J-PARC and the Prospective Neutron Sciences

PRAMANA °c Indian Academy of Sciences Vol. 71, No. 4 — journal of October 2008 physics pp. 629–638

J-PARC and the prospective neutron sciences

MASATOSHI ARAI J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan E-mail: [email protected]

Abstract. Overview of the neutron target system, instrument suite and perspective neutron sciences of J-PARC are described. The neutron facility of J-PARC, JSNS, will be operated from May 2008. JSNS will be a 1 MW pulsed spallation neutron source. About 10 high performance instruments are under construction to be ready by the Day-One.

Keywords. J-PARC; pulsed neutron source; neutron scattering.

PACS No. 61.05.-F

1. Introduction

J-PARC is an interdisciplinary facility with high power proton accelerator complex to be completed by 2008 (figure 1). Materials-Life Science Facility (MLF) will be a very intensive pulsed neutron and muon facility at 1 MW of the accelerated proton power. The neutron peak flux will be as high as several hundred times of existing high flux reactors. The accelerator accelerates protons at 3 GeV and the current of 333 µA, operated at 25 Hz with the pulse duration is about 1 µs. Protons of 2 × 1015 in one accelerated proton pulse bombard the mercury target and create neutrons of 1.2 × 1017 instantaneously. This is equivalent to 3 × 1018 neutrons/s in time average and is comparable to that of a 15 MW research reactor. However, the instantaneous flux, the pulse peak flux, is 100 times higher than that of ILL reactor, which is the most intensive research reactor in the world. Therefore, if we can prepare suitable instruments, such as time-of-flight instruments, to handle such a high peak flux in a proper way, the performance of instrument can be really 100 times higher for actual experiments. Pulsed neutron scattering technique was started in the electron LINAC of Tohoku University in 1970s in Japan, who is one of pioneers of this technique. It was followed by spallation neutron sources using proton accelerators, in the KENS facility in KEK in 1980, Japan, the IPNS facility in USA in 1982 and the ISIS facility in UK in 1985. In the early days for the pulsed neutron technique, its usefulness was not recognized. However, honest efforts by the foregoing facilities have proved abilities, efficiency and usefulness of a pulsed neutron source. Because of this the current world trend of futuristic neutron sources seems to be pulsed neutron sources as seen in the construction of SNS in USA starting operation in

629 Masatoshi Arai

Figure 1. Arial view of J-PARC. The picture was taken in May 2007.

2006, JSNS of J-PARC in Japan to be operated in 2008, CSNS under construction in China and ESS under discussion in Europe.

2. Neutron production principles for spallation sources

Spallation reactions effectively occur above 100 MeV of proton energies. High energy neutrons, pions, and smashed nuclei caused by this reaction make inter- nuclear cascades followed by low energy neutron evaporation from excited nuclei, giving a very high efficiency to create neutrons, having a maximum in spectrum at around 2 MeV for the evaporation neutrons. For instance, a 1 GeV proton produces about 25 neutrons from a lead target, with low heat deposition in the target (figure 2). Studies in 1990s proved that the neutron production yield by the spallation reaction is almost proportional to the accelerator power even higher than 12 GeV [1]. This resulted in the flexibility to optimize accelerator and neutron target design, where the key parameter is the power of accelerator by allowing various combina- tion of proton energies and currents. J-PARC uses 3 GeV protons of 333 µA in current to produce 1 MW in power.

3. Moderator performances

For neutron scattering experiments for materials research, it is necessary to reduce neutron energies created by spallation process down to a region of about 100 meV or 1 A˚ by thermalization process in moderators.

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Figure 2. Nuclear spallation by proton bombardment [2].

Figure 3. Neutron energy distribution (ﬂux) of J-PARC neutron source for coupled, decoupled and poisoned decoupled moderators. The ﬂux con- sists of a Maxwell distribution at low energies and a 1/E distribution at high energies [4].

Short pulsed spallation sources need to have a sharp pulse structure of thermalized neutrons. Hence the moderator dimension is small and optimized at around 10 × 10 × 10 cm3 to reduce time duration in scattering processes in the moderator. Thermalization process makes the Maxwell distribution associated with a 1/E distribution at the higher energy region (ﬁgure 3).

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Figure 4. Peak time width of neutrons for three kinds of moderators of JSNS. Ports 8 and 19 view the same poisoned moderator at the opposite surfaces, where the depths of poisoning plate from the moderator surfaces are 2.5 cm and 4 cm respectively. In the 1/E region for each moderator the pulse width is proportional as ∆t (µs) ∼ √ 2 ∼ 7λ (A)˚ [5]. E (eV)

The pulse width is a key parameter for short pulse spallation sources and it directly influences the instrument performance. It is almost proportional to the neutron wavelength in the 1/E region of flux and is broadened in the thermal equilibrium region, then saturated at the low energy region. The sharp nature can be extendable to the lower energy by cooling moderator. Hence, JSNS has chosen 20 K supercritical H2 moderators, in which broadening starts to occur below about 15 meV (2.5 A).˚ The proportionality between the pulse width and wavelength is of great importance for high resolution instruments, as we see in the later section (figure 4). JSNS, J-PARC has three kinds of 20 K hydrogen moderators: (1) Coupled moderator for high flux experiments. (2) Decoupled moderator for high resolution experiments. (3) Poisoned decoupled moderator for very high resolution experiments. The coupled moderator is composed of two layers. One layer is the 20 K hydrogen moderator at the core. Other layer, the so-called pre-moderator [3], is the ambient water moderator surrounding the core to effectively thermalize high energy neutrons entered from reflector and rid off the major heat load before neutrons entering the core moderator. This double-layered structure effectively produces very high flux cold neutrons (figure 5). ‘Coupling’ means that there is no barrier for neutron transport between reflector and moderator. So reflected neutrons from the reflector can return to the moderator without obstruction, and hence output flux from a beam port is very high

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Figure 5. Design of a coupled moderator for J-PARC. The hydrogen moderator is surrounded by water pre-moderator, which reduces neutron energy in the ﬁrst stage and removes major heat load before neutrons go into the hydrogen moderator [6].

(109 n/cm2/s), although the peak time width is wide, since neutrons can spend longer time in the reflector and the moderator before going out from the moderator. On the other hand, the decoupled structure has a neutron absorber between reflector and moderator. Low energy neutrons, which are thermalized in reflector, can be shut out from moderator, effectively the peak time width of neutrons be- comes sharp as a consequence. Poisoned moderator has additionally an absorber plate in the moderator to make the peak time width much sharper. Figures 3 and 4 show the expected neutron peak intensity and peak time width of those three moderators respectively. As we see the performance of moderators and scientific demands from users, we decided to have 11 beam ports for the coupled moderator, 6 beam ports for each of the decoupled and poisoned decoupled moderators out of the 23 total beam ports. This feature is very unique in spallation facilities.

4. Time-of-ﬂight methods in a spallation source

Since neutrons start to propagate after emission time when protons bombard neutron targets, the time-of-flight (TOF) measurement is a natural way to estimate neutron energy/wavelength. Neutrons propagating along a flight path, L1, are scattered by a sample and detected by a detector at a certain scattering angle, as shown in figure 6. Peak pulse width is conserved during the propagation from the source to the detector. Because of this, a sharp peak width from a moderator is preferable for a high resolution measurement with long flight path, separating peaks at the detector. 1 A˚ neutrons, whose speed is about 4000 m/s has a time width of about 10 µs. ∆d ∆t 10 µs Hence if L1 is 40 m, then the resolution can be d = t = 10000 µs = 0.001 for

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Figure 6. Time-of-flight–flight length diagram for elastic diffraction measurement. Neutrons are created at the time zero, and propagate along flight path by taking time according to their velocities. Longer flight length gives better peak separation and resolution, since the pulse peak width is conserved. a back scattering measurement. Hence, in a pulsed neutron source, flight path, is a good measure for resolution of instruments, longer the flight path, better is the resolution. For an inelastic measurement, one typical instrument is a chopper spectrometer. A fast rotating chopper is inserted in the incoming flight path before sample position, which monochromates the neutrons incident on the sample. Due to inelastic scattering in the sample, the scattered neutrons change their energy/speed and are detected by detector at a different time according to their energy while maintaining the initial peak time width. Hence, the length of the scattering flight path L2 is important to resolve the difference in the speeds of the scattered neutrons. If L2 ∆E ∆t 20 µs is 4 m, the energy resolution can be E = 2 t = 1000 µs = 0.02, for elastically scattered neutrons.

5. Neutron instrument suite and sciences in JSNS

Unique characteristics of neutrons enable scientists to study materials through atomic to macroscopic scale. Neutrons are scattered by nuclei in materials; this property gives comparable scattering cross-section from hydrogen, lithium etc. to iron and other heavy metals. Also isotope substitution contrast is a really unique character of neutron scattering technique. Spin of neutron is very suitable to understand magnetic structures. However, the most outstanding character of neutrons is that both wavelength and the energy matched with atomic spacing and the energy at the same time, and hence neutrons are one of the most suitable probes to study atomic structure and the dynamics at the same time, the so-called study space–time correlations, in any materials. The neutron ﬂux has been rather small to study dynamics in materials, since the inelastically scattered signal from dynamics is much less than elastically scattered neutron intensity. J-PARC will give an

634 Pramana – J. Phys., Vol. 71, No. 4, October 2008 J-PARC and the prospective neutron sciences excellent platform to do so for the ﬁrst time. Therefore, in this section we start with inelastic instrument suite in JSNS, J-PARC.

5.1 Inelastic instrument suite in JSNS

We are planning to have four inelastic scattering instruments. Two of them will be operational by December 2008. Figure 7 shows the energy–momentum space covered by those instruments. (1) High intensity chopper instrument (4SEASONS) [7] This spectrometer has been designed to study mechanism of high-Tc supercon- ductors and related materials. The resolution is relaxed to match with requirements for the target studies. The intensity will be very high at the sample position, which is surrounded by 2D detector system, and cover a conventional energy–momentum space. Multiplicity of incident energy in one measurement is planned by developing a new type of Fermi chopper system. (2) Low energy chopper instrument (AMATERAS) [8] This instrument is aiming at having very high performance below 20 meV for the incident energies. The energy resolution is about 1% of incident energy through 1 meV to 80 meV; for instance 15 µeV in resolution for 1 meV incidence. This instrument can be used in various scientific fields from biomaterials to highly correlated electron systems. (3) High resolution chopper instrument (HRC) [9] This is a conventional Fermi chopper instrument viewing decoupled moderator. The coverage in the energy–momentum space is very wide, up to 2 eV, with high energy resolution of 1% of incidence, and can be applicable in various scientific fields, such as magnetism, highly correlated electron systems etc. (4) Backscattering instrument for biomaterials (DNA) [10] DNA is a backscattering instrument with very high energy resolution of 2 µeV with very unique features. DNA views coupled moderator. A fast disc-chopper cut out a slice from the original broad peak with maintaining very high flux, and Si analyzer defines the final energy resolution. Hence, very good energy resolution can be realized with a fairly short flight path of instrument emphasizing very high intensity and efficiency. As the name shows, this instrument will be used for studying the mechanism of biomaterial’s functions, whose sample size is normally very small down to 1 mg range.

5.2 Diﬀractometers in JSNS

Pulsed neutron sources are naturally very suitable for diffraction measurements. As shown in figure 6, neutrons produced in the moderator propagate along the flight path and the energy/wavelength is analysed by the arriving time at the detector. Hence, without sacrificing incident neutrons, diffraction measurements can be achieved in a very effective way. Figure 8 shows a map of the performances of

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Figure 7. Energy–momentum space which is covered by inelastic instrument suite in MLF. renowned diffractometers in the world. Diffractometers under construction in JSNS are categorized as the so-called third-generation diffractometers in the map. Those have the highest resolution or are the most intense among diffractometers. (1) High resolution powder diffractometer (SHRPD) This diffractometer views the decoupled poisoned moderator and has 100 m flight path, realizing a resolution of ∆d/d = 0.03%, which is comparable to an intrinsic broadening width of diffraction peaks due to the surface effects of powder samples. Hence this is the ultimate resolution required for diffraction measurements. SHRPD can distinguish a tiny difference/distortion between lattice structure of nuclei and that of electrons. Lattice distortion is becoming the key element for functional materials, such as multiferroic materials and highly correlated electron systems. Hence, application of SHRPD can elucidate and find out relations between the atomic structure and functionality of materials. (2) High intensity powder diffractometer by Ibaraki prefecture (iMATERIA) [11] This diffractometer will be dedicated for the industrial use of neutrons. It has a very high throughput, 20,000 samples per year, and equipped with a sample exchange robot for quick measurements. The resolution is about 0.3% with very high flux. (3) Residual stress analysis diffractometer (TAKUMI) [12] This diffractometer equipped with a 1 ton gonio table treats mechanical components and measures stresses/strains with an accuracy of +/− 10−4–10−5 in a gauge

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Figure 8. Performance of diffractometers in JSNS. Resolution and relative intensity of diffractometers are illustrated. volume of 1 mm3. The detector system is a Li–ZnS scintillation detector, which was developed under collaboration with ISIS in UK. (4) Biomolecule diffractometer by Ibaraki prefecture (iBIX) [13] This diffractometer will be used for structural analysis of biomolecule crystals up to 135 A of lattice constants. More than 50% of atoms in biomolecules are hydrogen atoms. Therefore it is very important to refine crystal structure including hydrogen/protons and hydration water molecules, whose knowledge leads effective pharmaceutical developments. (5) High intensity total scattering diffractometer (NOVA) This instrument has been funded by the New Energy Development Organization (NEDO) aiming at studying the behaviour/structure of hydrogen atoms in materials to develop fuel cell of high performance. The instrument has a relaxed resolution but has one of the most intensive flux as a neutron diffractometer.

5.3 Other instruments under development

The small/wide angle diffractometer [14], reflectometer and spin echo instruments are of great demand. Design and developments of components for those instruments are under way. We are also making efforts to get construction budget from the government. The development of high pressure apparatus producing 30 GPa and

Pramana – J. Phys., Vol. 71, No. 4, October 2008 637 Masatoshi Arai pulsed high ﬁeld magnets producing 60 T with a time duration of 1 ms are also supported by grants.

Acknowledgements

I sincerely appreciate Prof. Nagamiya and all the colleagues who worked with me for the J-PARC construction.

References

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