- Email: [email protected]
Fast neutron and γ-ray dosimetry with imaging plates
Radiation Measurements 34 (2001) 513–516
www.elsevier.com/locate/radmeas
Fast neutron and -ray dosimetry with imaging plates A. Boukhaira; b , C. Heilmanna; ∗ , A. Nourreddinea , A. Papea , G. Portalc b Laboratoire
a Institut
de Recherches Subatomiques, 67037 Strasbourg Cedex 2, France de Physique et Techniques Nucl#eaires, Facult#e des Sciences, B.P. 20, El Jadida, Maroc c CEA=IPSN, 92260 Fontenay aux Roses, France
Received 28 August 2000; received in revised form 8 February 2001; accepted 8 March 2001
Abstract An imaging plate (IP) is a passive integrating detector capable of medium-term storage of ionizing energy. A visible laser scan can be used to liberate a number of visible photons proportional to the amount of energy trapped. An IP is sensitive to ionizing particles of any energy, whatsoever, to neutrons and to photons. It is linear in response over 4 –5 decades. We have studied the response of IPs to neutrons emitted by Pu–Be and Am–Be sources in di4erential measurements that correct for the -ray component from the neutron source. The dose threshold for fast neutrons, as well as for 60 Co -rays, could thus be c 2001 Elsevier Science Ltd. All rights reserved. determined. Keywords: Imaging plate; Photostimulated luminescence; Fast neutrons; -rays; Dosimetry
1. Introduction
2. Physical basis of imaging plates
Imaging plates (IPs) based on the photostimulation of phosphors were introduced several years ago, principally as a replacement for photographic :lms. Used extensively for biomedical autoradiography and for medical and dental imaging, IPs are capable of detecting charged particles and photons (; X; UV). More recently, they have been employed in crystallography, UV spectrometry, electron microscopy and non-destructive X-ray and neutron testing. For neutron imaging (B?ucherl et al., 1993; Niimura et al., 1994; Hofmann and Rausch, 1995; Rant et al., 1998; Thoms et al., 1999; Thoms, 1999; Tazaki et al., 1999), gadolinum-doped detectors, or normal detectors associated with a converter, are used. Recent recommendations and new norms for operational dosimetry by di4erent international commissions have led to changes in the methods used for individual dosimetry, implying the development of new types of dosimeters. It is in this context that we have begun a study of the response of IPs to fast neutrons and -rays.
IP detectors consist of a Dexible screen of BaFBr : Eu2+ about 1 mm thick composed of distinct layers: anti-abrasive front and back layers, a layer of dense inorganic phosphor crystals set in a polymer matrix to give a homogeneous distribution, and a polyethylene layer supporting the layer of phosphors. In some cases, the support is blackened with carbon to prevent laser light reDection and scattering during reading. Upon excitation of the phosphors by electromagnetic radiation or charged particles, the energy is stored. A number of Eu2+ ions proportional to the incident Dux on the IP loses an additional electron and the Eu3+ becomes trapped in a quasi-stable state by the Br − . This constitutes a latent image. To read the IP, a scan with a :nely focused laser beam frees the trapped electrons. The transition to the ground state of the phosphor is accompanied by a photon which is reDected by a mirror to optical :bers facing the scanned surface. The photon is converted to an electrical signal and ampli:ed by a photomultiplier (PM), then sampled and digitized by an analog-to-digital converter. The data are stored to be treated by an image processor. The intensity of the laser-induced luminescence varies with the wavelength of the reading laser, with the
∗ Corresponding author. Tel.: +33-3-88-10-63-54; fax: +33-3-88-10-62-34 E-mail address: [email protected] (C. Heilmann).
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 1 8 - 9
514
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
Fig. 1. Stimulation and luminescence spectrum of BaFBr : Eu 2+ . (Molecular Dynamics Technical Note # 53.)
maximum eIciency being obtained near the 633 nm wavelength of the He–Ne laser sometimes used. The light emitted by the phosphors is within the range of maximum eIciency (300 –500 nm) of the PM that captures the signal (Fig. 1). A :lter absorbs laser light from the stimulated light to avoid interference. An IP is reusable ad in:nitum. Its response is linear with dose over 4 –5 decades. Erasure of the residual image remaining after a reading is done by simple exposure to intense white light, a property which can also be an inconvenience because it requires protecting the IP from light during use.
3. Experimental The IPs used in this study were either of the dental type (Denoptix Gendex type 2) or the kind employed for medical X-rays (Fuji ST-VI) having respective thicknesess of 130 and 230 m. They di4er mainly in the thickness of the luminophor layer that determines their sensitivity. The detector marketed by Gendex has the same lateral dimensions as the photographic :lms used in ordinary personal dosimeters. For the neutron response, we have employed a differential method (FranKcois et al., 1982; Ott et al., 1985) which consists of subtracting the response of an IP (“A” in Fig. 2) placed in front of a hydrogen-containing converter from that of an IP (“B” in Fig. 2) behind the converter. In this con:guration, detector A registers the recoil protons from (n; p) scattering in the organic binder of the IP as well as -rays from the neutron source. Detector B registers the same things plus protons recoiling from the converter. The converter thickness was 1 mm but was not optimized. Aluminum foil was placed on the front and back faces of the stack to stop recoil protons coming from the polyethylene container. Stacks for the neutron measurements were placed inside light-tight containers, as were also detectors exposed to 60 Co for their -ray response.
Fig. 2. Schematic con:guration of a neutron detector.
Neutrons for the irradiations were obtained with an Am–Be source of the IPSN=DPHD=S.DOS at Fontenay aux Roses and with a Pu–Be source at the IReS=RAMSES. For the 60 Co, we used sources at both Institutes. A Molecular Imager FX reader from Bio.Rad, equipped with a 532 m wavelength diode laser, was used with a scanning resolution of 50 m (beam diameter, FWHM). The images obtained were coded on a 16-bit level of gray and stored for later analysis.
4. Results and discussion Fig. 3 shows the response of the Gendex 2 detector to fast neutrons from the Am–Be and Pu–Be sources. The response is clearly linear above 80 Sv and possibly below this value. The detection threshold at 2 of the detector background is 100 Sv with an uncertainty of 50% for the neutrons used in this work. The curves of Fig. 4 show the linear response to -rays for the two thicknesses of phosphor layers. The detection threshold calculated as above is 1:7 Sv with an uncertainty of 12% for 60 Co -rays. With the relatively low strength of both the neutron sources, we could not determine the saturation dose, although according to the literature (Thoms, 1997) it is high.
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
515
Fig. 3. Response of the Gendex 2 detector for fast neutrons from Am–Be and Pu–Be sources.
Fig. 4. Response to -rays from
60 Co
5. Conclusion This early study has shown that IPs can be used to detect -rays and neutrons with good sensitivity. There is promise that IPs can be used for personal dosimetry. We intend to study the response of IPs as a function of neutron energy and converter composition and thickness to :nd the conditions
for two thicknesses of imaging plates.
that will give a response independent of neutron energy. Medium- and long-term fading will also be investigated. Acknowledgements The authors wish to express their gratitude to the French Atomic Energy Commission BruyPeres le Chˆatel
516
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
for providing an imaging plate scanner. We are grateful to Professor J.C. Adlo4 IReS=ULP for his encouragement. Thanks to IReS=RAMSES and CEA=IPSN for providing irradiation facilities.
References B?ucherl, T., Rausch, C., Von Seggern, H., 1993. Quantitative measurements on thermal neutron detectors based on storage phosphors. Nucl. Instrum. Methods A 333, 502–506. FranKcois, H., Heilmann, C., Jung, M., 1982. Silver activation in emulsion for fast neutron dosimetry. Nucl. Instrum. Methods 200, 423–428. Hofmann, J., Rausch, C., 1995. Performance of a prototype detector system for thermal neutrons based on laser stimulated luminescence. Nucl. Instrum. Methods A 355, 494–500. Niimura, N., Karasawa, Y., Tanaka, I., Miyahara, J., Takahashi, K., Saito, H., Koizumi, S., Hidaka, M., 1994. An imaging
plate neutron detector. Nucl. Instrum. Methods A 349, 521–525. Ott, C., Jung, M., Heilmann, C., FranKcois, H., 1985. A code for fast neutron dosimertry. Radiat. Prot. 1, 541–546. Rant, J., Stade, J., Balasko, M., Kaling, M., 1998. New possibilities in neutron radiography with imaging plate. Proceedings of the Seventh European Conference on Non-Destructive Testing, Copenhagen, Denmark, May 1998, 2763–2770, ECNDT (1998), 2763–2770. Tazaki, S., Neriishi, K., Takahashi, K., Etoh, M., Karasawa, Y., Kumazawa, S., Niimura, N., 1999. Development of a new type of imaging plate for neutron detection. Nucl. Instrum. Methods A 424, 20–25. Thoms, M., 1997. The dynamic range of X-ray imaging with image plates. Nucl. Instrum. Methods A 389, 437–440. Thoms, M., 1999. Neutron detection with imaging plates, Part II. Detector characteristics. Nucl. Instrum. Methods A 424, 34–39. Thoms, M., Myles, D., Wilkinson, C., 1999. Neutron detection with imaging plates, Part I. Image storage and readout. Nucl. Instrum. Methods A 424, 26–33.
www.elsevier.com/locate/radmeas
Fast neutron and -ray dosimetry with imaging plates A. Boukhaira; b , C. Heilmanna; ∗ , A. Nourreddinea , A. Papea , G. Portalc b Laboratoire
a Institut
de Recherches Subatomiques, 67037 Strasbourg Cedex 2, France de Physique et Techniques Nucl#eaires, Facult#e des Sciences, B.P. 20, El Jadida, Maroc c CEA=IPSN, 92260 Fontenay aux Roses, France
Received 28 August 2000; received in revised form 8 February 2001; accepted 8 March 2001
Abstract An imaging plate (IP) is a passive integrating detector capable of medium-term storage of ionizing energy. A visible laser scan can be used to liberate a number of visible photons proportional to the amount of energy trapped. An IP is sensitive to ionizing particles of any energy, whatsoever, to neutrons and to photons. It is linear in response over 4 –5 decades. We have studied the response of IPs to neutrons emitted by Pu–Be and Am–Be sources in di4erential measurements that correct for the -ray component from the neutron source. The dose threshold for fast neutrons, as well as for 60 Co -rays, could thus be c 2001 Elsevier Science Ltd. All rights reserved. determined. Keywords: Imaging plate; Photostimulated luminescence; Fast neutrons; -rays; Dosimetry
1. Introduction
2. Physical basis of imaging plates
Imaging plates (IPs) based on the photostimulation of phosphors were introduced several years ago, principally as a replacement for photographic :lms. Used extensively for biomedical autoradiography and for medical and dental imaging, IPs are capable of detecting charged particles and photons (; X; UV). More recently, they have been employed in crystallography, UV spectrometry, electron microscopy and non-destructive X-ray and neutron testing. For neutron imaging (B?ucherl et al., 1993; Niimura et al., 1994; Hofmann and Rausch, 1995; Rant et al., 1998; Thoms et al., 1999; Thoms, 1999; Tazaki et al., 1999), gadolinum-doped detectors, or normal detectors associated with a converter, are used. Recent recommendations and new norms for operational dosimetry by di4erent international commissions have led to changes in the methods used for individual dosimetry, implying the development of new types of dosimeters. It is in this context that we have begun a study of the response of IPs to fast neutrons and -rays.
IP detectors consist of a Dexible screen of BaFBr : Eu2+ about 1 mm thick composed of distinct layers: anti-abrasive front and back layers, a layer of dense inorganic phosphor crystals set in a polymer matrix to give a homogeneous distribution, and a polyethylene layer supporting the layer of phosphors. In some cases, the support is blackened with carbon to prevent laser light reDection and scattering during reading. Upon excitation of the phosphors by electromagnetic radiation or charged particles, the energy is stored. A number of Eu2+ ions proportional to the incident Dux on the IP loses an additional electron and the Eu3+ becomes trapped in a quasi-stable state by the Br − . This constitutes a latent image. To read the IP, a scan with a :nely focused laser beam frees the trapped electrons. The transition to the ground state of the phosphor is accompanied by a photon which is reDected by a mirror to optical :bers facing the scanned surface. The photon is converted to an electrical signal and ampli:ed by a photomultiplier (PM), then sampled and digitized by an analog-to-digital converter. The data are stored to be treated by an image processor. The intensity of the laser-induced luminescence varies with the wavelength of the reading laser, with the
∗ Corresponding author. Tel.: +33-3-88-10-63-54; fax: +33-3-88-10-62-34 E-mail address: [email protected] (C. Heilmann).
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 1 8 - 9
514
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
Fig. 1. Stimulation and luminescence spectrum of BaFBr : Eu 2+ . (Molecular Dynamics Technical Note # 53.)
maximum eIciency being obtained near the 633 nm wavelength of the He–Ne laser sometimes used. The light emitted by the phosphors is within the range of maximum eIciency (300 –500 nm) of the PM that captures the signal (Fig. 1). A :lter absorbs laser light from the stimulated light to avoid interference. An IP is reusable ad in:nitum. Its response is linear with dose over 4 –5 decades. Erasure of the residual image remaining after a reading is done by simple exposure to intense white light, a property which can also be an inconvenience because it requires protecting the IP from light during use.
3. Experimental The IPs used in this study were either of the dental type (Denoptix Gendex type 2) or the kind employed for medical X-rays (Fuji ST-VI) having respective thicknesess of 130 and 230 m. They di4er mainly in the thickness of the luminophor layer that determines their sensitivity. The detector marketed by Gendex has the same lateral dimensions as the photographic :lms used in ordinary personal dosimeters. For the neutron response, we have employed a differential method (FranKcois et al., 1982; Ott et al., 1985) which consists of subtracting the response of an IP (“A” in Fig. 2) placed in front of a hydrogen-containing converter from that of an IP (“B” in Fig. 2) behind the converter. In this con:guration, detector A registers the recoil protons from (n; p) scattering in the organic binder of the IP as well as -rays from the neutron source. Detector B registers the same things plus protons recoiling from the converter. The converter thickness was 1 mm but was not optimized. Aluminum foil was placed on the front and back faces of the stack to stop recoil protons coming from the polyethylene container. Stacks for the neutron measurements were placed inside light-tight containers, as were also detectors exposed to 60 Co for their -ray response.
Fig. 2. Schematic con:guration of a neutron detector.
Neutrons for the irradiations were obtained with an Am–Be source of the IPSN=DPHD=S.DOS at Fontenay aux Roses and with a Pu–Be source at the IReS=RAMSES. For the 60 Co, we used sources at both Institutes. A Molecular Imager FX reader from Bio.Rad, equipped with a 532 m wavelength diode laser, was used with a scanning resolution of 50 m (beam diameter, FWHM). The images obtained were coded on a 16-bit level of gray and stored for later analysis.
4. Results and discussion Fig. 3 shows the response of the Gendex 2 detector to fast neutrons from the Am–Be and Pu–Be sources. The response is clearly linear above 80 Sv and possibly below this value. The detection threshold at 2 of the detector background is 100 Sv with an uncertainty of 50% for the neutrons used in this work. The curves of Fig. 4 show the linear response to -rays for the two thicknesses of phosphor layers. The detection threshold calculated as above is 1:7 Sv with an uncertainty of 12% for 60 Co -rays. With the relatively low strength of both the neutron sources, we could not determine the saturation dose, although according to the literature (Thoms, 1997) it is high.
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
515
Fig. 3. Response of the Gendex 2 detector for fast neutrons from Am–Be and Pu–Be sources.
Fig. 4. Response to -rays from
60 Co
5. Conclusion This early study has shown that IPs can be used to detect -rays and neutrons with good sensitivity. There is promise that IPs can be used for personal dosimetry. We intend to study the response of IPs as a function of neutron energy and converter composition and thickness to :nd the conditions
for two thicknesses of imaging plates.
that will give a response independent of neutron energy. Medium- and long-term fading will also be investigated. Acknowledgements The authors wish to express their gratitude to the French Atomic Energy Commission BruyPeres le Chˆatel
516
A. Boukhair et al. / Radiation Measurements 34 (2001) 513–516
for providing an imaging plate scanner. We are grateful to Professor J.C. Adlo4 IReS=ULP for his encouragement. Thanks to IReS=RAMSES and CEA=IPSN for providing irradiation facilities.
References B?ucherl, T., Rausch, C., Von Seggern, H., 1993. Quantitative measurements on thermal neutron detectors based on storage phosphors. Nucl. Instrum. Methods A 333, 502–506. FranKcois, H., Heilmann, C., Jung, M., 1982. Silver activation in emulsion for fast neutron dosimetry. Nucl. Instrum. Methods 200, 423–428. Hofmann, J., Rausch, C., 1995. Performance of a prototype detector system for thermal neutrons based on laser stimulated luminescence. Nucl. Instrum. Methods A 355, 494–500. Niimura, N., Karasawa, Y., Tanaka, I., Miyahara, J., Takahashi, K., Saito, H., Koizumi, S., Hidaka, M., 1994. An imaging
plate neutron detector. Nucl. Instrum. Methods A 349, 521–525. Ott, C., Jung, M., Heilmann, C., FranKcois, H., 1985. A code for fast neutron dosimertry. Radiat. Prot. 1, 541–546. Rant, J., Stade, J., Balasko, M., Kaling, M., 1998. New possibilities in neutron radiography with imaging plate. Proceedings of the Seventh European Conference on Non-Destructive Testing, Copenhagen, Denmark, May 1998, 2763–2770, ECNDT (1998), 2763–2770. Tazaki, S., Neriishi, K., Takahashi, K., Etoh, M., Karasawa, Y., Kumazawa, S., Niimura, N., 1999. Development of a new type of imaging plate for neutron detection. Nucl. Instrum. Methods A 424, 20–25. Thoms, M., 1997. The dynamic range of X-ray imaging with image plates. Nucl. Instrum. Methods A 389, 437–440. Thoms, M., 1999. Neutron detection with imaging plates, Part II. Detector characteristics. Nucl. Instrum. Methods A 424, 34–39. Thoms, M., Myles, D., Wilkinson, C., 1999. Neutron detection with imaging plates, Part I. Image storage and readout. Nucl. Instrum. Methods A 424, 26–33.