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2017

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. Mingrone1, M. Calviani1, M. Barbagallo2, E. Chiaveri1, 3, N. Colonna2, L. Cosentino4,P. Finocchiaro4, C. Massimi5, A. Perillo-Marcone1, V. Variale2, V. Vlachoudis1

1CERN, European Laboratory for Particle Physics, Geneva, Switzerland2INFN Section of Bari, Bari, Italy3University of Manchester, Manchester, United Kingdom4INFN Laboratori Nazionali del Sud, Catania, Italy5University of Bologna and INFN Section of Bologna, Bologna, Italy

Spokesperson: F. Mingrone (federica.mingrone@cern.ch)Technical coordinator: Oliver Aberle (oliver.aberle@cern.ch)

Abstract: Neutron Imaging is a well-developed radiographic testing method used fornondestructive inspection of inner parts of an object. Due to their peculiar interactionwith matter, neutrons act as probes penetrating thick-walled samples and providing an

image of the transmitted radiation, which intensity depends on the thickness of thematerial 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 areattenuated more effectively by heavier materials like metals, neutrons allow to imagelight materials, such as hydrogenous substances, with high contrast, making the two

imaging methods complementary to investigate the properties of object internalstructures. Several dedicated facilities are operating worldwide in order to provide highperformance neutron imaging stations, in particular at research nuclear reactors and atspallation 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 andseveral rods of different target-materials candidate irradiated at the HiRadMat facility.

Requested protons: 1 × 1018 protons on targetExperimental Area: EAR2

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1 Introduction and technological motivation

Following the successful validation of the neutron imaging method at the n TOF Experi-mental Area 2 during 2015 and 2016 [1], the n TOF Collaboration has decided to proposethe use of the n TOF neutron beam in EAR2 in order to execute neutron radiographymeasurements for technological applications on radioactive samples. The method provenin 2015-2016 is very powerful and allows reaching resolution of tens to hundreds of mi-crons, comparable to what can be obtained by standard X-ray radiography. It is thereforeproposing to apply the methods to three specific applications, which have an immediateimpact 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 ofthe 140 mm long pure iridium rods (right panel).

During November 2015, the HRMT27 experiment was carried out in the HiRadMat facilityat CERN. The objective has been to impact high-Z materials (including tungsten, iridiumand tantalum, see Fig. 1) with very high energetic beams in order to mimic the responseof 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 befound in a more comprehensive CDS note [3], under publication.Due to the thermal stresses induced by the proton beam, significant cracks occurred inmost 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 post-irradiation experiment. We propose therefore to inspect several of the high-Z rods withthe n TOF beam, in order to verify whether and to what extent cracks originated on thesurface 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 antiprotondecelerator (AD) target core to be fractured and damaged inside (see Fig. 2 for anexploded view of the assembly). Despite the very high dose rate in the order of severalmSv/h at 10 cm, EN/STI is planning to open one of the spent target in the radioactiveworkshop in building 867. In the light of this delicate intervention, a neutron radiographable to provide information on the status of the core, extrapolating whether it is intact,

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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 interventionplanning matters. The tests in EAR2 executed with the big collimator in 2016 on afresh, non-radioactive, antiproton decelerator target proved that the facility has enoughsensitivity to assess this point.

1.3 Neutron PIE of a graphite-embedded proton irradiated tan-talum 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 yieldstrength), another HiRadMat experiment will be executed in May 2017 to check howa 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 touse the n TOF neutron radiography station to evaluate the response of the discs to thebeam-induced thermal stresses before opening the Ti6Al4V vessel. X-ray will not be fullyadapted for this scope, due to the low sensitivity in the area between the Ta and thegraphite disks. For a direct comparison, we propose to carry out the neutron radiographyinspection before and after the irradiation.

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2 Experimental setup

Conventional neutron-imaging techniques are based on mapping the attenuation of aneutron beam when transmitted through a sample. The resulting intensity map can berepresented as an image with two main parameters, spatial resolution and contrast. Whilecontrast depends on the beam intensity, the radiograph resolution is determined by severalfactors [4], in particular by the beam divergence D/L, where D is the size of collimatorinlet aperture and L the distance between the inlet aperture and the point in the objectto be analyzed, and by the distance l between the point in the object to be analyzed andthe image detection plane. At n TOF, the diameter of the collimator inlet aperture couldhave two sizes, 218 mm and 667 mm. This parameter given, it is possible to adjust bothl and L in the experimental area to optimize the achievable resolution. The converterthickness and the distance between converter and film (preferably at contact) also playan important role for the resolution, and are optimized in the imaging system.Besides the geometrical resolution, there are several indicators that can describe the imagequality, 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 powerfultool to measure and predict system’s performances, representing the ratio of the magni-tude of the system output to the magnitude of the system input [5]. This method hasbeen 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 TOFsecond experimental area (EAR2) has been successfully proved [1]. The station doesnot imply any modification to the n TOF beam line, and it is positioned at a distanceof 220 cm from the exit of the collimator. This position has been chosen as the bestcompromise 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 downstreamin the vertical direction. The sample (stainless steel cylinder in the right picture) can bemoved in both the direction of the horizontal plane so to have a complete scan.

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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 beamitself. To perform a complete scan of the object under investigation, the measuring stationallows to shift it in the two directions of the horizontal plane. In Fig. 4 a picture of thestation is showed.A commercially available precise optic neutron imaging system from Photonic Scienceis used. It is based on an air-cooled SCMOS camera coupled with a ZnS/6LiF neutronscintillator, that emits at 520nm, of dimensions 100 × 100 mm2. The scintillator hasapproximately 100 µm thickness, optimized for resolution. The camera sensor has anactive input area of 13.3 × 13.3 mm2 mapped by 2048 × 2048 pixels, with an opticalpixel 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 frequencyand 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 thebackground coming from the experimental area and the radioactivity of the object underinvestigation.The station has been tested at the n TOF EAR2 with both the small - 218 mm innerdiameter - and the big - 667 mm inner diameter - collimator. The characteristics of theneutron beam have been evaluated through simulations. In particular, as can be seen inFig. 5, at the experimental location - 220 cm high from the collimator pinhole - about1 × 106 for the big and 6 × 105 for the small collimator thermal neutrons/cm2/pulseare expected. In the optimistic case of 1 pulse every 1.2 seconds, about 8 × 105 and5 × 105 thermal neutrons/cm2/pulse can be expected on the samples, respectively. Thebeam profile is expected to be between 4 and 6 cm diameter for the small collimator, andbetween 9 and 11 cm diameter for the big one, the difference depending on the sensitivityto the beam halo.During the tests in 2015 and 2016, a spare non-irradiated AD target has been investigatedin both the tests, and the results are shown in the two panels of Fig. 6 (left for the smalland right for the big collimator, respectively). As can be seen, in both cases the innerIridium 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 - 667 mm innerdiameter - (red line) and the small collimator - 218 mm inner diameter - (black line).

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Figure 6: Comparison of the image of a spare AD target obtained at the n TOF EAR2imaging station for the small (left panel) and big (right panel) collimator.

feasibility of the technique. While with the small collimator the highest spatial resolutionis reachable due to the lower D/L ratio, the advantage in using the big collimator is thata much larger surface is covered, with a higher instantaneous flux and thus improvedcontrast. As can be seen from Fig. 5 in fact, the highest neutron beam intensity coversa spot of about 8 cm diameter, while with the small collimator it covers a spot of only 2cm diameter.As previously mentioned, the quality of the imaging system has been analyzed througha MTF analysis performed with the software ImageJ [6]. The spatial resolution has beenfound 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 smallestdimensions of the inner structures of the objects under investigation are of the orders offew 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 theHRMT27 rods irradiated at the HiRadMat facility. In the light of the results obtainedfrom the test, the big collimator will be used, since it provides a still suitable spatialresolution and a much larger active area. Due to the geometry of the object underinvestigation, 4 to 5 scans are needed for the AD target and 3 to 4 for the HRMT27 rods.An average number of protons of 2× 1016 has been associated to each scan to accumulatesufficient statistics.Together with the scans of the object, background measurement should be foreseen. Theseare particularly important due to the high radioactivity of the irradiated objects, about5 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 importantto properly characterize the effects on the background to reach a satisfactory level ofresolution and contrast.Summary of requested protons: 1 × 1018 protons on target or or 1.5 weeks of mea-surements.

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References

[1] M. Calviani et al., CERN-INTC-2014-070 / INTC-I-160,http://cds.cern.ch/record/1979249/files/INTC-I-160.pdf (2014)

[2] C. Torregrosa et al., Proceedings of IPAC2016, Busan, Korea,http://jacow.org/ipac2016/papers/thpmy023.pdf (2016)

[3] C. Torregrosa et al., CERN-EN-2016-004, http://cds.cern.ch/record/2216475/files/CERN-EN-2016-004.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 radioscopiesystem characterization, Applied Optics 28 (1989) 5002

[6] https://imagej.nih.gov/ij/

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