Design and GEANT4 simulations of a neutron ...

Report n. AC-09-01

‘Design and GEANT4 simulations of a neutron collimator optimized for NRCI at the INES beamline’

Enrico Perelli Cippo

Integrating and Strengthening the European Research Area NEST - New and Emerging Science and Technology Report series editor: G. Gorini, Università degli Studi Milano Bicocca (Italy)

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Design and GEANT4 simulations of a neutron collimator optimized for NRCI at the INES beamline

Enrico Perelli Cippo

A neutron collimator has been designed for Neutron Resonance Capture Imaging (NRCI) applications at the INES beamline of the ISIS spallation neutron source. Use of a suitable collimator is needed in order to select a small region of the sample to be illuminated by a pencil neutron beam. The neutron resonant absorption and consequent prompt gamma emission are thus localized in a small region, and they can thus be reconstructed by scanning the various positions permitted by a motorized sample holder. Simulations have been performed with the GEANT4 code [1] in order to determine the optimal characteristics of the collimator in terms of neutron absorption and resolution. The optimized collimator allows for a 5 mm diameter neutron spot at the sample position.

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1. Introduction

A neutron collimator is under development within the Ancient Charm project. Main characteristics of this collimations should be: a) effective collimation of neutrons with energies up to 1 keV; b) low gamma-ray emission upon neutron capture; c) compatibility with the existing INES beamline. Previous experiments conducted at the VESUVIO and INES beamlines with a pencil beam collimation made of boron carbide have shown an increase in gamma background when the collimation was in place [2]. The reason of that is twofold: 1) the 478 keV gamma emission from boron upon neutron capture 2) the overall increase of neutrons scattered inside the block-house, determined by the collimation elements, resulting in gamma emission from various metal hardware. This resulted in a worse Signal/Background ratio than without the collimation, especially for those resonance energies that are located at high energy (typically E> 100 eV). This has led to the choice of lithium carbonate as an alternative material for the NRCI collimation. As it is known, 6Li has a relatively high neutron absorption cross section via the reaction: 3

Li 6 + 0 n1 →

2

He4 + 1 H 3

Q - value 4.78 MeV

that does not produce gamma-rays. Lithium carbonate is available in the form of powder, but it can be inserted in epoxy resin matrix up to a concentration of 60% by weight; the artefacts made with that mixture show reasonable mechanical characteristics and can be machined and drilled with standard tools (opposite, in example, to lithium nitride and sintered boron carbide).

2. Technical constraints

The pencil-beam collimation for ANCHIENT CHARM is to be provided for the INES beamline at the ISIS spallation neutron source. The layout of the existing INES/TOSCA collimation is presented in the ISIS drawing No. A 1-S1-2541-309-00-A . A simplified layout with the main characteristics underlined is presented in figure 1. The shape and dimensions of the umbra (the spot at the sample position in which the beam intensity is maximum) and penumbra (the maximum area of the beam) are determined by all the collimation elements along the beamline. We have to note that collimation elements for -3-

Primary collimation and TOSCA chopper umbra

TOSCA shutter

Moderator

TOSCA sample area

penumbra INES shutter

22800 mm

Figure 1: layout (simplified) of the TOSCA-INES beamline. The distance between the moderator surface and the INES sample position is 22.8 m. The umbra and penumbra of the beam at the INES sample position are determined by the geometry of the collimation elements along the beamline. The INES collimation allows for a square 40 mm x 40 mm umbra and a wider penumbra.

INES are also positioned inside the shutter, and are made of Boron Carbide and iron shot in epoxy matrix. Of the whole beamline, the part that was made available for modifications was the INES shutter. Moreover, some space is available in the INES block-hose before the main vacuum tank.

3. Simulations and design

GEANT4 was chosen as the most suitable simulation package, because it allows to simulate the full interaction of neutrons with matter. Other simulation packages usually only consider reflection or full absorption of (thermal) neutrons in matter, and do not consider the possibility of neutrons being transmitted through it. While this effect can be usually ignored for thermal neutrons, it becomes important in the epithermal energy range. As an energy distribution for the simulations, it was chosen the power function Φ( E ) = A

1 E 0 .9

This is the shape of the flux for undermoderated neutron beamlines like INES [3]. -4-

30

20

Y (mm)

10

0

-10

-20

-30 -30

-20

-10

0

10

20

30

X (mm)

Figure 2: result of a preliminary simulation of the neutron collimator for NRCI. The red circle is the calculated umbra, while the black dots area is the penumbra. The black dots actually represent simulated impacts of neutrons on the sample surface.

The main goal of the simulations was to obtain a collimator in which the full neutron spot (i. e. umbra and penumbra) of diameter 5 mm. Indeed, standard collimators in use at ISIS provide a penumbra usually far larger than the umbra; if this is not important for standard diffraction experiments, it is not acceptable in NRCI, where the dimensions of the neutron spot determine the resolution of the technique. In figure 2 it is shown the result of a preliminary simulation, in which the umbra is of the required diameter, but the penumbra is so intense that the overall neutron spot is of about 4 cm diameter. In order to squeeze the penumbra to a reasonable extent while allowing a full intensity beam in the umbra, the collimation was designed in two pieces: an in-shutter component to define the umbra and cut away most of the neutrons outside it, and a block-house component, positioned closer to the sample, in order to scrap the penumbra and reduce the amount of neutrons scattered at low angle by the previous element (see figure 3). Both collimation elements are designed with the structure of “irises” (i. e. wider and smaller alternated holes). As underlined by Winsor [4] the cylindrical hole surfaces “seen” by both moderator and sample area are the main cause of grazing angle scattered neutrons. This surface reduced by splitting the collimator into a series of relatively thin irises. For the epithermal neutron collimator, however, the space between the irises is occupied by wider-drilled Lithium carbonate elements (figure 4). Simulations were conducted with models with different diameters of the beam hole and positions along the available space. The final configuration was the one matching the requested performance, presenting a beam spot (umbra and penumbra) of about 5 mm -5-

INES shutter AC block-house collimation

Sample position Pre-existing INES collimation

AC shutter collimation

450 mm

400 mm

Figure 3: layout of the NRCI collimation for INES. The first collimation element is inside the INES shutter, along with the previous INES collimation. A further element is inside the block-house, on a positioning system.

INES shutter collimation (already present)

150 mm

50 mm

Variable diameter

AC shutter collimation 150 mm Variable diameter

AC block-house collimation 100 mm

100 mm

Figure 4: layout of the shutter collimation and block-house collimation for INES. It is made following the scheme of separate irises, in order to reduce grazing-angle scattering of neutrons. The block-house collimation follows the same principle.

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diameter (see figure 5). We have to note that the spot presents a quite sharp transition between the fully illuminated part and the non-illuminated part; in other words, the penumbra is reduced. This lead to a uniform beam inside the whole spot.

4. Conclusions

The neutron pencil beam collimator elements have been designed following the layout of the final simulated collimation. A spare collimation element was also made in low-antimony lead alloy, and it can be used as a further shielding for gamma rays associated with the neutron beam. The block-house AC collimation was installed on a remotely-controlled movable system [5] in order to be carefully aligned with the shutter collimation and sample. Preliminary tests on the pencil beam collimation were conducted in December 2008. After installation of the full collimation, the neutron beam was scanned with a 2x2 mm indium sample. The gamma rays emitted after neutron capture were detected by a series of gamma detectors and recorded in time-of-flight spectra, as in a realistic NRCI set-up. In order to isolate the contribution of the epithermal neutrons, a scan image was produced of the 9 eV resonance of In111. In other words, an image was produced of the space-related intensity of such resonance in a plane perpendicular to the beam, at sample position (see figure 6). Such an image is the requested map of intensity of the neutron beam at epithermal energy, and represents the proof of the effectiveness of the pencil-beam collimation. The tests have demonstrated the that an effective pencil beam of epithermal neutrons can be created by appropriate collimation.

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Figure 5: simulated neutron spot obtained with the final configuration for the AC collimation for NRCI. The diameter of the full spot (umbra + penumbra) is about 5 mm. Umbra and penumbra are virtually indistinguishable.

Figure 6: Scan of the neutron beam produced by the AC pencil-beam collimation at an energy of 9 eV (corresponding to the In111) resonance. The FWHM of the beam spot is about 5 mm.

References

[1] S. Agostinelli et al., “GEANT4: a simulation toolkit” Nucl. Instrum. Meth. A, 506 250 (2003) [2] E. Perelli Cippo, M. Tardocchi, “Effect on the background of the B4C collimation (“Insert Collimation”) in the VESUVIO beam line”, private communication [3] A. D. Taylor 1984 “SNS moderator performance predictions” Rutherford Appleton -8-

Laboratory Report RAL-84-120 (Oxford: Rutherford Appleton Laboratory) [4] C. G. Winsor “The degradation of a pulsed neutron beam by inelastically scattering collimators” Nucl. Instrum. Meth. A 243 470 (1986) [5] W. A. Kockelmann “Memo on ANCIENT CHARM pencil-beam collimation”, private communication

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