Beam charge integration in external beam PIXE-PIGE analysis utilizing proton backscattering with an extraction window

Beam charge integration in external beam PIXE-PIGE analysis utilizing proton backscattering with an extraction window 1 2 first-author 1 1 3 corresp Guang-Fu Wang guangfu_w@bnu.edu.cn Guang-Fu Wang guangfu_w@bnu.edu.cn guangfu_w@bnu.edu.cn 1 1 1 College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China China Nuclear Power Engineering Co., Ltd., Beijing 100840, China China Nuclear Power Engineering Co., Ltd., Beijing 100840, China Beijing Radiation Center, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China In this study we present a new method for the indirect integration of beam charges in external beam Proton Induced X-ray Emission and Proton Induced γ-ray Emission (PIXE-PIGE) analysis. We recorded proton spectra backscattered by a Kapton film extraction window in different sample situations and under different beam currents. We also simulated backscattering spectra using the simulation of backscattering spectra program(SIMNRA). We determined that in a specific geometrical arrangement, different sample situations did not significantly affect factor C Q (the ratio between integral backscattering proton counts and integral beam charges). We also studied the reproducibility and beam current dependence of factor C Q . The statistic factor of C Q was 28.95±0.6 kilo counts /μC, with an relative standard deviation (RSD) of 2.0%. Significantly, in external beam PIXE-PIGE analysis, we were able to calculate beam charge integration from the integral backscattering proton counts in an energy region. Beam charge integration External beam PIXE-PIGE Proton backscattering 1 INTRODUCTION 1 Quantitative external beam Proton Induced X-ray Emission and Proton Induced γ-ray Emission (PIXE-PIGE) analysis has yielded meaningful results in environtology, [ 1 1 , 2 2 ] biology, [ 3 3 ] art, archaeology, [ 4 4 - 7 7 4-7 ] and many other fields with the aid of integrated beam charges. Several methods have been adopted for measuring beam charge integration in external beam PIXE-PIGE analysis. The Faraday cup is the most traditional method, but it is not applicable to thick samples which beam line can’t penetrate. Past studies have also exploited the Rutherford Back Scattering (RBS) signal of protons (backscattered from helium atoms out of an exit window) to aim and measure beam charge integration. However, with the RBS approach helium pressure must be monitored due to its effect on proton spectrum background levels. [ 8 8 ] In the Laboratorio Beni Culturali laboratory (LABEC), beam current is inferred from the height of the proton backscattering spectra on the exit window. [ 9 9 ] One method for indirect charge integration employs the light emission from air molecules excited by the proton beam, where the light intensity is relevant to beam energy and gas pressure. [ 10 10 ] Other studies have employed proton-induced X-ray yields on Mylar film (coated with cobalt) [ 11 11 ] and exit windows for charge integration in the determination of the composition of glass [ 5 5 ] and low current measurements. [ 12 12 ] In proton single-event-effects experiments, film scintillators have been applied to detect the proton beam. [ 13 13 ] Finally, in the Japan National Institute of Radiological Sciences(NIRS), the beam current is monitored with a ceramic channel electron multiplier (CEM) to detect secondary electrons from a carbon film. [ 14 14 ] Traditionally, the Faraday cup method is used for beam charge integration in the External Beam PIXE-PIGE Analysis Facility at the GIC4117 Tandem Accelerator of Beijing Normal University; however, in thick samples situation, the beam charge cannot be integrated directly. Hence, quantitative analysis is difficult to achieve. To solve this problem, we assembled a new proton extraction window. During sample analysis, we installed an Au-Si surface barrier detector in a vacuum to detect the protons backscattered by the Kapton extraction window. At the same time, the actual beam charge ( Q F ) was integrated by the Faraday cup. Then, we compared the integration of backscattering proton total counts C B (corrected for dead time) in a certain energy region to the actual beam charge integration. We then obtained the ratio factor of C Q ( C B /Q F , kilo counts /μC, kc/μC). Finally, in sample situations where the beam charge could not be integrated directly, we calculated beam charge integration from the integration of backscattering proton counts and factor C Q . 2 EXPERIMENTAL SET-UP 1 The proton beam line used in this study was produced by the GIC4117 Tandem Accelerator of Beijing Normal University. (We have already provided details on the pre-existing external beam analysis facility.) [ 1 1 , 15 15 ] In this study, we replaced the extraction window with a 7.5 μm Kapton exit window. We positioned a 20 mm 2 Au-Si surface barrier detector (100 V bias, 0.5 μS shaping time, 302.6 amplifier gain) in vacuum behind the window. This barrier detector covered a fixed solid angle of the exit window. We placed a two-dimensional computer-controlled sample holder 1.2 cm from the exit window. We established a 600 cm 3 /min standard-state helium (He) flow between the window and the sample in order to maintain a stable low-Z gas atmosphere. Significantly, this method can lower beam attenuation and improve the detection efficiency of low energy X-ray. [ 16 16 ] We used a silicon drift detector (80 mm² area, 25 μm Dura Be window, FWHM 139 eV at 5.9 keV) and a Si (Li) detector (80 mm 2 area, 25 μm Be window, FWHM 159 eV at 6.4 KeV) for the detection of low-energy X-rays and medium-high energy X-rays, respectively. We detected high-energy γ-rays using an HPGe detector. Figure 1 Figure 1 shows the geometric arrangement of the apparatus. We vacuumed and installed a -350 V biased Faraday cup behind the sample holder. It is calibrated by X-ray yield of a thin Fe standard reference sample(49 μg•cm 2 , MicroMatter). [ 15 15 ] The proton energy was 2.5 MeV during routine analysis. The proton energy diminished by 147 KeV when processed through the Kapton film (C 22 H 10 O 5 N 2 ), and lost 32 KeV when processed through the helium flow (Geant4 simulation, n=1000). We adjusted the diameter of the beam spot to 6.8 mm to cover a sufficient sample area, and to satisfy the necessary requirements for the analysis of several types of routine aerosol samples. 3 RESULTS AND DISCUSSION 1 3.1. Backscattering Proton Spectra 2 In our experiment we demonstrate that the counting rate of backscattered protons is too high when the beam current exceeds 10 nA. In order to restrict the dead time of the proton detector to less than 3%, we attached an aluminum collimator (thickness 100 μm, diameter 0.7 mm) to the detector. First, we placed a 6.3-μm blank Mylar film on the sample holder. We recorded the spectra S under a 9-nA beam current for 1.20 μC Faraday cup charge integration. Figure 2 Figure 2 shows the proton spectrum and the SIMNRA simulation. The colored curves in Figure 2 Figure 2 are the general and elements-specific simulations. Peak ① indicates the protons backscattered by the helium atoms. The actual height of the helium (He) peak is lower than the simulation height and diminishes more rapidly in lower-energy regions. This can be attributed to the fact that when the distance from the exit window increases, the detector angle to the He volume decreases. The stacking of Peak②(carbon), Peak ③ (nitrogen), and Peak ④ (oxygen) indicates the protons backscattered by the Kapton film (Kapton film contains carbon). The air impurity in the He flow resulted in shifts of the nitrogen (Peak ⑤) and oxygen (Peak ⑥) peaks in lower-energy regions. Peak⑤ and Peak⑥ increased with decreases in the helium flow. The protons backscattered by the Mylar film were carbon (Peak ⑦) and oxygen (Peak ⑧). Because of the shelter provided by the exit device, the solid angle of the Mylar film was very small, which resulted in the peak height being much lower than those in the simulation. In order to confirm the sample effect on the spectra, we conducted our measurements using a thick lead plate sample. Figure 3(a) Figure 3(a) clearly shows that a lead (Pb) peak significantly affects the spectrum. Moreover, our detection and simulation results demonstrate that the air impurity in the He flow affected the nitrogen (N) and oxygen (O) peaks in low energy regions. In the initial solid angle arrangement, the samples significantly impacted the Kapton film backscattering proton peak. In other words, spectra were highly sample-dependent. In order to eliminate the effect of samples on the proton backscattering spectra, we adjusted the position of the Au-Si surface barrier proton detector so that it had no solid angle in relation to the sample. We then re-recorded spectra under sample vacancy, Mylar film, and lead (Pb) plate sample situations. Figure 3(b) Figure 3(b) clearly shows the effect of this change: no protons backscattered by samples were detected. Additionally, the background level for the Pb plate sample was slightly and continuously heightened. This difference is more noticeable in low energy sections (most likely because of multiple scattered protons). Beam-induced thinning of the exit window resulted in a shift of the trailing edges. [ 9 9 ] Furthermore, the PIGE cross-section was sensitive to beam energy, which was related to the thickness of the Kapton film. We artificially reduced the density of the Kapton film by 5% in the simulation, which resulted in shifts in both the He and Kapton peaks. The energy decrease at half of the maximum of the He peak was approximately 21KeV. Thus, in the future, beam-induced thinning of the exit window can be revealed by a shift in the He backscattering proton peak, as well as the FWHM of the exit window peak. Based on the above results and in order to minimize the effects of air impurity and exit window thinning, we selected the peak area in the channel region (450~555) for proton total count integration. 3.2 Reproducibility under Different Sample Situations 2 After adjusting the detector position, we recorded proton backscattering spectra under three types of sample situations: Sample vacancy, Mylar film, and Teflon film. The beam current was approximately 9 nA, and the Faraday cup beam charge integration was 1.20 μC for every sample. We recorded the spectra five times under each sample scenario. We integrated the total backscattered protons in channel 450~555. Figure 4(a) Figure 4(a) displays high levels of consistency for the specific results. Teflon film spectra were almost identical to Mylar film spectra; however, Teflon film was more fragile. In the sample vacancy situation, the helium atmosphere was diluted by air, and the N and O peaks in the low energy regions were sharpened. Significantly, different sample situations had no significant effect on factor C Q when the proton detector had no solid angle in relation to the sample. For comparison to Figure 4(a) Figure 4(a) , the integration of channel 400~555 is shown in Figure 4(b) Figure 4(b) . 3.3 Beam Current Dependence 2 Based on the above results, we only studied the beam current dependence of C Q under the Mylar sample situation. The beam current increased from 2 nA to 20 nA, which covers the commonly used beam current levels in our laboratory. We recorded five spectra under each beam current, and the beam charge integration was 1.20 μC for every measurement. As shown in Figure 5 Figure 5 , factor C Q varied from 27.60 kc /μC to 29.91 kc /μC. The mean value was 28.95±0.6 kc /μC, with a RSD of 2.0%. In this study, the primary sources for errors are detection errors and charge integration errors. We observed proton-beam induced thinning of the Kapton film exit window with the naked eye. Although this proton-beam induced thinning has not yet resulted in significant variations in the spectra or factor C Q , the Kapton film should be replaced periodically to prevent it from bursting and to maintain stable proton energy loss. Ultimately, our study determined that the value of C Q can change with different detector solid angles, exit window types, and beam energies; however, the value of C Q remains constant under a specific geometric arrangement and parameter setting. Additionally, factor C Q can be revised before analysis if necessary. In the future, we will study the precise measurement of exit window thinning and the beam energy dependence of C Q . 4 Conclusion 1 We studied proton backscattering through an extraction window under different sample situations and beam currents in order to determine indirect beam charge integration. By detection and simulation of proton backscattering spectra, we found that the type of sample, Kapton film thickness, He atom density, and air impurity all influence the spectra. By using a precise detector geometric arrangement and selecting the spectra range of interest, C Q (the ratio between the integral backscattering proton total count and the integral beam charge) tended to be highly consistent in all situations. The influences of the type of sample, air impurity in the helium atmosphere, and proton-induced exit window change were entirely eliminated. Ultimately, the C Q mean value was 28.95±0.6 kc/ μC, with an RSD of 2.0%. Generally, C Q is applicable for beam charge integration in external beam PIXE-PIGE analysis. Nondestructive quantitative PIXE-PIGE analysis of thick sample situations is achieved with this system. References Wang G F , Li X F , Chu J H , et al . External beam PIXE/PIGE analysis facility on GIC4117 tandem accelerator . 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An improved experimental setup for the simultaneous PIXE analysis of heavy and light elements with a 3 MeV proton external beam . Nucl. Instr. and Meth. B , 1996 , 109-110 : 125 - 128 . DOI: 10.1016/0168-583X(95)01209-5 http://doi.org/10.1016/0168-583X(95)01209-5 An improved experimental setup for the simultaneous PIXE analysis of heavy and light elements with a 3 MeV proton external beam 10.1007/s41365-016-0131-5 This work was supported by the Beijing Natural Science Funds (No. 1142006) . 2015-08-22 2015-11-06 2016-01-22 2016-10-12 2016-12-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0131-5 http://dx.doi.org/10.1007/s41365-016-0131-5 . 2016 Nuclear Science and Techniques 06 27