Proposal for a neutron imaging station at n TOF EAR2

Transcription

1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Proposal to the ISOLDE and Neutron Time-of-Flight Committee Proposal for a neutron imaging station at n TOF EAR2 January 11, 2017 F. Mingrone 1, M. Calviani 1, M. Barbagallo 2, E. Chiaveri 1, 3, N. Colonna 2, L. Cosentino 4, P. Finocchiaro 4, C. Massimi 5, A. Perillo-Marcone 1, V. Variale 2, V. Vlachoudis 1 1 CERN, European Laboratory for Particle Physics, Geneva, Switzerland 2 INFN Section of Bari, Bari, Italy 3 University of Manchester, Manchester, United Kingdom 4 INFN Laboratori Nazionali del Sud, Catania, Italy 5 University of Bologna and INFN Section of Bologna, Bologna, Italy Spokesperson: F. Mingrone (federica.mingrone@cern.ch) Technical coordinator: Oliver Aberle (oliver.aberle@cern.ch) CERN-INTC / INTC-P /01/2017 Abstract: Neutron Imaging is a well-developed radiographic testing method used for nondestructive inspection of inner parts of an object. Due to their peculiar interaction with matter, neutrons act as probes penetrating thick-walled samples and providing an image of the transmitted radiation, which intensity depends on the thickness of the material layers and on the specific attenuation properties of that material. In this regards, neutron radiography can be assimilated to X-ray. However, while X-rays are attenuated more effectively by heavier materials like metals, neutrons allow to image light materials, such as hydrogenous substances, with high contrast, making the two imaging methods complementary to investigate the properties of object internal structures. Several dedicated facilities are operating worldwide in order to provide high performance neutron imaging stations, in particular at research nuclear reactors and at spallation sources. We propose to exploit a neutron radiography testing station in the new n TOF Experimental Area 2 (EAR2), which feasibility has been already proved [1], to investigate the internal structure of a spent antiproton target for the AD facility and several rods of different target-materials candidate irradiated at the HiRadMat facility. Requested protons: protons on target Experimental Area: EAR2 1

2 1 Introduction and technological motivation Following the successful validation of the neutron imaging method at the n TOF Experimental Area 2 during 2015 and 2016 [1], the n TOF Collaboration has decided to propose the use of the n TOF neutron beam in EAR2 in order to execute neutron radiography measurements for technological applications on radioactive samples. The method proven in is very powerful and allows reaching resolution of tens to hundreds of microns, comparable to what can be obtained by standard X-ray radiography. It is therefore proposing to apply the methods to three specific applications, which have an immediate impact for the CERN s fixed target physics program. 1.1 Inspection of HRMT27 rods Figure 1: Image of one of the 140 mm long pure tungsten rods (left panel) and of one of the 140 mm long pure iridium rods (right panel). During November 2015, the HRMT27 experiment was carried out in the HiRadMat facility at CERN. The objective has been to impact high-z materials (including tungsten, iridium and tantalum, see Fig. 1) with very high energetic beams in order to mimic the response of the materials in the current antiproton production target located in the PS complex. Results have been reported in international conferences [2], and further details can be found in a more comprehensive CDS note [3], under publication. Due to the thermal stresses induced by the proton beam, significant cracks occurred in most of the materials. However, because of the very high residual dose rate of the targets (several hundreds of µsv/h), it is for the moment not possible to execute detailed postirradiation experiment. We propose therefore to inspect several of the high-z rods with the n TOF beam, in order to verify whether and to what extent cracks originated on the surface propagated towards the center of the rods. 1.2 Radiography of a spent antiproton decelerator (AD) target Based on the outcome of the HRMT27 experiment, we are expecting the antiproton decelerator (AD) target core to be fractured and damaged inside (see Fig. 2 for an exploded view of the assembly). Despite the very high dose rate in the order of several msv/h at 10 cm, EN/STI is planning to open one of the spent target in the radioactive workshop in building 867. In the light of this delicate intervention, a neutron radiograph able to provide information on the status of the core, extrapolating whether it is intact, 2

3 Figure 2: Exploded view of the current water-cooled antiproton decelerator target. or partially or fully fragmented, will be of great important for safety and intervention planning matters. The tests in EAR2 executed with the big collimator in 2016 on a fresh, non-radioactive, antiproton decelerator target proved that the facility has enough sensitivity to assess this point. 1.3 Neutron PIE of a graphite-embedded proton irradiated tantalum bar Figure 3: Cross-sectional view of the proposed flexible-graphite. Due to the excellent performance of tantalum (which is very ductile albeit with low yield strength), another HiRadMat experiment will be executed in May 2017 to check how a tantalum bar will react to a proton beam impact once fixed inside discs of flexible, low density, graphite, all contained in a Ti6Al4V tube (see Fig. 3). It is proposed to use the n TOF neutron radiography station to evaluate the response of the discs to the beam-induced thermal stresses before opening the Ti6Al4V vessel. X-ray will not be fully adapted for this scope, due to the low sensitivity in the area between the Ta and the graphite disks. For a direct comparison, we propose to carry out the neutron radiography inspection before and after the irradiation. 3

4 2 Experimental setup Conventional neutron-imaging techniques are based on mapping the attenuation of a neutron beam when transmitted through a sample. The resulting intensity map can be represented as an image with two main parameters, spatial resolution and contrast. While contrast depends on the beam intensity, the radiograph resolution is determined by several factors [4], in particular by the beam divergence D/L, where D is the size of collimator inlet aperture and L the distance between the inlet aperture and the point in the object to be analyzed, and by the distance l between the point in the object to be analyzed and the image detection plane. At n TOF, the diameter of the collimator inlet aperture could have two sizes, 218 mm and 667 mm. This parameter given, it is possible to adjust both l and L in the experimental area to optimize the achievable resolution. The converter thickness and the distance between converter and film (preferably at contact) also play an important role for the resolution, and are optimized in the imaging system. Besides the geometrical resolution, there are several indicators that can describe the image quality, providing the evidence that the technique used for a radiograph was satisfactory. Among them, the modulation transfer function (MTF) has been shown to be a powerful tool to measure and predict system s performances, representing the ratio of the magnitude of the system output to the magnitude of the system input [5]. This method has been exploited to analyze the performances of the imaging system tested at n TOF. As previously mentioned, the feasibility of a neutron imaging station in the new n TOF second experimental area (EAR2) has been successfully proved [1]. The station does not imply any modification to the n TOF beam line, and it is positioned at a distance of 220 cm from the exit of the collimator. This position has been chosen as the best compromise between the minimization of the ratio D/L, i.e. the maximization of the Figure 4: Pictures of the neutron imaging station, the neutron beam coming downstream in the vertical direction. The sample (stainless steel cylinder in the right picture) can be moved in both the direction of the horizontal plane so to have a complete scan. 4

5 achievable resolution, and the manageability of the station. The imaging system used provides a single radiograph of the object under investigation, mapping the neutron beam in two dimensions perpendicular to the direction of the beam itself. To perform a complete scan of the object under investigation, the measuring station allows to shift it in the two directions of the horizontal plane. In Fig. 4 a picture of the station is showed. A commercially available precise optic neutron imaging system from Photonic Science is used. It is based on an air-cooled SCMOS camera coupled with a ZnS/ 6 LiF neutron scintillator, that emits at 520nm, of dimensions mm 2. The scintillator has approximately 100 µm thickness, optimized for resolution. The camera sensor has an active input area of mm 2 mapped by pixels, with an optical pixel resolution of 6.5 µm. The camera operation is controlled completely via computer, and features that can be managed include gain, integration period, pixel clock frequency and image capture mode itself. The acquisition can be triggered by an external signal, characteristic that for a pulsed neutron beam as the one of n TOF allows to minimize the background coming from the experimental area and the radioactivity of the object under investigation. The station has been tested at the n TOF EAR2 with both the small mm inner diameter - and the big mm inner diameter - collimator. The characteristics of the neutron beam have been evaluated through simulations. In particular, as can be seen in Fig. 5, at the experimental location cm high from the collimator pinhole - about for the big and for the small collimator thermal neutrons/cm 2 /pulse are expected. In the optimistic case of 1 pulse every 1.2 seconds, about and thermal neutrons/cm 2 /pulse can be expected on the samples, respectively. The beam profile is expected to be between 4 and 6 cm diameter for the small collimator, and between 9 and 11 cm diameter for the big one, the difference depending on the sensitivity to the beam halo. During the tests in 2015 and 2016, a spare non-irradiated AD target has been investigated in both the tests, and the results are shown in the two panels of Fig. 6 (left for the small and right for the big collimator, respectively). As can be seen, in both cases the inner Iridium bar of the target is perfectly visible with a very good contrast, which proves the Figure 5: Radial profile of the n TOF neutron beam for the big collimator mm inner diameter - (red line) and the small collimator mm inner diameter - (black line). 5

6 Figure 6: Comparison of the image of a spare AD target obtained at the n TOF EAR2 imaging station for the small (left panel) and big (right panel) collimator. feasibility of the technique. While with the small collimator the highest spatial resolution is reachable due to the lower D/L ratio, the advantage in using the big collimator is that a much larger surface is covered, with a higher instantaneous flux and thus improved contrast. As can be seen from Fig. 5 in fact, the highest neutron beam intensity covers a spot of about 8 cm diameter, while with the small collimator it covers a spot of only 2 cm diameter. As previously mentioned, the quality of the imaging system has been analyzed through a MTF analysis performed with the software ImageJ [6]. The spatial resolution has been found to be between 10 and 12.5 µm for the image obtained with the small collimator, and between 25 and 125 µm for the image obtained with big one. Since the smallest dimensions of the inner structures of the objects under investigation are of the orders of few millimeters, both the setups are suitable for the proposed radiographs. 3 Beam time requirements The goal of the measurement is to perform a complete scan of the AD target and the HRMT27 rods irradiated at the HiRadMat facility. In the light of the results obtained from the test, the big collimator will be used, since it provides a still suitable spatial resolution and a much larger active area. Due to the geometry of the object under investigation, 4 to 5 scans are needed for the AD target and 3 to 4 for the HRMT27 rods. An average number of protons of has been associated to each scan to accumulate sufficient statistics. Together with the scans of the object, background measurement should be foreseen. These are particularly important due to the high radioactivity of the irradiated objects, about 5 msv at 10 cm for the AD target, and about 100s µsv at contact for the HRMT27 rods. The station has never been tested with radioactive samples, and it would be important to properly characterize the effects on the background to reach a satisfactory level of resolution and contrast. Summary of requested protons: protons on target or or 1.5 weeks of measurements. 6

7 References [1] M. Calviani et al., CERN-INTC / INTC-I-160, (2014) [2] C. Torregrosa et al., Proceedings of IPAC2016, Busan, Korea, (2016) [3] C. Torregrosa et al., CERN-EN , EN pdf (2016) [4] J. Domanus (Ed.), Practical Neutron Radiography, Kluwer Academic Publ., (1992) [5] K.W. Tobin, J.S Brenizer., J.N. Mait, An MTF technique for realtime radioscopie system characterization, Applied Optics 28 (1989) 5002 [6] 7