error in calibration that should not be difficult to correct. The dosimetric procedures, though, seem to be good. LiF Service n.8 (group 2) has a very high standard deviation, in fact, there were only three wrong results, but those were macroscopically

•9-wrong. Recalculating R and the standard deviation for the 73 valid evaluations, R grows and the standard deviation becomes very small. The diagnosis may be:

(1) incorrect calibration and

(2) à source of erratic serious mistakes that needs to be investigated. And so on.

A detailed analysis of R and standard deviation values may help the Services that have failed the tests detect the causes of errors and may also help those that have been accredited improve their performances.

300 600 Exposure (jiC/kg) 900 1200 2.5 r a 600 900 1200 Exposure (n-C/kg)

Figure 6 - R versus actual exposure for film dosimeters of two Services using the same method

0-A giaphic representation of the calculated figures adds to the picture, as illustrated in Figure 6, which refers to an accredited Service (n.3). and to a failed Service. They both use the same film-dosimetric method.

The" above suggests some remarks: TL dosimetry is easier than film dosimetry, and this is surely one of the reasons for its large development also for personal dosimetry. Aiso, there are few methods available; they are not very complicated, but require correct calibrations and care in following each suggested procedure.

Film dosimetry requires instead the choice of a right method among several possible ones, as well as longer and more difficult calibrations. Furthermore, the carrying out of the work itself is harder.

Nevertheless, the question is: should this type of dosimetry be completely replaced by TLD? The advantages offered by film dosimetry for mass personal survey, suggest that we should perhaps try to improve it, rather than abandon it.

The picture offered by the Italian Services that have undergone the tests is not yet complete, since the fifth session has just started. Until now, 60% of the tested organizations, which process about 52% of the yearly used dosimeters, have been accredited.

Any comment should wait until the end of the tests' sessions.

REFERENCES

1 ) Gruppo di Lavoro ENEA-EDP: Criteri per la verifica dell'affidabilità di un Servizio di dosimetria p: sonale per radiazioni X e gamma. ENEA-EDP 89/1

2 ) X and Gamma Reference Radiation for Calibrating Dosemeters and Dose Ratemeters and for Determining their Response as Function of Photon Energy, ISO 4037, 1979

3 ) Filmdosimetrie nach dem filteranalytischen Verfahren z'ir Strahlenschutzuber-wachung, DIN 6816, 1984

4 ) Dosimetri Individuali. Verifica dell'accuratezza. UNI 9104, 1I>88

-Edito dall'Enea Direzione Relazioni Esterne V. le Regina Margherita, 125 - 00198 Roma Finito Ji stampare nel mese di maggio 1994

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