An automatic evaluation system for NTA film neutron dosimeters

Radiation Measuroments PERGAMON

Radiation Measurements31 (1999)213-216

AN AUTOMATIC

EVALUATION SYSTEM NEUTRON DOSIMETERS

FOR NTA FILM

R. MI3LLER* AND T. OTTO* *CERN, TIS-RP, CH-1211 Geneva, Switzerland ABSTRACT At CERN, neutron personal monitoring for over 4000 collaborators is performed with Kodak NTA films, which have been shown to be the most suitable neutron dosimeter in the radiation environment around high-energy accelerators. To overcome the lengthy and strenuous manual scanning process with an optical microscope, an automatic analysis system has been developed. We report on the successful automatic scanning of NTA films irradiated with 23sPu-Be source neutrons, which results in densely ionised recoil tracks, as well as on the extension of the method to higher energy neutrons causing sparse and fragmentary tracks. The application of the method in routine personal monitoring is discussed.

KEYWORDS NTA films; automatic scanning; personal dosimetry; neutron dosimetry.

INTRODUCTION NTA films have been used for neutron personal monitoring around high-energy accelerators for over 30 years. Previous studies showed that the NTA film is the most suitable neutron dosimeter at accelerator centres such as CERN, where over 4000 persons have to be surveyed (H6fert, 1995). The Kodak film is composed of a 40 lun thick photoemulsion between two transparent protective covers (Stevenson, 1965). Recoil protons produced by the neutrons in the hydrogenous materials in and around the emulsion are recorded by ionisation of the photosensitive material. The number of recoil proton tracks per unit area of the emulsion has to be counted under an optical microscope in order to obtain a dose estimation. Sealing the films in aluminised plastic pouches in dry nitrogen protects the tracks against fading (Bartlett, 1977). The films thus treated can be worn for a period of two months. Analysing the films is lengthy and strenuous for the operator thus subjective influences cannot be avoided. Obviously there is a need for an automatic analysis system for the benefit of the person analysing the films as well as for the elimination of the subjective influences. Computerised image analysis systems already exist for many applications, e.g. in medicine or in highenergy physics for plastic detectors and nuclear emulsions (Rosa, 1997). However, to date no automatised track analysis for NTA films is available. The aim of this report is to present a short overview of the steps towards an automatised track analysis for NTA films in the Individual Dosimetry Service at CERN. O V E R V I E W OF IMAGE ANALYSIS SYSTEM In July 1997 CERN's Individual Dosimetry Service installed an image analysis system consisting of a Zeiss Axioskop microscope (40x Achroplan objective), motorised x-,y-,z-stage, Sony CCD Camera, Matrox Meteor Frame Grabber and standard PC with image analysis software Image-C (Imtronic, 1997). The software allows the manipulation of the acquired images with a range of subroutines such as filters and object recognition. For adaptation and automatisation the macro language VBA is used. 1350-4487/99/$ * see frontmatter© 1999 ElsevierScienceLtd.All rights reserved. PII: S1350-4487(99)00094-3

R. Miiller, T. Otto/Radiation Measurements 31 (1999) 213-216

214

Furthermore it is possible to control the movement of the stage in three directions from the software in order to automatically scan the surface of the film.

AUTOMATIC SCANNING AND TRACK ANALYSIS As a first step a scanning algorithm was tailored for films irradiated with a 238 Pu-Be radionuclide source giving rise to densely ionised recoil proton tracks. The algorithm (MOiler, 1997) consists of autofocus, correction of focus for oblique tracks by adding the images of three to five depth layers, contrast expansion, binarisation and object recognition, and finally their classification. In one standard field of 0.07 mm 2 more than a thousand objects are found out of which the recoil proton tracks have to be identified. For being classified as a track a total of 12 geometrical features e.g. shape, area and convexity must fall within defined limits. The suitability of the algorithm is shown by an analysis of unirradiated f i l l s and films irradiated on an ISO-phantom with dose equivalents Hp(10) = 0.25 to 4 mSv. The result is shown in Fig. 1. The scanned area on each NTA film amounts to a total of 3.4 nun 2. The track recognition rate of the analysis system is about 70 % in comparison with simultaneous analysis by a trained human operator. No deviation from linearity can be detected up to doses of 4 mSv.

i

50

i

i

i

I

.-"~

238pu_Be /i-

4O

E E

.../~'/"

Human operator 13.0

30

mm "2 mSv1 / /--"

f./--j

.

..... -

~ 2o

"O O

10

O-

j

g ~ z . ~ .... Automatic

~r j

scanning

9.2 mm "2 m S v 1

i

i

i

I

i

0

1

2

3

4

Dose equivalent Hp(10) (mSv)

Fig. 1. Proton recoil track densities on Kodak NTA films irradiated to different dose equivalents as evaluated by image analysis system and by trained human operator. The error bars are standard deviations, the sensitivity is evaluated from a straight-line fit to the data. The evaluation of NTA films irradiated by two different radionuclide neutron sources is summarised in Table 1. The lower track recognition rate of NTA films irradiated with 252Cf is caused by one of the present 12 geometrical features for object classification - the track length being smaller for the lower energy 252Cf than for 23s Pu-Be neutrons. Because of the occurrence of different kinds of background spots, of which appearance is irregular and strongly dependent on film quality, the object length has to exceed 13 jun, corresponding to a proton energy of about 1 MeV (Stevenson, 1966), for being classified as a recoil track. For a reduction of this length restriction and, subsequently the lowering of the threshold energy, different chemical developers and various developing times are currently under test. The track density of the background arises from falsely recognised photon background as well from cosmic ray neutrons and a particles emitted from naturally occurring radionuclides in the emulsion.

R. Miiller, T. Otto/Radiation Measurements 31 H999) 213-216

215

Table 1. Track densities and their standard deviation as determined on NTA films irrad, by different radionuclide neutron sources as evaluated by image analysis system and by trained human operator. Track Density (ram "2 mSv -1) Neutron Source Automatic Analysis

Human Operator

238pu-Be

9.2 + 0.3

13.0 + 0.3

252Cf

3.9 + 0.1

5.8 + 0.2

Background

1.0 + 0.1

0.6 + 0.2

The dominate contribution to personal neutron dose around high-energy accelerators comes from fast and relativistic neutronswith energies ranging from several MeV to several 100 MeV. Because of the smaller stopping power of the recoil protons generated by such neutrons the ionisation tracks are longer and less dense and often not contiguous. Therefore the described algorithna is expanded by subdividing the object recognition. The idea is to reconstruct long tracks from their fragments, which have been identified by the algorithm described for a radionuclide source. The orientation and coordinates of fragments unlimited by the length are taken, and distances and slopes between them are calculated. If the distance is lower than a specific threshold and slope and fragment orientation correspond, the fragments are counted as one track and the total track length is calculated. A final selection rejects all objects smaller than 13 ~tm. The results obtained with the expanded algorithm are given in Table 2 for an area of 1.5 nun 2 on the films both evaluated by the analysis system and the human operator. The mean projected track length distribution for all examined neutron energies evaluated by the image analysis system and the trained human operator are consistent. For neutrons with an energy of 15.1 MeV, the image analysis system is twice as sensitive as for 238pu-Be neutrons whereas the human operator achieves a gain in sensitivity of a factor of three.

Table 2. Track densities and mean projected track length and their standard deviation as determined on Kodak NTA films irradiated by different neutron energies as evaluated by image analysis system and by trained human operator. Neutron Energy

Track Density (mm "2 mSv ~)

Mean Projected Track Length 0tm) (Consideration of Tracks > 13 ~tm)

(MeV)

Automatic Analysis

Human Operator

Automatic Analysis

Human Operator

1.2 5.3 15.1

1.0 + 0.1 10.0 + 0.7 19.7 + 1.3

2.3 + 0.2 13.0 + 0.9 43.6 + 3.1

14.1 + 0.9 21.6 + 6.9 23.9 + 10.9

15.0 _+ 1.3 18.6 +_7.0 24.8 + 5.4

SENSITIVITY AND DETECTION LIMITS The detection limit according to DIN 25482 (DIN, 1992) can be evaluated. Using the calibration factor of 9.2 nun 2 mSv -~ for films irradiated with 238pu-Be and the density of background tracks of lmm 2 mSv ~, an area of 1.5 mm 2, corresponding to 22 standard fields 0Vliiller, 1998), must be scanned in order to detect a dose equivalent ofHN = 0.5 mSv with 95% confidence level (Swiss Ordonnance, 1998). The ordinance says "smallest dose that has to be measureable: Ho = 0.5 mSv" (original in french). We interpret this as a request for the detection level. For films irradiated with high-energy neutrons with as previously shown higher calibration factor, the confidence level will actually exceed 95%.

216

R. Miiller, T. Otto/RadiationMeasurements31 (1999)213-216

CONCLUSION The development of an automatic scanning and analysis system for the evaluation of Kodak NTA f i l l s is reported showing that the automatic scanning and analysis of NTA films is feasible. The evaluation of NTA films irradiated with neutrons in the energy range 0.5 to 1.4 MeV as well as with relativistic neutrons is improved by optimising both the object classification and the photographic development. An advantage of the automatised system is that the track length distribution is obtained at the same time as track counting and eventually enables a correction for energy dependence. The system takes approximately 5 minutes for scanning and analysis of each film for a detection limit corresponding to a dose equivalent ofHN = 0.5 mSv. With the present system it would be possible to evaluate the total of 4000 NTA films worn for a period of two months at CERN. For application in the Individual Dosimetry Service the comparatively slow macro-language algorithms will be rewritten in a compiled programming language in order to achieve significant gains in processing time.

Acknowledgement - The authors thank the staff of Imtronic GmbH for their technical support, especially the helpful advice of Mr. U. Spengler, and Mrs. H. Soubeyran of the Individual Dosimetry Service for contributing her knowledgeof developingand evaluationof NTA films gained in almost 10 years.

REFERENCES Bartlett T. and Creasy F.L. (1977) Latent Image Fading in Nuclear Emulsion. Phys. Med. Biol. 11, 736. DIN 25482, Beiblatt 1 (1992) Nachweisgrenze und Erkennungsgrenze bei Kemstrahlungsmessungen. H6fert M. and Stevenson G.R. (1995) Individual Monitoring in High-Energy Stray Radiation Fields. CERN/TIS-RP/95-19/CF. Miiller, R. and Otto, T. (1997) Automatic Scanning o f NTA Films Irradiated with 23Spu-Be Source Neutrons. CERN/TIS-RP/IR/97-37. Miiller, R. and Otto, T. (1998) Automatic Scanning o f NTA Film Dosimeters, Presented at 12 ~ Int. Conference on Solid State Dosimetry, 1998, Burgos, Spain. Imtroinc GmbH (1997) Systemhaus fiir Bildverarbeitung,. Rudower Chaussee 5, D-12489 Berlin. Rosa G., Di Bartomolomeo A., GreUa G.,and Romano G. (1997), Automatic Analysis of Digitized TV-Images by Computer-driven Optical Microscope, Univrsita di Salerno, Dipartimento di Scienze Fisiche. Nucl. lnstrum. Meth. A, 394 (3), 357-367. Stevenson G.R. (1965) A Personnel Dosimetry Service for fast Neutrons, Ministry of Health and Medical Research Council. R.P.S./I/5. Stevenson G.R. (1966) The Low Energy Threshold of the Kodak Personal Fast Neutron Film. Ministry of Health and Medical Research Council, R.P.S./I/15. Swiss Ordonnance sur la dosim6trie individuelle (1998), Le D6partement f6d6ral de l'interieur et le D6partement f6d6ral de 1' envriment, des transports, de 1' 6nergie et de la communication, Annexe 5