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Response from inorganic scintillation screens induced by high energetic ions
Nuclear Instruments and Methods in Physics Research B 365 (2015) 533–539
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Response from inorganic scintillation screens induced by high energetic ions q A. Lieberwirth a,b,⇑, W. Ensinger b, P. Forck a,c, S. Lederer a,b a
GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany TU Darmstadt (Materials Analysis), Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany c Goethe University (Physics), Max-von-Laue-Str. 1, 60438 Frankfurt, Germany b
a r t i c l e
i n f o
Article history: Received 19 May 2015 Received in revised form 21 July 2015 Accepted 24 July 2015 Available online 5 August 2015 Keywords: Scintillation screens Inorganic material High energetic beam Light output Emission spectra
a b s t r a c t Inorganic scintillation screens were irradiated with swift heavy ion beams at GSI accelator facility, using protons up to Uranium ions as projectiles. Beams were extracted from the synchrotron SIS18 with high energy (300 MeV/u) in slow and fast extraction mode. During irradiation the scintillation response of the screens was recorded by two different optical setups simultaneously to investigate the light output, profile characteristics and emission spectra. In principle fast extracted beams induce lower light output than slow extracted beams. The output per deposited energy shows a decreasing dependency with respect of the atomic number. Emission spectra showed no significant defects of the materials, neither at irradiation with increasing beam intensity nor during long time irradiation. Ó 2015 Elsevier B.V. All rights reserved.
1. Motivation In the last decades scintillation screens received an increasing interest as simple tool for beam alignment at accelerator facilities. In particular screens made of inorganic materials were proven to verify all kinds of radiation in direct or indirect manner [1]. For common ion beam alignment the screen is driven in intercepting way into the beam. Incident particles transfer energy into the material and induce an avalanche production of electron– hole-pairs. If the charges are not bound due to vacancies or color centers photons are emitted during recombination processes [2,3]. Depending on the chosen material, the photons are emitted within the visible spectrum of light and can thus be recorded by CCD cameras or given directly to the operating team as video stream. Former investigations with low energy ion beams had shown a non-linear behavior for the light output L, e.g. number of emitted photons, and two dimensional characteristics of the recorded beam profile with respect to the accumulated ion dose [4]. Different characteristics were already observed during measurements with high energy ion beams [5]. For high energy ion beams (few
q
Work is supported by BMBF, contract number 05P12RDRBJ.
⇑ Corresponding author at: GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany. E-mail address: [email protected] (A. Lieberwirth). http://dx.doi.org/10.1016/j.nimb.2015.07.111 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
hundred MeV/u) the question of a possible saturation effect for short pulse irradiation at large number of irradiated particles, as it was predicted for single particle ionization in [6,7], is rarely investigated. Thus inorganic materials were irradiated by ion beams requested in fast and slow extraction mode [8] from SIS18 in GSI. Light output and beam profiles were recorded for different beam intensities to investigate linearity and reproducibility for equal beam characteristics. During analysis a relative light yield was calculated in dependency of the atomic number of investigated projectiles. During all measurements the emission spectra of the targets were recorded simultaneously. In an additional survey the radiation hardness of the materials was studied.
2. Experimental setup Altogether seven different scintillation screens of inorganic material with diameters between 50 and 80 mm served for investigations. They were mounted in air with 45° bias angle to the beam axis on a movable target ladder that was controlled remotely. None of the screens were exchanged during the investigations. The targets consist of phosphor screens, single crystals and polycrystalline ceramics and their key parameters are given in Table 1. For targets #6 and #7 the decay times were not given by the supplier. For A999 the decay time under optical stimulation
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Table 1 Key parameters of investigated materials. Targets #3 and #4 are made from the same production line. For #6 and #7 decay times were not given by supplier, they can be found in reports [9–11]. Target material #1 #2 #3 #4 #5 #6 #7
P43 (Gd2O2S:Tb) P46 (Y3Al5O12:Ce) P46 (Y3Al5O12:Ce) YAG:Ce (Y3Al5O12:Ce) YAG:Ce (Y3Al5O12:Ce) A999 (Al2O3, 99.99 % purity) Al2O3:Cr (0.04 weight-% doping)
Phase
Supplier
Thickness
q½cmg 3 ]
Decay time
Powder crystal Powder crystal Powder crystal Single crystal Single crystal Polycrystalline Polycrystalline
ProxiVision [12] ProxiVision [12] Crytur [13] Crytur [13] SaintGobain [14] BCE [15] BCE [15]
50 lm 50 lm 20 lm 250 lm 1.0 mm 0.8 mm 0.8 mm
7.21 4.55 4.55 4.57 4.55 3.94 3.73
2.6 ms 300 ns 70 ns 60 ns 70 ns 35 ms >3.4 ms
is found as 35 ms [11]. Investigations at the FLASH experiment found multiple values for the decay time of the Chrome-doped Aluminum Oxide [9,10]: Here the authors measured the decay as sum of several exponential decays with decay times of 3.4 ms and 6.7 ms. Ion beams from protons (Z ¼ 1) till Uranium (Z ¼ 92) were extracted from SIS18 with intensities varied from 6106 particles per pulse (ppp) up to 21010 ppp. The pulse duration was 300– 400 ms for slow and 1 ls for fast extraction mode and the general requested beam energy was 300 MeV/u. A resonant transformer was used to measure the current of fast extracted beam pulses with an accuracy of 15% [16,17]. For measurements in slow extraction mode a diagnostic device was driven in the beam, that consists of an Ionization Chamber (6.5 mm Ar–CO2 gas mixture, separated in beam direction by two 100 lm stainless steel walls from the vacuum, measurement accuracy of 15%) and a SEM (three 100 lm Al plates, measurement accuracy of 15%) [18]. Additionally the beam passed a 50 lm thick stainless steel foil into air 72 cm before the target ladder. Thus the beam was stripped and the energies EB at the target surface were calculated numerically by LISE [19]. Calculation results are given in Table 2. To record the beam response an optical setup was mounted perpendicular to the target surface with object distance of 50 and 70 cm. A scheme is shown in Fig. 1. It consists of two cameras (visible in blue) with different assembling: The first camera (AVT Marlin or Stingray respectively, 1/2’’ CCD chip, 8 bit resolution, monochrome) was mounted slightly below the optic main axis and recorded the two dimensional beam response of the scintillation screens. The camera was equipped with a Pentax C1614ER lens of 16 mm focal length and remotely controlled iris. The spectral transmittance starts from 350 nm and ranges up to more than 1000 nm [20]. To further increase the dynamic range of the measurements a neutral-density-filter (Hoya ND03, 1.4 mm thick, 5% transmission) was mounted additionally. The recorded images served for investigations in light output L, the herefrom resulting light yield Y and second statistical moments of beam profiles as representative for horizontal and vertical beam width. The second camera (PCO 1600, 12.2 mm 9.0 mm CCD sensor size, 14 bit resolution, monochrome) was mounted on a Horiba CP140-202 spectrometer with an average dispersion of 50 nm/mm
Table 2 Beam energies EB at target surface for described setup, numeric calculation performed with LISE [19].
Projectile
Beam energy EB [MeV u ] Slow extr. Fast extr.
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
299.2 297 298 281 272
299.8 299 297 295 292
Fig. 1. Scheme of experimental setup. The distance between the air window and the target ladder is 72 cm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[21]. Here the emission spectra were recorded in the wavelength range from 230 to 800 nm. As lens a Pentax C2514-M of 25 mm focal length and a wavelength transmission from 350 nm into IR region was used. It was focused on an adjustable slit, mounted in horizontal orientation on the input of the spectrometer. To increase the light input the slit was opened until a wavelength resolution of 8.5 nm FWHM was reached. Images were recorded simultaneously from both cameras during irradiation. Integration times were set to at least 400 ms (depending on extraction time) to capture the full scintillation of the targets. The camera gain was kept at minimum. Background images with the same camera settings were recorded shortly before incoming pulses to use for noise reduction during analysis.
3. Description of analysis For records from camera #1 background was subtracted and for further noise reduction a Region of Interest (ROI) was set. These images were normalized by camera and iris settings and then analyzed with respect to:
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Light output L : The number of emitted photons per irradiated pulse, given here as sum of gray scale values ki within the ROI.
L¼
X
ki
ð1Þ
i ROI
For measurements with varying beam intensities a linear fit algorithm was performed and the slope mL was calculated. To reduce the influence of readout noise for low beam intensities [22,23] the regression line was forced to intercept abscissa at Zero. As factor for the goodness the coefficient of determination [24] was calculated with
R2 ¼ 1
RSS TSS
ð2Þ
where RSS means the residual and TSS the total sum of squares as defined in common literature (for example in [25]). An accuracy of minimum R2 P 95% was achieved for each analysis. Light Yield Y : In principle the light yield corresponds to the light output L per deposited energy DEion and particles per pulse ppp.
Y¼
L ppp DEion
ð3Þ
If the light output L is responding linearly to the number of irradiated particles the factor L=ppp can be exchanged by the previous calculated slope mL . This changes Eq. (3) to
Y¼
mL DEion
Fig. 2. Light output L of all investigated targets under irradiation with fast extracted Uranium beam as function of irradiating particles per pulse. Each 10th data point is plotted.
ð4Þ
Statistical moments: The second statistical moments of the beam profiles in horizontal direction rhorz and vertical direction rv ert were calculated as defined in literature [25]. rhorz and rv ert served as corresponding factors for horizontal and vertical beam width. Recorded images from the camera #2 were background subtracted as well and the resulting spectra were folded with the transmission efficiency of the optical system. These spectra were folded with the transmission curve of the used neutral-density-filter Hoya ND03 to calculate a realistic value for the transmission with respect to camera #1 (find more details in [26]). Transmission values between 4.4% (for P43 phosphor target) and 10.3% (for Al2O3:Cr target) were calculated and integrated into analysis.
5 more light (black septagons). Investigations in slow extraction mode show similar results. Regarding the complete range of investigated projectiles, the light output is shown for the material P43 phosphor in Fig. 3. Each 15th data point is plotted with errors between 5% and 25% for L. The errors in the horizontal axis accord to 15% of the number of irradiated particles per pulse for measurements in both extraction mode. All projectiles were requested with the same energy per nucleon. The beam energies are summarized in Table 2. The energy deposition in the scintillators was calculated with LISE [19]. The screen thickness of the P43 phosphor target was multiplied with the bias angle and set to 70.71 lm and the results are listed in Table 4. As can be seen protons induce the lowest light output (red and green diamonds) and for increasing atomic number of the projectile the slope increases up to the light output induced by Uranium ions (green and blue pentagons). The analysis showed that with increasing atomic number Z more light output is observed at irradiation with slow extracted beams than for irradiation with fast extracted beams. For P43 phosphor the difference is of maximum a factor 1.5 for measurements with Xenon beams (see Table 3). It is so far not understood
4. Results 4.1. Light output A comparison of light output for all investigated targets under irradiation of fast extracted Uranium beam is shown in Fig. 2. Each 10th data point is plotted with errors between 5% and 20% for L, originated by the signal-to-noise-ratio in the recorded images and 15% error for the number of irradiated particles. The extraction time was 1 ls. In the investigated ranges of 3106 till 108 ppp all investigated materials respond linear to the number of irradiated particles per pulse and a saturation effect as predicted for single particle ionization [6,7] was not observed. From the investigated phosphor screens the highest light output is observed from the P43 material (blue circle). Only YAG:Ce from SaintGobain (purple squares) showed an even higher light output. It is supposed that the screen thickness of 1 mm and the resulting large energy deposition is responsible for this observation. The lowest light output is observed from pure Aluminum Oxide (yellow hexagons). Chrome-doping induces here a factor of
Fig. 3. Light output L of P43 phosphor screen induced by investigated range of projectiles as function if irradiating particles per pulse.
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Table 3 Calculated slopes mL of light output as function ppp for P43 phosphor. Dm was calculated by help of covariance matrix. The coefficient of determination R2 [24] is given in brackets. Projectile
mL;slow DmL;slow (R2 )
mL;fast DmL;fast (R2 )
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
(1.2 ± 0.3)103 (91.18%) (5.4 ± 0.2)102 (94.10%) 0.64 ± 0.02 (94.05%) 1.89 ± 0.07 (87.90%) 5.94 ± 0.3 (89.18%)
(1.3 ± 0.1)103 (99.34%) (5.8 ± 0.1)102 (99.54%) 0.47 ± 0.02 (99.34%) 1.22 ± 0.03 (96.66%) 4.25 ± 0.2 (84.71%)
why the difference is so small and the findings can neither be explained by the differences in deposited energy nor by the ratio of pulse durations (105). 4.2. Light yield The light yield was calculated by Eq. (4) and normalized to the yield for protons. Fig. 4 shows the results for P43 phosphor, A999 screen and YAG:Ce single crystal #5 for the investigated projectiles. The given error bars correspond to relative values to the data points of 5–25% for investigations in fast extraction and 20–35% for investigations in slow extraction. Data, that was taken in previous investigations in slow extraction mode normalized to the data points for the Uranium beam and are given in the plots as well. For both extraction types a decrease is clearly visible with increasing atomic number. For measurements with fast extracted Xenon beam the light yield of P43 phosphor and A999 shows a minimum of 40% and for YAG:Ce single crystal at 20%. This behavior is assigned to the dependency of light yield from the ionization density, also called scintillator non-proportionality. Nevertheless, the origins of this scintillators characteristic belong to recent research [6,27–31].
Fig. 4. Calculated light yield Y of P43 phosphor, A999 screen and YAG:Ce #5 single crystal for investigated projectiles, the light yield induced by protons serves as reference. Data of reference [5] is given as well.
4.3. Beam profile measurements From the recorded images the beam profiles were calculated in horizontal and vertical direction. Fig. 5 shows a comparison of beam profiles from the investigated targets during irradiation with fast extracted Xenon beam. The phosphor and ceramic screens as well as single crystal # 4 show the same projections, while on single crystal # 5 the beam appears broader in horizontal direction. This is most likely assigned to the thickness of the target and the increased scattering in the screens. The beam profiles were analyzed with respect to second statistical moments rhorz and rv ert as factors corresponding to horizontal and vertical beam width, but no noticeable changes occurred with increasing beam intensity. 4.4. Emission spectra Fig. 6 shows spectra of P43 phosphor emission under irradiation with Nitrogen beam (left plot) and Uranium beam (right plot).
Table 4 Deposited energies in P43 phosphor target for investigated projectiles, calculated with LISE [19] for a target thickness of 70.71 lm Deposited energy DE [MeV] Projectile
Slow extr.
Fast extr.
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
0.11 5.37 88.98 340.24 985.78
0.11 5.35 87.79 332.43 956.38
Fig. 5. Comparison of beam profiles record by scintillation screens under irradiation with a fast extracted Xenon beam at 109 ppp.
They are averaged over a range of beam intensities between 108 till 109 ppp. A significant difference was not observed within the accuracy of the used setup. Although the deposited energy per paticle ranged up to 1 GeV for investigated Uranium beam, a material modification could not be proven and the characteristics of the recorded spectra are similar to those in literature, e.g. for irradiation with X-rays [32]. This outcome is confirmed additionally by the different investigated Y3Al5O12:Ce targets as shown for irradiation with slow extracted Uranium beam in Fig. 7. The spectra for investigated YAG:Ce single crystals show the same characteristics as the investigated P46 phosphor screens. The slight shift of P46 phosphor #2 is assigned upcoming fluctuations in the operational alignment that causes the beam position to deviate out of the optical focus of camera #2. The recorded spectra is in accordance to those found in previous studies under X-ray and c irradiation [13,33,34].
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Fig. 6. Emission spectra of P43 phosphor, irradiated by Nitrogen (left) and Uranium (right) in fast and slow extraction mode respectively.
Fig. 7. Emission spectra of targets #2–#5 (Y3Al5O12:Ce as single crystals or phosphor screens) under irradition with slow extracted Uranium beam.
4.5. Radiation hardness tests In an additional investigation all scintillation screens were irradiated with 1000 pulses at 2109 ppp each. As projectile Nickel ions were used as listed in Table 2. Afterwards the irradiation was stopped for 15 min and than continued by 100–150 pulses to check for any permanent modification of the materials. The light yield was calculated by dividing the recorded light output by the deposited number of particles per pulse and the deposited energy of the beam. The result is plotted for a selection of targets in Fig. 8. The data were binned to a beam intensity of 51010 ppp and statistic deviations are used as error bars. A decrease of less than 5% is observed for P43 phosphor after a deposited flux of more than 21012 ions. A larger effect was found for Al2O3:Cr with 10% decrease in light output. P43 phosphor starts with from the initial value of light yield after the 15 min break, but decreases again after a few pulses. The increased error bars after break for P46 phosphor #3 are assigned to fluctuations of the beam properties. Fig. 9 shows the second statistical moments in horizontal rhorz and vertical direction rv ert . The data were binned to a beam intensity of 51010 ppp and statistic deviations are used as error bars. The targets show similar beam sizes for long time irradiation. The recorded beam size is unchanged after the break of irradiation for three of four targets. For P46 #3 a fluctuation in operational alignment caused a change of beam size after break.
Fig. 8. Light yield during radiation hardness test with fast extracted Nickel beam.
In the recorded emission spectra the material stability of the targets is observed as well. Fig. 10 shows a sequence of spectra from P43 phosphor emission during long time irradiation (blue) and after break (red). No specific changes occurred even under the harsh conditions of applied radiation hardness test. These outcomes differ from investigations with ions in low energy regime [35]. A corresponding behavior can be modeled by TRIM simulations [36] to calculate the number of vacancies produced by Nickel ions in an undoped Gd2O2S material with a density of 7.21 g/cm3. Similar to the calculations for Table 2 the screen thickness was set to 70.71 lm. Comparing the calculations for a specific beam energy of EB ¼ 300:0 MeV=u with those of EB ¼ 3:0 MeV=u the number of vacancies increases with decreasing beam energy up to a factor of 88. Thus the material is damaged less under irradiation with high beam energy. 5. Conclusion Inorganic scintillation screens, including phosphor screens, single crystals and polycrystalline ceramics, were investigated with high energetic ion beams of 300 MeV/u of different projectiles from protons till Uranium. Such screens offer great material stability during irradiation with varying beam intensity for all
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Fig. 9. Calculated second statistical moments during radiation hardness test with a fast extracted Nickel beam,
rhorz for horizontal and rv ert for vertical beam profile.
Acknowledgements This work is supported by BMBF, Grant No. 05P12RDRBJ. The authors would like to thank the members of beam instrumentation departement of GSI for support and help during investigations. Especially mentioned may be B. Walasek-Höhne, A. Reiter and M. Hartung for personal support during preparation and analysis. It is also a pleasure to acknowledge members of the GSI operating team and detector laboratory. References
Fig. 10. Sequence of recorded emission spectra of P43 phosphor during radiation hardness test with a slow extracted Nickel beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
investigated projectiles and during radiation hardness test with Nickel beam. While the light output for similar beam conditions differ by a factor of up to 60 for different targets (Fig. 2) the light output and yield show an increasing difference with increasing atomic number (Figs. 3 and 4). The recorded beam profiles result in the same beam sizes rhorz and rv ert for six of seven targets (Fig. 5). The outcomes of the present proceeding differ from investigations in low-energy regions from 1.4 to 11.4 MeV/u [1,4,35]. The targets were studied concerning radiation hardness under irradiation with Nickel beam up to 21012 accumulated ions. Analysis of light yield showed only slight decrease. The largest decrease is found to be 10% for Chrome-doped Aluminum Oxide. It is not necessary to apply a thermal annealing step as found for low energy beam instrumentation and reported within issue [37]. Horizontal and vertical beam widths are constant for three of four targets. The drop of beam width for P46 #3 after irradiation break is assigned to fluctuations in the ion source. In summary inorganic scintillation screens serve as reliable beam diagnostic tool for alignment of ion beams with specific energies of EB few hundred MeV/u. For moderate beam intensities a replacement of the screens is not necessary.
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Response from inorganic scintillation screens induced by high energetic ions q A. Lieberwirth a,b,⇑, W. Ensinger b, P. Forck a,c, S. Lederer a,b a
GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany TU Darmstadt (Materials Analysis), Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany c Goethe University (Physics), Max-von-Laue-Str. 1, 60438 Frankfurt, Germany b
a r t i c l e
i n f o
Article history: Received 19 May 2015 Received in revised form 21 July 2015 Accepted 24 July 2015 Available online 5 August 2015 Keywords: Scintillation screens Inorganic material High energetic beam Light output Emission spectra
a b s t r a c t Inorganic scintillation screens were irradiated with swift heavy ion beams at GSI accelator facility, using protons up to Uranium ions as projectiles. Beams were extracted from the synchrotron SIS18 with high energy (300 MeV/u) in slow and fast extraction mode. During irradiation the scintillation response of the screens was recorded by two different optical setups simultaneously to investigate the light output, profile characteristics and emission spectra. In principle fast extracted beams induce lower light output than slow extracted beams. The output per deposited energy shows a decreasing dependency with respect of the atomic number. Emission spectra showed no significant defects of the materials, neither at irradiation with increasing beam intensity nor during long time irradiation. Ó 2015 Elsevier B.V. All rights reserved.
1. Motivation In the last decades scintillation screens received an increasing interest as simple tool for beam alignment at accelerator facilities. In particular screens made of inorganic materials were proven to verify all kinds of radiation in direct or indirect manner [1]. For common ion beam alignment the screen is driven in intercepting way into the beam. Incident particles transfer energy into the material and induce an avalanche production of electron– hole-pairs. If the charges are not bound due to vacancies or color centers photons are emitted during recombination processes [2,3]. Depending on the chosen material, the photons are emitted within the visible spectrum of light and can thus be recorded by CCD cameras or given directly to the operating team as video stream. Former investigations with low energy ion beams had shown a non-linear behavior for the light output L, e.g. number of emitted photons, and two dimensional characteristics of the recorded beam profile with respect to the accumulated ion dose [4]. Different characteristics were already observed during measurements with high energy ion beams [5]. For high energy ion beams (few
q
Work is supported by BMBF, contract number 05P12RDRBJ.
⇑ Corresponding author at: GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany. E-mail address: [email protected] (A. Lieberwirth). http://dx.doi.org/10.1016/j.nimb.2015.07.111 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.
hundred MeV/u) the question of a possible saturation effect for short pulse irradiation at large number of irradiated particles, as it was predicted for single particle ionization in [6,7], is rarely investigated. Thus inorganic materials were irradiated by ion beams requested in fast and slow extraction mode [8] from SIS18 in GSI. Light output and beam profiles were recorded for different beam intensities to investigate linearity and reproducibility for equal beam characteristics. During analysis a relative light yield was calculated in dependency of the atomic number of investigated projectiles. During all measurements the emission spectra of the targets were recorded simultaneously. In an additional survey the radiation hardness of the materials was studied.
2. Experimental setup Altogether seven different scintillation screens of inorganic material with diameters between 50 and 80 mm served for investigations. They were mounted in air with 45° bias angle to the beam axis on a movable target ladder that was controlled remotely. None of the screens were exchanged during the investigations. The targets consist of phosphor screens, single crystals and polycrystalline ceramics and their key parameters are given in Table 1. For targets #6 and #7 the decay times were not given by the supplier. For A999 the decay time under optical stimulation
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Table 1 Key parameters of investigated materials. Targets #3 and #4 are made from the same production line. For #6 and #7 decay times were not given by supplier, they can be found in reports [9–11]. Target material #1 #2 #3 #4 #5 #6 #7
P43 (Gd2O2S:Tb) P46 (Y3Al5O12:Ce) P46 (Y3Al5O12:Ce) YAG:Ce (Y3Al5O12:Ce) YAG:Ce (Y3Al5O12:Ce) A999 (Al2O3, 99.99 % purity) Al2O3:Cr (0.04 weight-% doping)
Phase
Supplier
Thickness
q½cmg 3 ]
Decay time
Powder crystal Powder crystal Powder crystal Single crystal Single crystal Polycrystalline Polycrystalline
ProxiVision [12] ProxiVision [12] Crytur [13] Crytur [13] SaintGobain [14] BCE [15] BCE [15]
50 lm 50 lm 20 lm 250 lm 1.0 mm 0.8 mm 0.8 mm
7.21 4.55 4.55 4.57 4.55 3.94 3.73
2.6 ms 300 ns 70 ns 60 ns 70 ns 35 ms >3.4 ms
is found as 35 ms [11]. Investigations at the FLASH experiment found multiple values for the decay time of the Chrome-doped Aluminum Oxide [9,10]: Here the authors measured the decay as sum of several exponential decays with decay times of 3.4 ms and 6.7 ms. Ion beams from protons (Z ¼ 1) till Uranium (Z ¼ 92) were extracted from SIS18 with intensities varied from 6106 particles per pulse (ppp) up to 21010 ppp. The pulse duration was 300– 400 ms for slow and 1 ls for fast extraction mode and the general requested beam energy was 300 MeV/u. A resonant transformer was used to measure the current of fast extracted beam pulses with an accuracy of 15% [16,17]. For measurements in slow extraction mode a diagnostic device was driven in the beam, that consists of an Ionization Chamber (6.5 mm Ar–CO2 gas mixture, separated in beam direction by two 100 lm stainless steel walls from the vacuum, measurement accuracy of 15%) and a SEM (three 100 lm Al plates, measurement accuracy of 15%) [18]. Additionally the beam passed a 50 lm thick stainless steel foil into air 72 cm before the target ladder. Thus the beam was stripped and the energies EB at the target surface were calculated numerically by LISE [19]. Calculation results are given in Table 2. To record the beam response an optical setup was mounted perpendicular to the target surface with object distance of 50 and 70 cm. A scheme is shown in Fig. 1. It consists of two cameras (visible in blue) with different assembling: The first camera (AVT Marlin or Stingray respectively, 1/2’’ CCD chip, 8 bit resolution, monochrome) was mounted slightly below the optic main axis and recorded the two dimensional beam response of the scintillation screens. The camera was equipped with a Pentax C1614ER lens of 16 mm focal length and remotely controlled iris. The spectral transmittance starts from 350 nm and ranges up to more than 1000 nm [20]. To further increase the dynamic range of the measurements a neutral-density-filter (Hoya ND03, 1.4 mm thick, 5% transmission) was mounted additionally. The recorded images served for investigations in light output L, the herefrom resulting light yield Y and second statistical moments of beam profiles as representative for horizontal and vertical beam width. The second camera (PCO 1600, 12.2 mm 9.0 mm CCD sensor size, 14 bit resolution, monochrome) was mounted on a Horiba CP140-202 spectrometer with an average dispersion of 50 nm/mm
Table 2 Beam energies EB at target surface for described setup, numeric calculation performed with LISE [19].
Projectile
Beam energy EB [MeV u ] Slow extr. Fast extr.
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
299.2 297 298 281 272
299.8 299 297 295 292
Fig. 1. Scheme of experimental setup. The distance between the air window and the target ladder is 72 cm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[21]. Here the emission spectra were recorded in the wavelength range from 230 to 800 nm. As lens a Pentax C2514-M of 25 mm focal length and a wavelength transmission from 350 nm into IR region was used. It was focused on an adjustable slit, mounted in horizontal orientation on the input of the spectrometer. To increase the light input the slit was opened until a wavelength resolution of 8.5 nm FWHM was reached. Images were recorded simultaneously from both cameras during irradiation. Integration times were set to at least 400 ms (depending on extraction time) to capture the full scintillation of the targets. The camera gain was kept at minimum. Background images with the same camera settings were recorded shortly before incoming pulses to use for noise reduction during analysis.
3. Description of analysis For records from camera #1 background was subtracted and for further noise reduction a Region of Interest (ROI) was set. These images were normalized by camera and iris settings and then analyzed with respect to:
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Light output L : The number of emitted photons per irradiated pulse, given here as sum of gray scale values ki within the ROI.
L¼
X
ki
ð1Þ
i ROI
For measurements with varying beam intensities a linear fit algorithm was performed and the slope mL was calculated. To reduce the influence of readout noise for low beam intensities [22,23] the regression line was forced to intercept abscissa at Zero. As factor for the goodness the coefficient of determination [24] was calculated with
R2 ¼ 1
RSS TSS
ð2Þ
where RSS means the residual and TSS the total sum of squares as defined in common literature (for example in [25]). An accuracy of minimum R2 P 95% was achieved for each analysis. Light Yield Y : In principle the light yield corresponds to the light output L per deposited energy DEion and particles per pulse ppp.
Y¼
L ppp DEion
ð3Þ
If the light output L is responding linearly to the number of irradiated particles the factor L=ppp can be exchanged by the previous calculated slope mL . This changes Eq. (3) to
Y¼
mL DEion
Fig. 2. Light output L of all investigated targets under irradiation with fast extracted Uranium beam as function of irradiating particles per pulse. Each 10th data point is plotted.
ð4Þ
Statistical moments: The second statistical moments of the beam profiles in horizontal direction rhorz and vertical direction rv ert were calculated as defined in literature [25]. rhorz and rv ert served as corresponding factors for horizontal and vertical beam width. Recorded images from the camera #2 were background subtracted as well and the resulting spectra were folded with the transmission efficiency of the optical system. These spectra were folded with the transmission curve of the used neutral-density-filter Hoya ND03 to calculate a realistic value for the transmission with respect to camera #1 (find more details in [26]). Transmission values between 4.4% (for P43 phosphor target) and 10.3% (for Al2O3:Cr target) were calculated and integrated into analysis.
5 more light (black septagons). Investigations in slow extraction mode show similar results. Regarding the complete range of investigated projectiles, the light output is shown for the material P43 phosphor in Fig. 3. Each 15th data point is plotted with errors between 5% and 25% for L. The errors in the horizontal axis accord to 15% of the number of irradiated particles per pulse for measurements in both extraction mode. All projectiles were requested with the same energy per nucleon. The beam energies are summarized in Table 2. The energy deposition in the scintillators was calculated with LISE [19]. The screen thickness of the P43 phosphor target was multiplied with the bias angle and set to 70.71 lm and the results are listed in Table 4. As can be seen protons induce the lowest light output (red and green diamonds) and for increasing atomic number of the projectile the slope increases up to the light output induced by Uranium ions (green and blue pentagons). The analysis showed that with increasing atomic number Z more light output is observed at irradiation with slow extracted beams than for irradiation with fast extracted beams. For P43 phosphor the difference is of maximum a factor 1.5 for measurements with Xenon beams (see Table 3). It is so far not understood
4. Results 4.1. Light output A comparison of light output for all investigated targets under irradiation of fast extracted Uranium beam is shown in Fig. 2. Each 10th data point is plotted with errors between 5% and 20% for L, originated by the signal-to-noise-ratio in the recorded images and 15% error for the number of irradiated particles. The extraction time was 1 ls. In the investigated ranges of 3106 till 108 ppp all investigated materials respond linear to the number of irradiated particles per pulse and a saturation effect as predicted for single particle ionization [6,7] was not observed. From the investigated phosphor screens the highest light output is observed from the P43 material (blue circle). Only YAG:Ce from SaintGobain (purple squares) showed an even higher light output. It is supposed that the screen thickness of 1 mm and the resulting large energy deposition is responsible for this observation. The lowest light output is observed from pure Aluminum Oxide (yellow hexagons). Chrome-doping induces here a factor of
Fig. 3. Light output L of P43 phosphor screen induced by investigated range of projectiles as function if irradiating particles per pulse.
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Table 3 Calculated slopes mL of light output as function ppp for P43 phosphor. Dm was calculated by help of covariance matrix. The coefficient of determination R2 [24] is given in brackets. Projectile
mL;slow DmL;slow (R2 )
mL;fast DmL;fast (R2 )
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
(1.2 ± 0.3)103 (91.18%) (5.4 ± 0.2)102 (94.10%) 0.64 ± 0.02 (94.05%) 1.89 ± 0.07 (87.90%) 5.94 ± 0.3 (89.18%)
(1.3 ± 0.1)103 (99.34%) (5.8 ± 0.1)102 (99.54%) 0.47 ± 0.02 (99.34%) 1.22 ± 0.03 (96.66%) 4.25 ± 0.2 (84.71%)
why the difference is so small and the findings can neither be explained by the differences in deposited energy nor by the ratio of pulse durations (105). 4.2. Light yield The light yield was calculated by Eq. (4) and normalized to the yield for protons. Fig. 4 shows the results for P43 phosphor, A999 screen and YAG:Ce single crystal #5 for the investigated projectiles. The given error bars correspond to relative values to the data points of 5–25% for investigations in fast extraction and 20–35% for investigations in slow extraction. Data, that was taken in previous investigations in slow extraction mode normalized to the data points for the Uranium beam and are given in the plots as well. For both extraction types a decrease is clearly visible with increasing atomic number. For measurements with fast extracted Xenon beam the light yield of P43 phosphor and A999 shows a minimum of 40% and for YAG:Ce single crystal at 20%. This behavior is assigned to the dependency of light yield from the ionization density, also called scintillator non-proportionality. Nevertheless, the origins of this scintillators characteristic belong to recent research [6,27–31].
Fig. 4. Calculated light yield Y of P43 phosphor, A999 screen and YAG:Ce #5 single crystal for investigated projectiles, the light yield induced by protons serves as reference. Data of reference [5] is given as well.
4.3. Beam profile measurements From the recorded images the beam profiles were calculated in horizontal and vertical direction. Fig. 5 shows a comparison of beam profiles from the investigated targets during irradiation with fast extracted Xenon beam. The phosphor and ceramic screens as well as single crystal # 4 show the same projections, while on single crystal # 5 the beam appears broader in horizontal direction. This is most likely assigned to the thickness of the target and the increased scattering in the screens. The beam profiles were analyzed with respect to second statistical moments rhorz and rv ert as factors corresponding to horizontal and vertical beam width, but no noticeable changes occurred with increasing beam intensity. 4.4. Emission spectra Fig. 6 shows spectra of P43 phosphor emission under irradiation with Nitrogen beam (left plot) and Uranium beam (right plot).
Table 4 Deposited energies in P43 phosphor target for investigated projectiles, calculated with LISE [19] for a target thickness of 70.71 lm Deposited energy DE [MeV] Projectile
Slow extr.
Fast extr.
Proton (Z ¼ 1) Nitrogen (Z ¼ 7) Nickel (Z ¼ 28) Xenon (Z ¼ 54) Uranium (Z ¼ 92)
0.11 5.37 88.98 340.24 985.78
0.11 5.35 87.79 332.43 956.38
Fig. 5. Comparison of beam profiles record by scintillation screens under irradiation with a fast extracted Xenon beam at 109 ppp.
They are averaged over a range of beam intensities between 108 till 109 ppp. A significant difference was not observed within the accuracy of the used setup. Although the deposited energy per paticle ranged up to 1 GeV for investigated Uranium beam, a material modification could not be proven and the characteristics of the recorded spectra are similar to those in literature, e.g. for irradiation with X-rays [32]. This outcome is confirmed additionally by the different investigated Y3Al5O12:Ce targets as shown for irradiation with slow extracted Uranium beam in Fig. 7. The spectra for investigated YAG:Ce single crystals show the same characteristics as the investigated P46 phosphor screens. The slight shift of P46 phosphor #2 is assigned upcoming fluctuations in the operational alignment that causes the beam position to deviate out of the optical focus of camera #2. The recorded spectra is in accordance to those found in previous studies under X-ray and c irradiation [13,33,34].
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Fig. 6. Emission spectra of P43 phosphor, irradiated by Nitrogen (left) and Uranium (right) in fast and slow extraction mode respectively.
Fig. 7. Emission spectra of targets #2–#5 (Y3Al5O12:Ce as single crystals or phosphor screens) under irradition with slow extracted Uranium beam.
4.5. Radiation hardness tests In an additional investigation all scintillation screens were irradiated with 1000 pulses at 2109 ppp each. As projectile Nickel ions were used as listed in Table 2. Afterwards the irradiation was stopped for 15 min and than continued by 100–150 pulses to check for any permanent modification of the materials. The light yield was calculated by dividing the recorded light output by the deposited number of particles per pulse and the deposited energy of the beam. The result is plotted for a selection of targets in Fig. 8. The data were binned to a beam intensity of 51010 ppp and statistic deviations are used as error bars. A decrease of less than 5% is observed for P43 phosphor after a deposited flux of more than 21012 ions. A larger effect was found for Al2O3:Cr with 10% decrease in light output. P43 phosphor starts with from the initial value of light yield after the 15 min break, but decreases again after a few pulses. The increased error bars after break for P46 phosphor #3 are assigned to fluctuations of the beam properties. Fig. 9 shows the second statistical moments in horizontal rhorz and vertical direction rv ert . The data were binned to a beam intensity of 51010 ppp and statistic deviations are used as error bars. The targets show similar beam sizes for long time irradiation. The recorded beam size is unchanged after the break of irradiation for three of four targets. For P46 #3 a fluctuation in operational alignment caused a change of beam size after break.
Fig. 8. Light yield during radiation hardness test with fast extracted Nickel beam.
In the recorded emission spectra the material stability of the targets is observed as well. Fig. 10 shows a sequence of spectra from P43 phosphor emission during long time irradiation (blue) and after break (red). No specific changes occurred even under the harsh conditions of applied radiation hardness test. These outcomes differ from investigations with ions in low energy regime [35]. A corresponding behavior can be modeled by TRIM simulations [36] to calculate the number of vacancies produced by Nickel ions in an undoped Gd2O2S material with a density of 7.21 g/cm3. Similar to the calculations for Table 2 the screen thickness was set to 70.71 lm. Comparing the calculations for a specific beam energy of EB ¼ 300:0 MeV=u with those of EB ¼ 3:0 MeV=u the number of vacancies increases with decreasing beam energy up to a factor of 88. Thus the material is damaged less under irradiation with high beam energy. 5. Conclusion Inorganic scintillation screens, including phosphor screens, single crystals and polycrystalline ceramics, were investigated with high energetic ion beams of 300 MeV/u of different projectiles from protons till Uranium. Such screens offer great material stability during irradiation with varying beam intensity for all
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Fig. 9. Calculated second statistical moments during radiation hardness test with a fast extracted Nickel beam,
rhorz for horizontal and rv ert for vertical beam profile.
Acknowledgements This work is supported by BMBF, Grant No. 05P12RDRBJ. The authors would like to thank the members of beam instrumentation departement of GSI for support and help during investigations. Especially mentioned may be B. Walasek-Höhne, A. Reiter and M. Hartung for personal support during preparation and analysis. It is also a pleasure to acknowledge members of the GSI operating team and detector laboratory. References
Fig. 10. Sequence of recorded emission spectra of P43 phosphor during radiation hardness test with a slow extracted Nickel beam. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
investigated projectiles and during radiation hardness test with Nickel beam. While the light output for similar beam conditions differ by a factor of up to 60 for different targets (Fig. 2) the light output and yield show an increasing difference with increasing atomic number (Figs. 3 and 4). The recorded beam profiles result in the same beam sizes rhorz and rv ert for six of seven targets (Fig. 5). The outcomes of the present proceeding differ from investigations in low-energy regions from 1.4 to 11.4 MeV/u [1,4,35]. The targets were studied concerning radiation hardness under irradiation with Nickel beam up to 21012 accumulated ions. Analysis of light yield showed only slight decrease. The largest decrease is found to be 10% for Chrome-doped Aluminum Oxide. It is not necessary to apply a thermal annealing step as found for low energy beam instrumentation and reported within issue [37]. Horizontal and vertical beam widths are constant for three of four targets. The drop of beam width for P46 #3 after irradiation break is assigned to fluctuations in the ion source. In summary inorganic scintillation screens serve as reliable beam diagnostic tool for alignment of ion beams with specific energies of EB few hundred MeV/u. For moderate beam intensities a replacement of the screens is not necessary.
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