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The optimization of the neutron sensitivity of image plates
Nuclear Instruments and Methods in Physics Research A 384 (1997) 457-462
__-----. I-S &METNoDs IN PNVSICS RRsEARcN Section A
ELSEVIER
The optimization of the neutron sensitivity of image plates M. Thornsa**, M.S. Lehmannb,
C. Wilkinson”
“University of Erlangen-Niimberg, Institute of Material Science VI, Martensstr. 7, 91058 Erlangen, Germany blnstitute hue-Lmagevin, Av. des Martyrs, F38042 Grenoble 9, France ‘EMBL, Av. des Martyrs, F38042 Grenoble 9, France Received 9 August 1996; revised form received 3 September 1996
Abstract Image plates containing varying concentrations of the neutron converter Gd,O, and the storage phosphor BaFBr:Eu” have been prepared. Their neutron sensitivity was measured for a neutron wavelength of 3 w and compared with a new model, which is in good agreement with the measured data, showing that the sensitivity, which is the photostimulated light output per irradiated neutron, of image plates is always higher if the neutron irradiation and the readout are done from the same side. In this case an optimum in sensitivity can be obtained for volume fractions of Gd,O, between 20% and 30% of the total volume of storage phosphor and converter in the material. In the case of neutron irradiation and readout from opposite sides the volume fraction of Gd,O, should be varied reciprocally with the thickness d of the phosphor layer in order to obtain a maximum of sensitivity. The experimental results and the model indicate that the maximum sensitivity is lower by a factor between 2 and 4 compared with the readout from the same side. The relation between sensitivity and detective quantum efficiency is discussed.
Keywords:
BaFBr:Eu, Gd, Gd,O,; Image plate; Image readout; Light scattering; Neutron absorption; Neutron sensibilisation; Neutron sensitivity; Photon diffusion; Photostimulation; Photostimulated luminescence; Sensitivity; Storage phosphor
1. Introduction During the last decade detectors which are based on image plates were introduced in various fields of radiographic X-ray imaging such as medical X-ray diagnostics, crystallography and X-ray testing. Compared to other detectors image plates have a lot of advantages as for example high sensitivity, high dynamic range in exposure (more than 6 orders of magnitude), large area, high resolution and reusability. Therefore it is desirable to utilize image plates also for the imaging of neutrons. Since the image plates are composed of the storage phosphor BaFBr:Eu, which is embedded in an organic binder and coated onto a substrate made of polymers, the neutron absorption of such image plates is very low. Therefore various methods have been used to sensitize the image plates to neutron radiation [l-4]. One method uses a thin gadolinium-foil, which is mounted in close contact to a conventional image plate. During the neutron irradiation the gadolinium absorbs the *Corresponding author. Tel. +49 9131 857683; fax +49 9131 858495.
neutrons and converts them into a secondary radiation [5], which is composed of conversion electrons and X-ray quanta, the latter having an energy above 80 keV On average about 0.7 conversion electrons having an energy of the order of 100 keV are produced per absorbed neutron. This secondary radiation in the form of conversion electrons is then partly absorbed in the image plate while most of the X-rays are transmitted because of the low absorption coefficient of the image plate for the high energy X-ray radiation. By the absorption of the conversion electrons, F-centers and hole storage centers are generated in the storage phosphor in a concentration which is proportional to the applied neutron dose. For X-ray irradiation it was found, that on average 125 eV energy is needed to form a photostimulable F-center [6]. Therefore it can be expected that roughly 800 F-centers are generated per absorbed conversion electron. Subsequent to the neutron irradiation the image information can be recovered by removing the Gd-foil and scanning the image plate with a red laser beam. Thereby the electrons which are stored in the F-centers are liberated and recombine with the hole storage centers emitting blue light. The intensity of the luminescence of this recombina-
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M. Thorns et (11. / Nucl. Instr. und Meth. in Phys. Res. A 384 (1997) 457-462
tion process is proportional to the applied dose of the secondary radiation since it is proportional to the concentration of the radiation induced storage centers. Therefore the neutron image can be recovered by measuring this intensity during the scan. Since the above described method has the drawback of a considerable self-absorption of the conversion electrons in the Gd-foil itself the method of Rausch et al. [I] has been further investigated and the results are presented in this paper. This method uses special image plates, which are sensitized by the incorporation of small Gd,O, grains into the phosphor layer. Due to the close contact of the phosphor grains with the Gd,O, grains a larger fraction of the conversion electrons is absorbed in the storage phosphor during the neutron irradiation and therefore a higher quantum efficiency and therewith recombination luminescence intensity results. For the practical application of these image plates several questions are open: What is the optimum concentration of the Gd?O,-powder in the storage phosphor and what is the best thickness of the image plate in order to gain highest sensitivity? What is the difference in sensitivity if the image plate is scanned from the side of the neutron irradiation or from the opposite side? It is the intention of this paper is to answer these questions. In Section 2 measurements of the variation of the emitted photostimulated luminescence (PSL) intensity output per irradiated neutron with varying concentration of Gd,O, grains, are presented. The results of the measurements of this quantity, which is called in the following sensitivity, are compared in Section 3.2 with a new model described in Section 3.1. This model is in good agreement with the measured data. The implications of this model on the variation of the sensitivity with the thickness of the phosphor layer and the Gd,O.X concentration are discussed in Section 3.2 and summarized in Section 4.
2. Experimental Several image plates with varying Gd,O, concentration have been prepared in the following way. The Gd,O, powder of 2 ,um average grain size was dispersed in a mixture of BaFBr:Eu storage phosphor of 7 pm grain size and an organic binder. The ratio of the three components was varied in such a way that the volume ratio of the organic binder and the powder components remained constant. The mixtures were then coated on a transparent polymer substrate having a thickness of 100 pm. After coating the layers were dried. The resulting thickness of the dried phosphor layers were measured to be 120 pm. Using this method image plates with concentrations of O%, lo%, 15%, 20%. 30%. 40%, 50% and 70% of weight GdzO, compared with the total powder weight were prepared. These weight fractions convert to volume frac-
tions of 7.l%, 10.8%. l4.7%, 18.7%. 22.8%. 31.5%, 40.8% and 6 I .7% of the total volume of the powder. This quantity is more appropriate for the analysis of the results presented in Section 3. In order to measure the sensitivity a neutron beam having a wavelength of 3 A and a linewidth Ah/h = 8% was irradiated on the phosphor side (front) in the direction normal to the image plates. In total 103 000 neutrons were incident on an area of about 1 cm* on each plate. Immediately after the irradiation the image plates were scanned from the phosphor side on a drum scanner described in Refs. [7,8]. The intensity of the neutron images was then integrated. Subsequently the experiment was repeated for all the image plates with the neutron beam incident on the rear of the plates, through the substrate in the direction normal to the image plate. The experimental results are shown in Fig. I together with the measured neutron absorption of the image plates. Besides irregularities, which are caused by the manual production of the image plates, it can be seen that with increasing Gd,O, concentration the intensity of the photostimulated luminescence of the front irradiated phosphor layers first increases until a maximum is reached at about 35% Gd,O, concentration. At this concentration the neutron absorption is 98.5%. For larger concentrations the intensity of light output decreases. In the case of neutron irradiation through the substrate the maximum of the PSL-intensity is reached at a GdzO, concentration of 15%. Comparing the maxima of the PSL-intensities for the different directions of the neutron irradiation it is obvious that about twice the sensitivity can be reached if the image plate is neutron irradiated and read out from the same side.
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i% L 0.15 ix .g
0,io -
F! d .g w5 $ a
o,oo
-
0.0
0.1
0,2
0.3
0.4
0,5
0.6
volume fraction Gd203 Fig. 1. Photostimulated luminescence intensities (squares and circles) and neutron absorption (triangles) of image plates of 120 /.~rn thickness prepared with various weight fractions of Gd,O,,. The squares and circles represent data that are obtained by irradiating the phosphor layer side (front) and the backside of the image plates with neutrons, respectively. The image plates were always scanned from the front (phosphor-) side.
M. Thorns et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462 3. The variation of the neutron sensitivity plates with their thickness and converter concentration
459
phosphor layer and the penetration depth of conversion electrons is short compared to the dimensions of the image plate the number of conversion electrons at the location r’ is proportional to the locally absorbed neutron dose
of image
3.1. A new model n,m-dD(r’)ldz. The above presented experimental results can be understood in terms of a model that is based on four basic processes, which are relevant to the generation and the emission of photostimulated luminescence and will be discussed in this section. Within this model the measured PSL-intensity Z,,, g enerated in a small volume of the phosphor layer is assumed to be given by l,,,(i)
= aQ)&,(r3P(i).
(1)
where a is a factor of proportionality, n, the number of conversion electrons, which have been generated by the neutron radiation, f, the fraction of conversion electrons that have been absorbed in the storage phosphor grains, I,,, the laser intensity exciting the storage centers, P the probability that the generated PSL is collected and measured by the photodetector, and i the position in the phosphor layer. According to the model the first important process is the production of secondary radiation - here conversion electrons - in the converter by the absorption of the primary radiation - here neutrons. For the experimentally relevant case of neutron radiation impinging from one direction onto the image plate the absorbed neutron dose is given by
Wr’) -___ dz
d = -D ,--,t+ ‘dz
=D
(y ee”‘,,i 0 eff
(2)
where D,, is the incident neutron dose, z the penetration depth of the neutrons into the phosphor layer, and akrr the effective neutron absorption coefficient of the image plate. The effective neutron absorption coefficient oerr is related to the neutron absorption coefficient cu,+,~ of Gd,O,, which is equal to 120 mm-‘, the average volume filling factor of grains in the coated layer f and the volume fraction of Gd,O, grains x in the total grain volume by the relation
This factor was calculated to be a,,, = 84x mm-’ for the prepared image plates by fitting the exponential decay of dose in Fq. (2) for the various volume fractions n of the Gd,O, grains to the experimentally observed change of the neutron transmission. A comparison of this value with the neutron absorption coefficient add++ of Gd,O, shows that the powder grains fill about 70% of the coated layer volume in these image plates, while the rest, 30%, is filled with organic binder or air. Since the Gd,O, grains are uniformly dispersed in the
(4)
Therefore it can be assumed within the model that the number of generated photostimulable storage centers and therewith the sensitivity of the image plate is proportional to the locally absorbed neutron doses as stated in Eq. (I). The next factor which has to be known in order to calculate the PSL-intensity according to Eq. (I) is the fraction of the generated conversion electrons that tinally are absorbed in the storage phosphor in order to generate photostimulable storage centers. For the model presented here two simplifications are made. Firstly, it is assumed that the conversion electrons are either absorbed in the phosphor grains or in the Gd,O, grains, since the organic binder is made of atoms having a comparably small nuclear charge Z. Secondly it is assumed, that the range of the secondary radiation - here conversion electrons - is much larger than the grain size of the converter, in this case Gd,O,. Thus the fraction of secondary radiation that is absorbed in the storage phosphor is proportional to the volume fraction of the storage phosphor compared to the total volume of grains in the layer, f,m(l -.X1.
(5)
In order to justify the simplifications in some cases a grinding of the converter to the required grain size which is smaller than the penetration depth may be necessary. For the experimentally used image plates the simplifications can be applied, since the range of the conversion electrons is of the order of 10 pm, which is several times larger than the grain size of the Gd,O,-powder. The variation of the excitation intensity with the depth of the penetration into the layer can be described by a diffusion-like propagation process of the exciting photons in the layers as pointed out in Refs. [9-l 1). Within this model it is assumed that the direction of propagation of the exciting photons is randomly changed after traveling a scattering length 1, which is correlated with the grain size of the phosphor layer. In the case of negligible photon absorption in the phosphor layer, as for the white image plates in this experiment, an analysis of the diffusion process results in a variation of the excitation intensity with the penetration depth z as
(6) where d is the thickness of the phosphor layer. A similar variation of the escape probability of the PSL-photons from the front surface of the phosphor layer
460
M. Thornset al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462
(7) is found for their diffusion-like propagation process in the case of negligible absorption [ll]. For details on the diffusion approximation and the above presented results the reader is referred to the original literature [9- II]. In total all the above listed processes give after an integration over the coordinate z a measure of the PSLintensity, which is I,,,@,
d, I) = b( I- xl
d %d203~'i(!?&?)2dZ Xa cd,o,fieI b(l -x)
=
af *x2(21+ d)* _ 2)e-“f*d
+
[(--af 2x2Iz+ 2afxl
af *x2(1 + d)’ - 2cyfx(l + d)
+ 21
(8)
for the irradiation and the readout of a neutron image from the same side of the image plate. In this formula b is a constant of proportionality and cuf = CY~~,,J For the irradiation from one side of the image plate and the scan of the image plate from the other side a PSL-intensity
LL(x, d, I) = b( 1 -
=
x)
b(1 -x)
af ‘x*(21 + d)’ -e +$f
[fff =x212+ 2afxl + 2
‘x*(1 + d)’ + 2cufx(Z+ d) + 2)] (9)
can be deduced.
3.2. Discussion of experimental results and comparison with the model The above deduced functions are plotted for varying Gd,O, concentration in Fig. 2 for a scattering length 1= 40 pm, which is approximately four times the grain size of the storage phosphor, and the other parameters of the experimentally used image plates. It is evident, that the calculated values fit the experimental data very well and reproduce the maxima of the PSL-intensity at 30% and 15% volume fraction Gd,O,. As discussed in Ref. [lo] the difference between the assumed scattering length 1 and the grain size is caused by the relatively small scattering power of the grains, which are embedded in the organic binder. Therefore, several grain boundaries have to be penetrated by the photons until a scattering probability,
Fig. 2. The calculated variation of the PSL-intensity according to the model presented in Section 3 for neutron sensitized image plates having a thickness d= 120 pm, a neutron absorption coefficient a;,, = 84 mm’ X volume fraction Gd,O, and a scattering length l= 40 pm for photons in the phosphor layer. Squares represent data points for neutron irradiation and readout from the same side of the image plate and circles represent the data points for the readout of the image plate from the opposite direction. Additionally shown is the calculated neutron absorption of the image plates (triangles).
which is independent of the scattering angle is reached, as is assumed by the diffusion approximation. Using smaller values for the scattering length 1 results in a larger difference of the PSL-sensitivities for both types of irradiation and readout. Additional calculations varying both parameters, the thickness and the fraction of Gd,O, of the image plates, are presented in Fig. 3 for the same factor of proportionality b = 1. The plots clearly show that the light output is higher if the side of neutron irradiation and readout is the same. In this mode of operation the volume fraction of Gd,O, compared to the total powder volume should be between 20% and 30% to achieve highest sensitivity. With the increase of the layer thickness, the maximum sensitivity increases because a lower fraction of Gd,O, is needed to achieve the same neutron absorption of the layer, which reduces simultaneously the self-absorption of the conversion electrons in the Gd,O,. Since the increase of the layer thickness results in a simultaneous loss of resolution as described in detail in Ref. [9], the possible increase in sensitivity is limited by the requirements of resolution in the particular practical application. In order to achieve optimum light output by a neutron sensitized image plate detector, it is therefore necessary to optimize the thickness of the phosphor layer together with the concentration of the converter - here it is Gd - for the particular application. In the case of the readout from the opposite side of neutron irradiation the preparation of image plates having a maximum of sensitivity becomes more critical with increasing thickness of the phosphor layer. The reason for
hf. Thorns et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462
(a)
I.0
sensitivity [a.u.] 0 6 ._ 0.7 0.5 -- 0.6 -0.4
.. 0.5
-0.3
.. 04
-0.2 lo.1 -0
-. 0.3 -- 0.2 -. 0.1
~ 0;2
0;4
0;6
0.8
1.0
volume fraction Gd203 (b)
1.0
‘-i
sensitivity [a.u.]
0.8
C 2 0.6 E 0.4 E
volume fraction Fig. 3.
plots of sensitivity of for varying of Gd,O, phosphor layer irradiation and opposite sides A scattering
this behavior
sensitized image thickness of from the side (a) I= pm was
the following. order to a high there must a considerable of the of generated as described the exponential in Eq. and the of the of the of the intensity I_ (6) and emission probability Eq. (7). the latter very small z = d it is necessary to adjust the neutron absorption coefficient in such a way that a large fraction of the neutrons is absorbed at lower values of z. Therefore the product acd,O,fnd should be close to 1, or x=l/cy ,,+Jd~I/d. This results in a hyperbolic function for the position of the maximum sensitivity in the contour plot (Fig. 3b). For practical application of neutron sensitized image plates the detected quantum efficiency (DQE) is a measure of image quality. It is defined as the ratio of the squared signal to noise ratios of the measured and the irradiated image. In the case of image plate detectors this quantity depends on both image plate related parameters, such as the sensitivity, and on parameters of the readout system, such as the PSL-light collection efficiency of the photodetector and the intensity of the readout light. However, the investigation of these scanner related parameters and therefore the investigation of the DQE is beyond the scope of this paper. Nevertheless it is useful to discuss qualitatively the relation of the here investigated sensitivity and the DQE.
461
For the irradiated image the signal to noise ratio is given by A, where n is the number of neutrons. If in the transmission line of image information during the scan the number of “image’‘-quanta is reduced to a lower number ni, the signal to noise ratio of the image will be reduced to -\/;;;, resulting in DQE = n,ln. In the other case of no reduction of the number of image quanta the DQE will be unity. In the process of image recovering the main bottleneck with respect to the number of image quanta is the number of photoelectrons np that are created per irradiated neutron at the photocathode of the photomultiplier, which detects the PSL. This number is proportional to parameters such as the sensitivity of the image plate, the light collection efficiency of the PSL in the readout optics of the scanner and the intensity of the readout light. In the case of np < 1 an increase of the sensitivity of the image plate results in a simultaneous increase of the DQE. In the other case of np > 1 an increase of the sensitivity of the image plate results in no further increase of the DQE. Therefore it can not be stated in general whether the optimization of the neutron sensitivity of the image plates results in a larger DQE. For the image plate reader and image plates utilized here an analysis of the image noise showed that for image plates with high sensitivity the increase of the DQE was lower than the increase of the image plate sensitivity. Therefore it is assumed, that in this case the number of photons that is generated per absorbed neutron is close to one. It should be noted, that the model presented here for the sensitization of image plates can be applied to other types of radiation if the following requirements are fulfilled: 1) The secondary radiation that is emitted by the converter has to be absorbed in the phosphor layer within a distance that is much smaller than the thickness of the image plate. 2) The grain size of the converter has to be much smaller than the penetration depth of the secondary radiation. In the case of image plates that are sensitized to neutron radiation both requirements are fulfilled. The secondary radiation, which generates PSL-centers in the storage phosphor, is composed of conversion electrons and eventually some X-ray fluorescence that is emitted from the Gd-atoms. This radiation is assumed to have a penetration depth of about 10 pm in the phosphor layer, which is an order of magnitude shorter than the thickness of the image plate and an order of magnitude larger than the grain size of the utilized Gd,O,-powder.
4. summary The optimization of the sensitivity of image plates with respect to neutron radiation is investigated. A series of image plates with varying concentration of the converter, in this case Gd,O,, has been prepared. Their sensitivity
462
M. Thorns et al. I Nucl. Instr. and Meth.
with respect to neutron radiation has been measured and compared with a model that is based on four basic processes: 1) Absorption of the neutrons by the converter and generation of secondary radiation. 2) Absorption of the secondary radiation nearby the point of generation. 3) Light diffusion of the readout light in the phosphor layer during the readout, resulting in a linear decrease of the readout light intensity with increasing penetration depth. 4) Light diffusion of the emitted photostimulated luminescence in the phosphor layer during the readout, resulting in a linear decrease of the probability to be emitted from the surface of the image plate for increasing distance to the surface. Good agreement between the measured and the calculated data is found. These data show that the sensitivity of image plates is always higher if the neutron irradiation and the readout are done from the same side. In this case an optimum of the sensitivity can be obtained for volume fractions of Gd,O, between 20% and 30% compared to the sum of the volumes of storage phosphor and converter in the material. In the case of irradiation and readout from opposite sides the volume fraction of GdzO, x should be varied with the thickness d of the phosphor layer as XK I ld in order to obtain a maximum of sensitivity. However, the maximum sensitivity is lower by a factor between 2 and 4 compared with the readout from the same side. The relation between the DQE, which depends on both image plate and scanner associated parameters, and the sensitivity of the image plates is qualitatively discussed. For the utilized image plate reader and image plates with high sensitivity the increase of the DQE is lower than the increase of the image plate sensitivity. This indicates, that the number of photoelectrons, which is generated per irradiated neutron at the photocathode of the photodetector during the readout, is close to unity.
in Phys. Res. A 384 (1997)
457-462
Acknowledgments The authors would like to acknowledge the help of Dip].-Ing. N. Gloser, Dipl.-Ing. R. Fasbender and H. Rieck at the university of Erlangen, in the preparation of the image plates. They are also grateful to F. Cipriani and J.C. Castagna of EMBL for technical assistance.
References [I] C. Rausch, T. Biicherl, R. Gabler, H. von Seggem and A. Winnacker, SPIE Proc. 1737 (1992) 255. [2] T. Biicherl, C. Rausch and H. von Seggem, Nucl. Instr. and Meth. A 333 (1993) 502. [3] C. Wilkinson, A. Gabriel, MS. Lehmann, T. Zemb and F. Nt, SPIE Proc. 1737 (1992) 324. ]4] N. Niimura, Y. Karasawa, I. Tanaka, J. Miyahara, K. Takahashi, H. Saito, S. Koizumi and M. Hid&a, Nucl. Instr. and Meth. A 349 (1994) 521. [5] C. Rausch, J. Hofmann, M. Thorns and H. von Seggem, Neutron image plates - Part II: neutron sensibilization and imaging properties, to be published. [6] M. Thorns and H. von Seggem, J. Appl. Phys. 75 (9) (1991) 4658. [7] F. Cipriani, F. Dauvergne, A. Gabriel, C. Wilkinson and MS. Lehmann, Biophys. Chem. 53 (1994) 5. [8] F. Cipriani, J.-C. Castagna, M.S. Lehmann and C. Wilkinson, Physica B 213 & 214 (1995) 975. [9] M. Thorns, Appl. Opt. 35 (19??) 3702. [lo] M. Thorns and H. von Seggem, Radiographic imaging with image plates: The influence of the readout intensity on the image quality, to be published in J. Appl. Phys. [ 1 l] M. Thorns, Nucl. Instr. and Meth. A 378 (1996) 598.
__-----. I-S &METNoDs IN PNVSICS RRsEARcN Section A
ELSEVIER
The optimization of the neutron sensitivity of image plates M. Thornsa**, M.S. Lehmannb,
C. Wilkinson”
“University of Erlangen-Niimberg, Institute of Material Science VI, Martensstr. 7, 91058 Erlangen, Germany blnstitute hue-Lmagevin, Av. des Martyrs, F38042 Grenoble 9, France ‘EMBL, Av. des Martyrs, F38042 Grenoble 9, France Received 9 August 1996; revised form received 3 September 1996
Abstract Image plates containing varying concentrations of the neutron converter Gd,O, and the storage phosphor BaFBr:Eu” have been prepared. Their neutron sensitivity was measured for a neutron wavelength of 3 w and compared with a new model, which is in good agreement with the measured data, showing that the sensitivity, which is the photostimulated light output per irradiated neutron, of image plates is always higher if the neutron irradiation and the readout are done from the same side. In this case an optimum in sensitivity can be obtained for volume fractions of Gd,O, between 20% and 30% of the total volume of storage phosphor and converter in the material. In the case of neutron irradiation and readout from opposite sides the volume fraction of Gd,O, should be varied reciprocally with the thickness d of the phosphor layer in order to obtain a maximum of sensitivity. The experimental results and the model indicate that the maximum sensitivity is lower by a factor between 2 and 4 compared with the readout from the same side. The relation between sensitivity and detective quantum efficiency is discussed.
Keywords:
BaFBr:Eu, Gd, Gd,O,; Image plate; Image readout; Light scattering; Neutron absorption; Neutron sensibilisation; Neutron sensitivity; Photon diffusion; Photostimulation; Photostimulated luminescence; Sensitivity; Storage phosphor
1. Introduction During the last decade detectors which are based on image plates were introduced in various fields of radiographic X-ray imaging such as medical X-ray diagnostics, crystallography and X-ray testing. Compared to other detectors image plates have a lot of advantages as for example high sensitivity, high dynamic range in exposure (more than 6 orders of magnitude), large area, high resolution and reusability. Therefore it is desirable to utilize image plates also for the imaging of neutrons. Since the image plates are composed of the storage phosphor BaFBr:Eu, which is embedded in an organic binder and coated onto a substrate made of polymers, the neutron absorption of such image plates is very low. Therefore various methods have been used to sensitize the image plates to neutron radiation [l-4]. One method uses a thin gadolinium-foil, which is mounted in close contact to a conventional image plate. During the neutron irradiation the gadolinium absorbs the *Corresponding author. Tel. +49 9131 857683; fax +49 9131 858495.
neutrons and converts them into a secondary radiation [5], which is composed of conversion electrons and X-ray quanta, the latter having an energy above 80 keV On average about 0.7 conversion electrons having an energy of the order of 100 keV are produced per absorbed neutron. This secondary radiation in the form of conversion electrons is then partly absorbed in the image plate while most of the X-rays are transmitted because of the low absorption coefficient of the image plate for the high energy X-ray radiation. By the absorption of the conversion electrons, F-centers and hole storage centers are generated in the storage phosphor in a concentration which is proportional to the applied neutron dose. For X-ray irradiation it was found, that on average 125 eV energy is needed to form a photostimulable F-center [6]. Therefore it can be expected that roughly 800 F-centers are generated per absorbed conversion electron. Subsequent to the neutron irradiation the image information can be recovered by removing the Gd-foil and scanning the image plate with a red laser beam. Thereby the electrons which are stored in the F-centers are liberated and recombine with the hole storage centers emitting blue light. The intensity of the luminescence of this recombina-
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M. Thorns et (11. / Nucl. Instr. und Meth. in Phys. Res. A 384 (1997) 457-462
tion process is proportional to the applied dose of the secondary radiation since it is proportional to the concentration of the radiation induced storage centers. Therefore the neutron image can be recovered by measuring this intensity during the scan. Since the above described method has the drawback of a considerable self-absorption of the conversion electrons in the Gd-foil itself the method of Rausch et al. [I] has been further investigated and the results are presented in this paper. This method uses special image plates, which are sensitized by the incorporation of small Gd,O, grains into the phosphor layer. Due to the close contact of the phosphor grains with the Gd,O, grains a larger fraction of the conversion electrons is absorbed in the storage phosphor during the neutron irradiation and therefore a higher quantum efficiency and therewith recombination luminescence intensity results. For the practical application of these image plates several questions are open: What is the optimum concentration of the Gd?O,-powder in the storage phosphor and what is the best thickness of the image plate in order to gain highest sensitivity? What is the difference in sensitivity if the image plate is scanned from the side of the neutron irradiation or from the opposite side? It is the intention of this paper is to answer these questions. In Section 2 measurements of the variation of the emitted photostimulated luminescence (PSL) intensity output per irradiated neutron with varying concentration of Gd,O, grains, are presented. The results of the measurements of this quantity, which is called in the following sensitivity, are compared in Section 3.2 with a new model described in Section 3.1. This model is in good agreement with the measured data. The implications of this model on the variation of the sensitivity with the thickness of the phosphor layer and the Gd,O.X concentration are discussed in Section 3.2 and summarized in Section 4.
2. Experimental Several image plates with varying Gd,O, concentration have been prepared in the following way. The Gd,O, powder of 2 ,um average grain size was dispersed in a mixture of BaFBr:Eu storage phosphor of 7 pm grain size and an organic binder. The ratio of the three components was varied in such a way that the volume ratio of the organic binder and the powder components remained constant. The mixtures were then coated on a transparent polymer substrate having a thickness of 100 pm. After coating the layers were dried. The resulting thickness of the dried phosphor layers were measured to be 120 pm. Using this method image plates with concentrations of O%, lo%, 15%, 20%. 30%. 40%, 50% and 70% of weight GdzO, compared with the total powder weight were prepared. These weight fractions convert to volume frac-
tions of 7.l%, 10.8%. l4.7%, 18.7%. 22.8%. 31.5%, 40.8% and 6 I .7% of the total volume of the powder. This quantity is more appropriate for the analysis of the results presented in Section 3. In order to measure the sensitivity a neutron beam having a wavelength of 3 A and a linewidth Ah/h = 8% was irradiated on the phosphor side (front) in the direction normal to the image plates. In total 103 000 neutrons were incident on an area of about 1 cm* on each plate. Immediately after the irradiation the image plates were scanned from the phosphor side on a drum scanner described in Refs. [7,8]. The intensity of the neutron images was then integrated. Subsequently the experiment was repeated for all the image plates with the neutron beam incident on the rear of the plates, through the substrate in the direction normal to the image plate. The experimental results are shown in Fig. I together with the measured neutron absorption of the image plates. Besides irregularities, which are caused by the manual production of the image plates, it can be seen that with increasing Gd,O, concentration the intensity of the photostimulated luminescence of the front irradiated phosphor layers first increases until a maximum is reached at about 35% Gd,O, concentration. At this concentration the neutron absorption is 98.5%. For larger concentrations the intensity of light output decreases. In the case of neutron irradiation through the substrate the maximum of the PSL-intensity is reached at a GdzO, concentration of 15%. Comparing the maxima of the PSL-intensities for the different directions of the neutron irradiation it is obvious that about twice the sensitivity can be reached if the image plate is neutron irradiated and read out from the same side.
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s 3L
-
0,20 -
i% L 0.15 ix .g
0,io -
F! d .g w5 $ a
o,oo
-
0.0
0.1
0,2
0.3
0.4
0,5
0.6
volume fraction Gd203 Fig. 1. Photostimulated luminescence intensities (squares and circles) and neutron absorption (triangles) of image plates of 120 /.~rn thickness prepared with various weight fractions of Gd,O,,. The squares and circles represent data that are obtained by irradiating the phosphor layer side (front) and the backside of the image plates with neutrons, respectively. The image plates were always scanned from the front (phosphor-) side.
M. Thorns et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462 3. The variation of the neutron sensitivity plates with their thickness and converter concentration
459
phosphor layer and the penetration depth of conversion electrons is short compared to the dimensions of the image plate the number of conversion electrons at the location r’ is proportional to the locally absorbed neutron dose
of image
3.1. A new model n,m-dD(r’)ldz. The above presented experimental results can be understood in terms of a model that is based on four basic processes, which are relevant to the generation and the emission of photostimulated luminescence and will be discussed in this section. Within this model the measured PSL-intensity Z,,, g enerated in a small volume of the phosphor layer is assumed to be given by l,,,(i)
= aQ)&,(r3P(i).
(1)
where a is a factor of proportionality, n, the number of conversion electrons, which have been generated by the neutron radiation, f, the fraction of conversion electrons that have been absorbed in the storage phosphor grains, I,,, the laser intensity exciting the storage centers, P the probability that the generated PSL is collected and measured by the photodetector, and i the position in the phosphor layer. According to the model the first important process is the production of secondary radiation - here conversion electrons - in the converter by the absorption of the primary radiation - here neutrons. For the experimentally relevant case of neutron radiation impinging from one direction onto the image plate the absorbed neutron dose is given by
Wr’) -___ dz
d = -D ,--,t+ ‘dz
=D
(y ee”‘,,i 0 eff
(2)
where D,, is the incident neutron dose, z the penetration depth of the neutrons into the phosphor layer, and akrr the effective neutron absorption coefficient of the image plate. The effective neutron absorption coefficient oerr is related to the neutron absorption coefficient cu,+,~ of Gd,O,, which is equal to 120 mm-‘, the average volume filling factor of grains in the coated layer f and the volume fraction of Gd,O, grains x in the total grain volume by the relation
This factor was calculated to be a,,, = 84x mm-’ for the prepared image plates by fitting the exponential decay of dose in Fq. (2) for the various volume fractions n of the Gd,O, grains to the experimentally observed change of the neutron transmission. A comparison of this value with the neutron absorption coefficient add++ of Gd,O, shows that the powder grains fill about 70% of the coated layer volume in these image plates, while the rest, 30%, is filled with organic binder or air. Since the Gd,O, grains are uniformly dispersed in the
(4)
Therefore it can be assumed within the model that the number of generated photostimulable storage centers and therewith the sensitivity of the image plate is proportional to the locally absorbed neutron doses as stated in Eq. (I). The next factor which has to be known in order to calculate the PSL-intensity according to Eq. (I) is the fraction of the generated conversion electrons that tinally are absorbed in the storage phosphor in order to generate photostimulable storage centers. For the model presented here two simplifications are made. Firstly, it is assumed that the conversion electrons are either absorbed in the phosphor grains or in the Gd,O, grains, since the organic binder is made of atoms having a comparably small nuclear charge Z. Secondly it is assumed, that the range of the secondary radiation - here conversion electrons - is much larger than the grain size of the converter, in this case Gd,O,. Thus the fraction of secondary radiation that is absorbed in the storage phosphor is proportional to the volume fraction of the storage phosphor compared to the total volume of grains in the layer, f,m(l -.X1.
(5)
In order to justify the simplifications in some cases a grinding of the converter to the required grain size which is smaller than the penetration depth may be necessary. For the experimentally used image plates the simplifications can be applied, since the range of the conversion electrons is of the order of 10 pm, which is several times larger than the grain size of the Gd,O,-powder. The variation of the excitation intensity with the depth of the penetration into the layer can be described by a diffusion-like propagation process of the exciting photons in the layers as pointed out in Refs. [9-l 1). Within this model it is assumed that the direction of propagation of the exciting photons is randomly changed after traveling a scattering length 1, which is correlated with the grain size of the phosphor layer. In the case of negligible photon absorption in the phosphor layer, as for the white image plates in this experiment, an analysis of the diffusion process results in a variation of the excitation intensity with the penetration depth z as
(6) where d is the thickness of the phosphor layer. A similar variation of the escape probability of the PSL-photons from the front surface of the phosphor layer
460
M. Thornset al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462
(7) is found for their diffusion-like propagation process in the case of negligible absorption [ll]. For details on the diffusion approximation and the above presented results the reader is referred to the original literature [9- II]. In total all the above listed processes give after an integration over the coordinate z a measure of the PSLintensity, which is I,,,@,
d, I) = b( I- xl
d %d203~'i(!?&?)2dZ Xa cd,o,fieI b(l -x)
=
af *x2(21+ d)* _ 2)e-“f*d
+
[(--af 2x2Iz+ 2afxl
af *x2(1 + d)’ - 2cyfx(l + d)
+ 21
(8)
for the irradiation and the readout of a neutron image from the same side of the image plate. In this formula b is a constant of proportionality and cuf = CY~~,,J For the irradiation from one side of the image plate and the scan of the image plate from the other side a PSL-intensity
LL(x, d, I) = b( 1 -
=
x)
b(1 -x)
af ‘x*(21 + d)’ -e +$f
[fff =x212+ 2afxl + 2
‘x*(1 + d)’ + 2cufx(Z+ d) + 2)] (9)
can be deduced.
3.2. Discussion of experimental results and comparison with the model The above deduced functions are plotted for varying Gd,O, concentration in Fig. 2 for a scattering length 1= 40 pm, which is approximately four times the grain size of the storage phosphor, and the other parameters of the experimentally used image plates. It is evident, that the calculated values fit the experimental data very well and reproduce the maxima of the PSL-intensity at 30% and 15% volume fraction Gd,O,. As discussed in Ref. [lo] the difference between the assumed scattering length 1 and the grain size is caused by the relatively small scattering power of the grains, which are embedded in the organic binder. Therefore, several grain boundaries have to be penetrated by the photons until a scattering probability,
Fig. 2. The calculated variation of the PSL-intensity according to the model presented in Section 3 for neutron sensitized image plates having a thickness d= 120 pm, a neutron absorption coefficient a;,, = 84 mm’ X volume fraction Gd,O, and a scattering length l= 40 pm for photons in the phosphor layer. Squares represent data points for neutron irradiation and readout from the same side of the image plate and circles represent the data points for the readout of the image plate from the opposite direction. Additionally shown is the calculated neutron absorption of the image plates (triangles).
which is independent of the scattering angle is reached, as is assumed by the diffusion approximation. Using smaller values for the scattering length 1 results in a larger difference of the PSL-sensitivities for both types of irradiation and readout. Additional calculations varying both parameters, the thickness and the fraction of Gd,O, of the image plates, are presented in Fig. 3 for the same factor of proportionality b = 1. The plots clearly show that the light output is higher if the side of neutron irradiation and readout is the same. In this mode of operation the volume fraction of Gd,O, compared to the total powder volume should be between 20% and 30% to achieve highest sensitivity. With the increase of the layer thickness, the maximum sensitivity increases because a lower fraction of Gd,O, is needed to achieve the same neutron absorption of the layer, which reduces simultaneously the self-absorption of the conversion electrons in the Gd,O,. Since the increase of the layer thickness results in a simultaneous loss of resolution as described in detail in Ref. [9], the possible increase in sensitivity is limited by the requirements of resolution in the particular practical application. In order to achieve optimum light output by a neutron sensitized image plate detector, it is therefore necessary to optimize the thickness of the phosphor layer together with the concentration of the converter - here it is Gd - for the particular application. In the case of the readout from the opposite side of neutron irradiation the preparation of image plates having a maximum of sensitivity becomes more critical with increasing thickness of the phosphor layer. The reason for
hf. Thorns et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 457-462
(a)
I.0
sensitivity [a.u.] 0 6 ._ 0.7 0.5 -- 0.6 -0.4
.. 0.5
-0.3
.. 04
-0.2 lo.1 -0
-. 0.3 -- 0.2 -. 0.1
~ 0;2
0;4
0;6
0.8
1.0
volume fraction Gd203 (b)
1.0
‘-i
sensitivity [a.u.]
0.8
C 2 0.6 E 0.4 E
volume fraction Fig. 3.
plots of sensitivity of for varying of Gd,O, phosphor layer irradiation and opposite sides A scattering
this behavior
sensitized image thickness of from the side (a) I= pm was
the following. order to a high there must a considerable of the of generated as described the exponential in Eq. and the of the of the of the intensity I_ (6) and emission probability Eq. (7). the latter very small z = d it is necessary to adjust the neutron absorption coefficient in such a way that a large fraction of the neutrons is absorbed at lower values of z. Therefore the product acd,O,fnd should be close to 1, or x=l/cy ,,+Jd~I/d. This results in a hyperbolic function for the position of the maximum sensitivity in the contour plot (Fig. 3b). For practical application of neutron sensitized image plates the detected quantum efficiency (DQE) is a measure of image quality. It is defined as the ratio of the squared signal to noise ratios of the measured and the irradiated image. In the case of image plate detectors this quantity depends on both image plate related parameters, such as the sensitivity, and on parameters of the readout system, such as the PSL-light collection efficiency of the photodetector and the intensity of the readout light. However, the investigation of these scanner related parameters and therefore the investigation of the DQE is beyond the scope of this paper. Nevertheless it is useful to discuss qualitatively the relation of the here investigated sensitivity and the DQE.
461
For the irradiated image the signal to noise ratio is given by A, where n is the number of neutrons. If in the transmission line of image information during the scan the number of “image’‘-quanta is reduced to a lower number ni, the signal to noise ratio of the image will be reduced to -\/;;;, resulting in DQE = n,ln. In the other case of no reduction of the number of image quanta the DQE will be unity. In the process of image recovering the main bottleneck with respect to the number of image quanta is the number of photoelectrons np that are created per irradiated neutron at the photocathode of the photomultiplier, which detects the PSL. This number is proportional to parameters such as the sensitivity of the image plate, the light collection efficiency of the PSL in the readout optics of the scanner and the intensity of the readout light. In the case of np < 1 an increase of the sensitivity of the image plate results in a simultaneous increase of the DQE. In the other case of np > 1 an increase of the sensitivity of the image plate results in no further increase of the DQE. Therefore it can not be stated in general whether the optimization of the neutron sensitivity of the image plates results in a larger DQE. For the image plate reader and image plates utilized here an analysis of the image noise showed that for image plates with high sensitivity the increase of the DQE was lower than the increase of the image plate sensitivity. Therefore it is assumed, that in this case the number of photons that is generated per absorbed neutron is close to one. It should be noted, that the model presented here for the sensitization of image plates can be applied to other types of radiation if the following requirements are fulfilled: 1) The secondary radiation that is emitted by the converter has to be absorbed in the phosphor layer within a distance that is much smaller than the thickness of the image plate. 2) The grain size of the converter has to be much smaller than the penetration depth of the secondary radiation. In the case of image plates that are sensitized to neutron radiation both requirements are fulfilled. The secondary radiation, which generates PSL-centers in the storage phosphor, is composed of conversion electrons and eventually some X-ray fluorescence that is emitted from the Gd-atoms. This radiation is assumed to have a penetration depth of about 10 pm in the phosphor layer, which is an order of magnitude shorter than the thickness of the image plate and an order of magnitude larger than the grain size of the utilized Gd,O,-powder.
4. summary The optimization of the sensitivity of image plates with respect to neutron radiation is investigated. A series of image plates with varying concentration of the converter, in this case Gd,O,, has been prepared. Their sensitivity
462
M. Thorns et al. I Nucl. Instr. and Meth.
with respect to neutron radiation has been measured and compared with a model that is based on four basic processes: 1) Absorption of the neutrons by the converter and generation of secondary radiation. 2) Absorption of the secondary radiation nearby the point of generation. 3) Light diffusion of the readout light in the phosphor layer during the readout, resulting in a linear decrease of the readout light intensity with increasing penetration depth. 4) Light diffusion of the emitted photostimulated luminescence in the phosphor layer during the readout, resulting in a linear decrease of the probability to be emitted from the surface of the image plate for increasing distance to the surface. Good agreement between the measured and the calculated data is found. These data show that the sensitivity of image plates is always higher if the neutron irradiation and the readout are done from the same side. In this case an optimum of the sensitivity can be obtained for volume fractions of Gd,O, between 20% and 30% compared to the sum of the volumes of storage phosphor and converter in the material. In the case of irradiation and readout from opposite sides the volume fraction of GdzO, x should be varied with the thickness d of the phosphor layer as XK I ld in order to obtain a maximum of sensitivity. However, the maximum sensitivity is lower by a factor between 2 and 4 compared with the readout from the same side. The relation between the DQE, which depends on both image plate and scanner associated parameters, and the sensitivity of the image plates is qualitatively discussed. For the utilized image plate reader and image plates with high sensitivity the increase of the DQE is lower than the increase of the image plate sensitivity. This indicates, that the number of photoelectrons, which is generated per irradiated neutron at the photocathode of the photodetector during the readout, is close to unity.
in Phys. Res. A 384 (1997)
457-462
Acknowledgments The authors would like to acknowledge the help of Dip].-Ing. N. Gloser, Dipl.-Ing. R. Fasbender and H. Rieck at the university of Erlangen, in the preparation of the image plates. They are also grateful to F. Cipriani and J.C. Castagna of EMBL for technical assistance.
References [I] C. Rausch, T. Biicherl, R. Gabler, H. von Seggem and A. Winnacker, SPIE Proc. 1737 (1992) 255. [2] T. Biicherl, C. Rausch and H. von Seggem, Nucl. Instr. and Meth. A 333 (1993) 502. [3] C. Wilkinson, A. Gabriel, MS. Lehmann, T. Zemb and F. Nt, SPIE Proc. 1737 (1992) 324. ]4] N. Niimura, Y. Karasawa, I. Tanaka, J. Miyahara, K. Takahashi, H. Saito, S. Koizumi and M. Hid&a, Nucl. Instr. and Meth. A 349 (1994) 521. [5] C. Rausch, J. Hofmann, M. Thorns and H. von Seggem, Neutron image plates - Part II: neutron sensibilization and imaging properties, to be published. [6] M. Thorns and H. von Seggem, J. Appl. Phys. 75 (9) (1991) 4658. [7] F. Cipriani, F. Dauvergne, A. Gabriel, C. Wilkinson and MS. Lehmann, Biophys. Chem. 53 (1994) 5. [8] F. Cipriani, J.-C. Castagna, M.S. Lehmann and C. Wilkinson, Physica B 213 & 214 (1995) 975. [9] M. Thorns, Appl. Opt. 35 (19??) 3702. [lo] M. Thorns and H. von Seggem, Radiographic imaging with image plates: The influence of the readout intensity on the image quality, to be published in J. Appl. Phys. [ 1 l] M. Thorns, Nucl. Instr. and Meth. A 378 (1996) 598.