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Design of imaging system for CSNS near-target beam diagnostics

  • Zhirong Zeng
  • Shanhua Zhang
  • Weilin Cheng
  • Quanzhi Yu
  • Shaohong Wei
  • Bin Zhou
  • Donghui Zhu
  • Quan Ji
  • Aijun Zeng
  • Tianjiao Liang
  • Yuanbo Chen
Original Paper
  • 166 Downloads

Abstract

Introduction

 China Spallation Neutron Source (CSNS) is an accelerator-based pulsed neutron source which produces neutron with spallation reaction induced by proton bombarding tungsten target. With increasing beam powers and influences on target, near-target monitoring becomes extreme necessary. In this situation, an optical imaging system for proton beam diagnostics and monitoring near the target is being developed at CSNS, which can provide real-time images of the beam on target and beam distribution information.

Target imaging system design and development

 In the design of CSNS target imaging system, coating of \(\mathrm{Cr}^{3+}\!\!:\!\mathrm{Al}_{2}\mathrm{O}_{3}\) is used to convert particle radiation into emission light. According to the geometry limits of CSNS target station, a special optical system was designed and fabricated to collect the emission light. When the proton beams strike on the target, the coating on the target will be excited, emitting luminescence at the same time. The mirrors and lenses of the optical system image the distribution of emission light into a radiation-hard imaging fiber, which transmits the images to the GigE camera located at low-dose area outside of the target station. Software was written on the LabView platform to control the camera and analyze the images on line. Mock-up of the imaging system was manufactured to test and evaluate the performances of the system. Some important characteristics of the system were obtained and studied.

Conclusion

 Tests on the mock-up of the system present reliably expectation for beam diagnostics. The imaging system has been installed at CSNS recently. More work will be continued to improve the properties of the system.

Keywords

Beam diagnostics Beam profile Imaging system Luminescent coating 

PACS

41.85.Qg 41.85.Ew 42.82.Bq 29.40.-n 

Introduction

China Spallation Neutron Source (CSNS) has been built in Dongguan city, Guangdong province. The proton beam is accelerated up to 1.6 GeV at 25 Hz repletion rate by a LINAC and a rapid cycling synchrotron (RCS). After extraction from the RCS, the high-energy and intensity proton beam is transferred to the tungsten target to produce spallation neutrons. Twenty neutron beamlines will be built for neutron-scattering experiments [1].
Fig. 1

Location of target and PBW attached by imaging system at CSNS target station

Continuously monitoring of beam on target is needed for beam controlling in beam target shooting system. On the other hand, effects or damages of the beam to the target are extensively studied [2, 3]. Radiation damages on the target and the target vessel dependence on incident beam intensity are one of the issues concerned. In the unexpected situation of the focused beam, the focusing effect on the target will cause the target out of operation rapidly. The study of the relationship between the characteristics of the incident beam and the target damages will be benefit for the target commissioning, operation and protection. Due to these reasons, devices of beam monitor near the target are necessary. Lots of beam diagnostics devices have already been developed to measure and determine the basic parameters of particle beams in the target area at spallation neutron source in the world [4, 5]. In the target station at CSNS, the nearest position for laying out of the accelerator beam diagnostic devices is the aluminum proton beam window (PBW), which is about 1934 mm distance from the nose of the target. Multiwire monitor system has been designed to place on the side of the window which is away from the target. In order to comprehend the interaction of the beam and the target, a beam imaging system based on luminescence coating sprayed on the target is designed and fabricated to observe the beam on target online.

Imaging system overview and configuration

Diagnostic devices of the proton beams based on the light transformation method are often used in the accelerator community [6, 7, 8, 9]. In general, scintillation screens are used to be excited by the incident particles. It emits photons by the de-excitation process. The characteristics of the incident beam can be determined by the distribution of the de-excited light. Typical imaging systems include the luminescent material, a suitable optical system and an image acquisition system. In CSNS target station as shown in Fig. 1, due to the high-radiation environment near the target, the target and the proton beam channel are surrounded by thick shielding, which is composed of stainless steel and heavy concrete. The proton beam window assembly provides the interface between the accelerator and the target station. It separates the helium atmosphere of the target station from the high vacuum of the accelerator beam transportation tube. To get the real-time view of the beam on target and be compatible with the geometry of the CSNS target station, the luminescence material of the target imaging system is sprayed on the front face of the target. The optical system is then designed and attached to the PBW. Some channels and narrow spaces in the shielding of the PBW assembly are reserved to lay out the imaging system. A radiation-hard imaging fiber is required to transfer the focused image to the camera, which is located in the radiation safe area. The location of the imaging system in the target station is shown in Fig. 1.
Fig. 2

Design and functional components of target imaging system

Figure 2 shows the detailed layout of the proposed imaging system and its functional components. The luminescent material of \(\hbox {Cr}^{3+}\):\(\hbox {Al}_{2}\hbox {O}_{3}\) is chosen as the proton radiation to light convertor, which is widely used as scintillation screen with good thermo and radiation hardness properties for beam monitoring applications. Usually, as a destructive method, this kind of scintillation screen with 1 mm thickness is inserted into the beam path by mechanical devices located at many places of the accelerator and beam lines during the measurements. It is then retracted when irradiated by high-power beams causing high heating deposition. The dissipation of heating deposited by the incident high-energy particles limits the use of the screen for a long time. The concept of a thin luminescent coating sprayed on the target vessel was adopted in our design. The target vessel worked as the substrate of the coating is cooled by water.

To allow the emitted light passing through, a vertical view tube was reserved in the PBW shielding. Optical mirrors, including spherical mirror, plane mirror and lenses, are set near the PBW assembly to collect and focus the image into the 11-m fiber. A relay lens connects the fiber to the camera. Data are acquired and analyzed by a local controller.

The geometries of the Ring to Target Beam Transport (RTBT) and the Target-Moderator-Reflector (TMR) at CSNS are similar to that of the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). This conceptual design of the CSNS target imaging system is motivated by the similar system and the successful experience at SNS [10, 11, 12]. However, the specific size of the target station at CSNS is totally different from that of the SNS. In this study, to realize the conceptual design of the target imaging system, the detailed technical design adopts some different methods to try to simplify the installation and adjustment. To collect and reflect the luminescence light, instead of the parabolic mirror at SNS, a spherical mirror is used in the optical system. Special mechanical structures designed to make optical adjustment are considered. The mock-up of the optical system and auxiliary tooling are designed to test and evaluate the optical performance. Since the recipe of the luminescent material is hard to obtain, the fabrication process of the coating was explored. Some interesting results were observed from the tests on the coating samples. The technical design and development are described in the following sections.

Target imaging system development

Fabrication of luminescent coating

\(\hbox {Cr}^{3+}\):\(\hbox {Al}_{2}\hbox {O}_{3}\) is a well-known fluorescent material that is widely used in beam observations in accelerators because of its good performance in terms of thermal stability, linearity, radiation hardness and high conversion efficiency [13, 14, 15]. Luminescent screens of \(\hbox {Cr}^{3+}\):\(\hbox {Al}_{2}\hbox {O}_{3}\) in sheets of 1 mm thickness can be obtained from companies, usually called ‘chromox-6.’ In this case, due to the special shape of the target container, the coating could only be fabricated by a spraying process. Three kinds of powders with different chromium concentrations and spraying processes were explored and developed. Diagnostic methods including photoluminescence, radioluminescence, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the characteristics of the sprayed samples. It was found that the luminescence intensity of the flame sprayed sample was higher than that of the plasma and D-gun-sprayed samples. From the XRD results, it appears that the samples with more \(\alpha -\hbox {Al}_{2}\hbox {O}_{3}\) have a higher luminescence intensity than those with lower \(\alpha -\hbox {Al}_{2}\hbox {O}_{3}\). After the spraying process, part of the \(\alpha -\hbox {Al}_{2}\hbox {O}_{3}\) in the powders changed into another phase, such as \(\gamma -\hbox {Al}_{2}\hbox {O}_{3}\). The flame spraying process, especially with relatively lower ethyne contents, helped to keep more \(\alpha -\hbox {Al}_{2}\hbox {O}_{3}\) in the samples. By performing these tests and analyses, it was observed that the coating luminescence intensity was improved. More details were presented in another article [16].

According to the photoluminescence tests on the sprayed samples performed with a 532 nm laser, all of the coating samples had strong emission lines at 692.9 and 694.3 nm, which were called the ‘Ruby lines,’ and they were generated by the de-excitation process of \(\hbox {Cr}^{3+}\) [17, 18, 19]. Considering the peak position of the emission spectrum, a center wavelength of 694 nm and a wavelength width of 20 nm were chosen as the primary working bandwidth of the imaging system when the optical system was designed.

Many types of coating samples were prepared by spraying processes during the coating development. To evaluate the future work status under proton beam irradiation at CSNS, four samples were selected and tested at Heavy Ion Research Facility in Lanzhou (HIRFL). A deuterium beam of energy 300 MeV, with a pulse length of 3 s, and intensity of \(10^{5}\sim 10^{7}\) per pulse per 25 s, was used to irradiate the samples. The coating samples showed successfully fluoresced results. It was speculated that the emission intensities would be higher when irradiated by a 1.6 GeV, \(\sim \)10\(^{13}\) n/pulse proton beams. Using the confirmed flame spraying process, the nose of the target container was coated with a \(\hbox {Cr}^{3+}\):\(\hbox {Al}_{2}\hbox {O}_{3}\) layer in the area of 160 mm \(\times \) 60 mm, as shown in Fig. 3.
Fig. 3

The luminescence coating sprayed on the target, with designed thickness of 0.25 mm. The Cr concentration is 0.22%

Design and fabrication of optical system assembly

The imaging system collects luminescent light that is emitted from the coating material, and it acquires images online. The closest point for optical access to the target coating is at PBW assembly, which includes a thin aluminum window and a 505-mm-thick shielding plug above the window. The reserved view tube in the shielding plug is 28 mm in diameter and 945 mm in length to permit the light to pass through. The PBW is 1934 mm away from the target coating. The radiation dose is \(\sim \)10\(^{6}\) Gy/h at the PBW and decreases to \(\sim \)1 Gy/h above the cylinder shielding plug. Only the metal mirror can survive in such a high-radiation area of the PBW. Due to the manufacturing level, a spherical mirror was chosen at the PBW instead of a parabolic mirror. The lenses of high-purity fused silicon could be used in the narrow space above the cylinder shielding plug. At the top of the shielding plug, the tube terminates with a CF35 flange and a fused silica window to isolate the helium environment from the ambient as shown in Fig. 2. Limited by the geometry and the spaces, the plane mirror and the optical lenses were used and mounted on the shielding plug to focus the image. The light emitted from the coating was reflected by the spherical mirror at the PBW, passes through the tube, and is reflected again by the plane mirror to turn toward the horizontal direction to ensure the placement of the optical lenses. The image was focused into a fiber which is 1.4 mm in diameter.

The designed object has a rectangular area of 170 mm \(\times \) 70 mm, centered with the face of the coating on the target and accommodates a 10 mm allowance in the object plane. The focused image should be less than 1 mm in size to couple with the imaging fiber easily and to accommodate for the slight misalignment. The absolute magnification of the system is less than 0.0054. The schematic drawing of the beam channel from the target to the PBW is shown in Fig. 4. The proton beam channel in the moderator and reflector plug, as well as the helium vessel of the target station, will significantly limit the position of the spherical mirror. According to the geometry of the tunnel and the PBW assembly, the center of the spherical mirror is in line with the view tube to maximize the light transmission and the field of view (FOV). The effective sizes of the mirror in vertical is determined by the beam size and the relative position of the tunnel and the coating, which were calculated to vary from 30 mm to 43.7 mm in vertical, as shown in the red area in Fig. 5. This is done so that the mirror collects the desired rays and does not interfere with the proton beam.
Fig. 4

The approximate optical system model for evaluations of the spherical mirror parameters

The maximum FOV and the size of the spherical mirror are severely limited by the view tube and the proton beam channel. When designing the optical system, the off-axis optical path was ignored in order to evaluate the radius of curvature of the spherical mirror. The aperture stop is supposed to be laid in the view tube. It is designed to be 25 (d) mm in diameter and \(l_1 \) mm from the center of the spherical mirror in vertical. The maximum value of \(l_1 \) is 1010 mm when the aperture stop is on top of the view tube. The entrance pupil is \({d}'\) mm in diameter and \(l_2 \) mm from the center of the spherical mirror. The entrance pupil is the image of the aperture stop. The relationships between them could be described by Eqs. (1) and (2).
$$\begin{aligned} \frac{1}{l_1 }+\frac{1}{l_2 }= & {} \frac{1}{f} \end{aligned}$$
(1)
$$\begin{aligned} \frac{d}{{d}^\prime }= & {} \frac{l_1 }{l_2 } \end{aligned}$$
(2)
$$\begin{aligned} \frac{d_m -{d}^\prime }{l_2}= & {} \frac{D-d_m }{l_0 } \end{aligned}$$
(3)
The focal length of the spherical mirror is designated as f and is half of the radius of curvature of the mirror. The entrance pupil limits the marginal rays that are emitted from the marginal object and reflected by the marginal mirror. The relationship between the marginal rays, the object, and the mirror are approximately described by Eq. (3). D is the size of the object, which is \(\sqrt{170^{2}+70^{2}}\) mm. \(l_0 \) is the distance from the object to the center of the mirror and is 1884 mm. \(d_m \) is the size of the spherical mirror. The maximum effective value of \(d_m \) is 26.36 mm and is calculated form the geometry of the proton beam channel and the object.
$$\begin{aligned} \hbox {f}=\frac{d_m l_1 l_0 }{l_1 \left( {D-d_m } \right) +l_0 \left( {d+d_m } \right) } \end{aligned}$$
(4)
By solving Eqs. (13), the focal length f of the spherical mirror is given by Eq. (4). It is clear that that f is the increasing function of \(d_m\) and \(l_1\). The maximum absolute focal length of the spherical mirror is calculated to be 195.95 mm when the aperture stop is laid on top of the view tube, and the spherical mirror has the maximum effective size. So the maximum radius of curvature of the mirror is 391.9 mm. This value is calculated from the approximate on-axis model. It just provides a guideline for choosing the spherical mirror which plays an important role in the optical system. In fact, the spherical mirror with a smaller radius of 296.5 mm was chosen in the optical design and simulation. The magnification of the spherical mirror system is calculated to be 0.0729. Therefore, the magnification of the focusing lenses should be less than 0.074 to keep the image size within 1 mm, enabling good coupling with the fiber. In addition, the size of the lenses should be compatible with the geometry of the narrow space above the PBW cylinder shielding.

Furthermore, the resolution and the numerical aperture (NA) are important for the optical system. The FIGR-20 imaging fiber was manufactured by Fujikura Corporation with a resolution of 35 lp/mm (lines pairs per mm) and an NA of 0.22. It introduces the limitations in terms of the resolution and the NA of the optical systems fore-end and rear-end. The fore-end optical system of the fiber, which is composed of the mirrors and the focusing lenses, should have a resolution of \(\ge \) 35 lp/mm and an NA of \(\le \) 0.22. The luminous flux density is proportional to the square of the NA . The value of the NA of the fore-end optical system should be as close as possible to 0.22. The rear-end optical system of the fiber, which is composed of the relay lenses and the CCD should have a resolution of \(\le \) 35 lp/mm and an NA of \(\ge \) 0.22. The resolution limit of the target imaging system is 35 lp/mm, as determined by the fiber.

Based on the physical considerations and available mounting spaces of the target station, the combinations of mirrors and lenses with suitable structures were chosen from the lens data. The optimization of the optical system was performed using optics design software. The design of the fore-end optical system inside of the target station results in a total optical path length of 3073.618 mm, an effective focal length of 8.515 mm, and a paraxial magnification of − 0.00423. According to the peaks of the emission spectrum, the optical system was designed for use at 694 nm, with a 20 nm wavelength width. Optical components include the spherical mirror, plane mirror, silicon window and four lenses, which have a combined total of 12 optical surfaces in total. Figure 5 shows the simulated performance of the fore-end optical system. The MTF of the center and the marginal FOV are better than 0.48 at the resolution of 35 lp/mm. The spherical and chromatic aberrations are the major factors that influence the image quality. The geometrical distortion is obvious in the grid distortion simulated using software, as shown in Fig. 5b. The elongation in one direction would have a serious impact on the proton profile measurement precision, which should be calibrated and corrected by image processing. The simulated maximum distortion is 2.018%. The slight pincushion distortion of the system is present mainly in the corners of the image, where the proton beam is unlikely to exit. Due to the limitation of the geometry of the target station and the high-radiation environment, the optical system has a modest design and can satisfy the requirements of the target imaging system.
Fig. 5

Simulated performance of the optical system, a MTF of the fore-end optical system, b geometrical distortion of an ideal grid. The maximum distortion is 2.018%

In the mechanical and engineering design of optical system, the spherical mirror is mounted at the corner of the PBW, without interference with the incident proton beam. The center of the surface is located 1884 mm from the target coating, and is off-axis from the proton beam by 35 mm vertically and 93 mm horizontally. The materials of RSA Al 6061 are polished to form the spherical mirror and the plane mirror using diamond-turned process [20, 21]. The reflective film is not coated on the surface due to the ‘bilateral effect’ under high irradiation. The aluminum, off-axis spherical mirror can be micro-adjusted in five dimensions using six screws to provide the requisite FOV, and to help to accommodate the vertical displacement of the optical system and the tube, as shown in Fig. 6. However, due to the narrow space near the PBW, the system geometry may result in some difficulties during the installation and adjustment.
Fig. 6

The spherical mirror mounted on the proton beam window assembly. The mechanical design of the six regulated screws would allow the micro-adjustment in five dimensions

The lenses above the cylinder shielding plug focus the image into a small image-point, providing an adjustable interface between the imaging fiber. Corning 7980 ArF grade fused silicon glass [22, 23, 24] was recommended to polish the lenses. Any anti-reflective film is not coated on the surface to avoid radiation effect. The radiation-hard imaging fiber is attached to the optical lenses via a specially designed adjustable connector. The other end of the fiber terminates into the designed relay lens, which couples with the C-mount GigE camera. The magnification of the relay lens is 5x, which accommodates the CCD utilization and installation allowance. The relay lens includes five lenses, which have a combined total of 10 optical surfaces and is made of ZK and ZF series glasses with anti-reflective films. To reduce the chromatic aberration, a filter with a bandwidth of 680–710 nm was designed and placed in front of the camera sensor. The filter can be optionally used depending on the intensity of the luminescence.

Image acquisition and data analysis

The images of the proton beam on the target are acquired by the GigE camera out of the target station. The Prosilica GC750 industrial grade camera was used during the test stage. Due to the high noise level of the industrial grade camera, a scientific CCD camera is used instead for the detection of weak light. The acquisition software was written on the LabView platform, which includes two parts of the camera control and the data analysis. Users can set up the exposure time, trigger mode, and gain and can save the images to disk. The camera is triggered by the external delayed accelerator T0 signal which is sent out when the proton beam pulse strikes on the target. The repetition rate of the proton beam is 25 Hz. The exposure time of each image is usually within 40 ms.

Generally, for the accelerator, if the lattice functions of the \(\upbeta \)-function \({\upbeta }\)(t), momentum spread \(\frac{\Delta p}{p}\) and the dispersion D(t) at the target location t are well known, the relationship between the beam width \(\upsigma _{\mathrm{x}}\), \(\upsigma _{\mathrm{y}}\) on the target and beam emittance \(\upepsilon \) is given by Eq. (5).
$$\begin{aligned} \sigma _x^2 \left( t \right)= & {} \epsilon _x \beta _x \left( t \right) +\left( {D\left( t \right) \frac{\Delta p}{p}} \right) ^{2} \end{aligned}$$
(5a)
$$\begin{aligned} \sigma _y^2 \left( t \right)= & {} \epsilon _y \beta _y \left( t \right) \end{aligned}$$
(5b)
Fig. 7

Program to project and fit the data online. The fitted beam size is in unit of pixel, which will be converted to mm by multiplying the calibrated scaling factors

Nevertheless, due to the dispersion and the imprecise fixed lattice functions, the beam profile width is less stringently related to the emittance. Besides monitoring the image view of the proton beam on the target, the goal of the data analysis of the imaging system is to project the transverse beam profile and to fit the beam width. The peak density is also needed to be monitored. So that the accelerator could adjust the proton beam position on the target according to the results rather than the calculations. Damages to the target could also be subsequently studied. Transverse beam profiles are given by Eq. (6), which expresses the variation in the proton densities with the position. The average densities in columns or rows are sometimes used instead of the integrated densities in Eq. (6). Typically, the protons are distributed according to the Gaussian function, double Gaussian function or super Gaussian function [25]. The beam width could be calculated by fitting the profile curve.
$$\begin{aligned} \hbox {Profile}_H\left( x \right)= & {} \mathop \int \limits _{-\infty }^{+\infty } i\left( {x,y} \right) dy \end{aligned}$$
(6a)
$$\begin{aligned} \hbox {Profile}_V\left( y \right)= & {} \mathop \int \limits _{-\infty }^{+\infty } i\left( {x,y} \right) dx \end{aligned}$$
(6b)
The process of the data analysis includes image processing, profile drawing, and fitting. The images are preprocessed by the average filtering method. Regions of interest of the images are optionally chosen in order to draw beam profile in vertical and horizontal. The Gaussian function in the form of Eq. (7) is fitted to match the slope of the profiles and to calculate the beam size. The function could be changed if necessary. The parameter of ‘a’ in Eq. (7) is referred to the noise level. ‘b’ is referred to the dark noise structure which is mainly caused by the charge transfer process in CCD and the background. The beam size is obtained from the fitted value of ‘\(\upsigma \).’ Figure 7 shows the interface of the data projection and fitting program. The fitted parameters should be multiplied by the calibrated scaling factors of the imaging system in order to convert them to the real size in millimeters. This has been completed after calibration of the imaging system. The images and fitted parameters of the beam spot are shared and integrated with the CSNS control system via the Experimental Physics and Industrial Control System (EPICS) [26].
$$\begin{aligned} \hbox {y}=\hbox {a}+\hbox {b}\cdot \hbox {x}+\hbox {c}\cdot e^{\frac{\left( {x-x_0 } \right) ^{2}}{2\sigma ^{2}}} \end{aligned}$$
(7)

Mock-up of the imaging system and optical performance tests

Because the imaging system is mounted on the PBW at a much later time and it is impossible to obtain the optical characteristics after installation, the mock-up of the imaging system was constructed to evaluate the system performance. Figure 8 shows the assembly of the mock-up of the imaging system, including the optical system, the simulators of the proton beam channel and the PBW assembly. In the tests of the assembly, as shown in Fig. 8, a 650-nm LED source was used instead of the emitted luminescence. Although the wavelength was different from the luminescence of the coating, the optical properties to be tested could be considered to be the same at these two wavelengths. The interfaces with the proton beam channel and the PBW were the same as the real ones. The Prosilica GC 750 camera was connected to a laptop and was controlled by the LabView program directly without EPICS.
Fig. 8

Mock-up of imaging system and optical performance tests in the laboratory

Images of the grids were acquired to observe the aberrations and evaluate the scaling factor. A negligible distortion was observed, as shown in Fig. 9a. Elongation in horizontal was also obvious, and was mainly caused by the off-axis system. It is also shown in the simulated result of Fig. 5b. This could be recovered by the image processing. The rough imaging scaling factors of 217.4 \(\mu \)m/pixel in horizontal and 392.2 \(\mu \)m/pixel in vertical were established without considering the distortion. The FOV was measured to be 76 mm \(\times \) 168 mm in the object plane.
Fig. 9

Acquired images in optical system assembly tests, a image of graph paper without fiber, b image of resolution chart without fiber, c image of ‘CSNS’ letters with fiber

In order to test the system resolution, a black bar chart with different spatial frequencies was printed. The minimum spatial frequency was 0.5 lp/mm. The chart was placed at the object plane and was illuminated by the 650 nm LED source. Images were acquired without the fiber as shown in Fig. 9b. The resolution of the optical system without the imaging fiber was better than 0.5 lp/mm in the object plane, corresponding to 33 lp/mm in the image plane. Figure 9c shows an image of the letters ‘CSNS’ letters acquired by the optical system with fiber. Although the degradation of the image quality caused by the fiber was evident, the performance of the whole system was acceptable for beam diagnostics.

The transmittance was measured at several interfaces with a He–Ne laser (632.8 nm) and a laser power meter. The test results are presented in Table 1. The transmittance from the spherical mirror to the image plane of the optical system is close to a theoretical level, which was evaluated to be 48.5%. The attenuation of the fiber is \(\le \) 32 dB/km at 700 nm, which implies that the transmittance of the fiber is >90% per meter. It was observed that most of the light power was lost at the coupling interface between the lenses and the fiber.
Table 1

Transmittance of the optical system

Spherical mirror reflectivity

82.5%@45\({^{\circ }}\)

Transmittance from the spherical mirror to the image plane of the focusing lenses

42.5%

11 m imaging fiber transmission (including coupling loss and attenuation)

10.6%

Transmittance from the spherical mirror to the camera sensor plane

4.1%

Conclusions

The target imaging system was designed and fabricated for the proton beam diagnostics near the target at CSNS. It will serve not only to provide online image view of the proton beam on the target, but also to confirm the beam characteristics to assure the target lifetime at CSNS. Tests on mock-up of the system present reliably expectation for providing guidance of beam controlling. The measured FOV, resolution and the image quality of the optical system are acceptable. Radioluminescence test on the sprayed coatings gives acceptable evaluations of the luminescence emission. The imaging system has been installed and tested with the proton beams at CSNS. Some useful images have been acquired recently. Further works including data analysis will be continued and discussed after more images acquired. In addition, since the luminescence intensity and the optical aberration are related to the resolution of the beam size and the peak density, studies on key techniques to improve the luminescence intensity and optical system properties will be continued, with the goal of beam projecting in more accurate and making this primary tool near the target more reliable to align beam on the target.

Notes

Acknowledgements

This work was supported by the China Spallation Neutron Source project, the National Science Foundation of China (Grant Nos. 11575289) and the Project on the Integration of Industry, Education & Research of Guangdong Province, China (Grant No. 2015B090901048).

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Copyright information

© Institute of High Energy Physics, Chinese Academy of Sciences; Nuclear Electronics and Nuclear Detection Society and Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Institute of High Energy PhysicsChinese Academy of Sciences (CAS)BeijingChina
  2. 2.Dongguan Neutron Science CenterDongguanChina
  3. 3.Shanghai Institute of Optics and Fine MechanicsChinese Academy of ScienceShanghaiChina
  4. 4.Beijing National Laboratory for Condensed Matter Physics, Institute of PhysicsChinese Academy of ScienceBeijingChina

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