Recent Developments of Instruments in a Spallation

Proc. QENS/WINS 2012 J. Phys. Soc. Jpn. 82 (2013) SA024 ©2013 The Physical Society of Japan

Recent Developments of Instruments in a Spallation Neutron Source at J-PARC and Those Prospects in the Future Masatoshi Arai1 , Ryoichi Kajimoto2 , Mitsutaka Nakamura1 , Yasuhiro Inamura1 , Kenji Nakajima1 , Kaoru Shibata1 , Nobuaki Takahashi1 , Junich Suzuki2 , Shinichi Takata1 , Takeshi Yamada2 , Shinichi Itoh3 1

Materials Life Science Division, J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 2 Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan 3 Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan E-mail: [email protected] (Received December 24, 2012) The paper describes developments of neutron scattering instruments in J-PARC neutron facility, which is a short pulse spallation source operated at a repetition rate 25 Hz and has a proton power 1 MW. New concept has been adopted to utilize high intensity coupled moderator even for a high resolution inelastic scattering instruments by taking a pulse shaping chopper system. Advanced technique has been also realized by taking an ability of event-recording system altogether with an improvement of a chopper system, realizing multi-Ei method. The paper also touches upon a general design concept for diffractometers in accordance with the source characteristics. KEYWORDS: pulsed neutron, instrumentation, chopper, backscattering, pulse shaping

1.

Introduction

In these years new pulsed spallation neutron sources have been come on-line with an enormous increase of neutron flux of two orders of magnitude. At the same time new ideas/deliberation for instrumentation have proposed/done and implemented as an actual user instruments. Those achievements are not only due to improvement of hardware components but to advancement of software, data acquisition electronics and computing power. Use of pulse shaping chopper applied to a broad but intense pulse peak can enhance neutron flux with a reasonable resolution and has given a great flexibility to an instrument associated with an ability of the multi-Ei measurement (multi repetition rate). Event recording data acquisition has enabled a real-time, in-situ measurement, asynchronous chopper phasing and even continuous sample rotation measurement to observe a full image of S (Q, E) in the four dimensional phase space. A large area detector gives huge amount of data-set and data analysis cannot be done by pencil & graph-paper work anymore. Instead, Giga Bytes of pixilated picture of scattering image has petitioned a new dimension of data reduction treatment and sophisticated data analysis software. When we consider a design of instrument in a pulsed source, especially in a short pulsed source, it is quite important to have a mutual matching among accelerator, target and instrument specification. All of those are components to be optimized to have a satisfactory performance of instrument. In this paper we review the recent developments of pulsed neutron instrumentation at J-PARC neutron source and foresee prospects in the coming future. Note that the current power of accelerator 1 SA024-1

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is 300 kW, and we are planning to ramp up to 1 MW, final goal, in several years.

2.

Neutron Source Characteristics and Importance of Phase Synchronization with Accelerators

J-PARC Materials Life Science Facility (MLF) neutron target station is 1 MW source operated at 25 Hz [1]. The mercury target will accept 333 µA of 3 GeV protons with an effective time width of 1 µs, two bunches with 150 ns width separated by 750 ns. Details of the target station performance are shown in Ref. 2. One of the most important cooperation between the accelerator team and the neutron team is an introduction of a master clock for the accelerator control. Now the accelerator is extremely accurately controlled with a master clock of 12 MHz with time jitter of 1.7 ns [3]. This scheme is quite important for instrument control, especially chopper control. We require a time jitter less than 0.3 µs for a Fermi chopper phase-control above 1 eV neutrons, where the intrinsic time width from moderator follows √ ∆tm [µs] = 2/ E[eV] [4]. Synchronization to the main is much easier technique for accelerator, but it gives 20 µs, 0.1% of the main frequency, in time jitter. Accelerator needed additional power feedback system to follow the master clock, however, control timing for whole accelerator system became very stable as well as control for whole chopper system of neutron instruments. There can be an alternative method to achieve the required timing accuracy, however, once a master clock is introduced; it becomes very simple and easy for all kind of in-phase device not only for chopper system but for a real time measurement etc. to have a precise synchronization even among them. We took the repetition rate 25 Hz for accelerator. It gives a wide dynamical range in one frame for time-of-flight instruments and high pulse-peak intensity. This choice again gave a burden for accelerator and target technology; one proton bunch contains larger number of protons than that for higher frequency, giving a higher space-charge effect and a worse damage in the target. For typical instrument with 20 m flight path, the dynamical range is about 8 Å in the wave length, or 1 meV to 10 eV in energy. This repetition rate is a great compromise to satisfy both of high energy spectroscopy and slow neutron experiments, and it is also good for high resolution instruments with a long flight path.

3.

Neutron Pulse Structure from the Source

When protons bombard the target, high energy neutrons are created, go out radially, some of neutrons scattered back by reflector and go into a moderator, where neutrons loose energy by scattering with hydrogenous materials, and are thermalized to the moderator temperature. Slowed down neutrons are lead to instruments and used for a scattering experiment. MLF has three cryogenic super-critical hydrogen moderators (20 K, 1.5 MPa) [5]. The coupled moderator (CM) gives high intensity with a broad peak structure, which is surrounded by an ambient water pre-moderator to slow down high energy neutrons and remove unnecessary heat deposit before neutrons go into CM. Since CM occupies the optimized space by itself below the target, hence the performance is quite outstanding. A decoupled moderator (DM) and a poisoned decoupled moderator (PM) produce sharp pulse structure and are suited to medium resolution and high resolution instruments, although the intensity is sacrificed considerably. Details of the pulse shape from the moderators are described in Ref. 6. Figure 1 shows calculated peak structure for each moderator at various neutron energies. Broadness and peak intensity of CM are much enhanced at low energy below around 30 meV. At higher energy the peak structure becomes similar to each other among moderators. 2

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Fig. 1. Pulse structures of neutrons from CM, DM, PM at 5, 20 and 80 meV respectively [blue: CM, green: DM, black: PM (4 cm depth), red: PM (2.5 cm depth)] [5].

The pulse width from PM is almost proportional to the wavelength (This is independent from moderator material [7].); √ ∆tm [µs] = 7 · λ[Å] = 2/ E[eV]. (1) This feature is quite important for high resolution diffraction, especially powder diffraction, where constant resolution is required in a wide d-spacing. We return to this point later.

4.

TOF Diffraction

4.1

Principle of TOF diffraction The energy/wavelength of neutrons flying to diffractometer is analyzed by time-of-flight recorded at detectors. Hence all of neutrons emitted from moderator through the flight path come to a sample, scattered and then detected by detector. Therefore, it is a quite effective use of neutrons. We do not need monochrometer before the sample. Figure 2 shows an illustration of principle of a TOF diffractometer. Pulse structure does not change during flight. Hence either sharp peak or long flight path gives high resolution. Spallation neutron source is very rich with epithermal neutrons, for which neutron guide is not effective to transport. Therefore, if an instrument, like a liquid-amorphous diffractometer, needs to utilize such higher energy neutrons, then the instrument length should choose a short flight path. The resolution of diffractometer is described as; ( ( )2 )2 ∆Q ∆t 2 = (cot θ · ∆θ) + . (2) Q t

There are only two important terms. One is angular term and another is timing term. The relation

Flight distance

Bragg peaks

Scattering Angle 2θ θ

detecotor L2

L2

sample

detectors

L1

L1 Time of Flight

Fig. 2.

Source

3

Layout of time-of-flight diffraction.

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Fig. 3. Black line for detector arrangement gives constant resolution at any scattering angle. However it needs infinite detector tail along the small angle direction and unrealistic. Therefore compromised arrangement becomes as drawn in red.

Fig. 4. Detector arrangement of SHRPD.

between time-of-flight and wavelength is; t[µs] =

mλL λ[Å] · L[cm] = . 2π 0.3956

(3)

Here, we note importance of relation (1) in order to realize a constant Q-resolution along Q; Eq. (1) realizes the second term constant in Eq. (2). Because of this character diffractometers are traditionally installed at a port viewing either DM or PM. Assuming an instrument viewing a PM, then from Eqs. (2) and (3) with Eq. (1) it is easily estimated a possible resolution at a high angle detector at repetition frequency of 25 Hz. For instance, for a flight path length L1 = 100 m, then the resolution becomes Q/Q = 0.04% with the condition of Eq. (1), and the wavelength band λ = 1–2.5 Å, resulting the dynamical range of Q = 0.1–10 Å−1 . 4.2

Detector arrangement for a resolution oriented instrument (powder diffractometer) If we want to have an instrument, which gives constant-Q resolution at any detector angle, a balance between the angular term and the timing term (path length) gives the layout of detector arrangement as seen as black line in Fig. 3 [8]. However, this needs enormous cost for detector and is unrealistic because of infinite tail at the small angle direction. If the source frequency is high enough, say 50 or 60 Hz, detector should be continuously arranged through scattering angle to cover a required Q-range as shown in POWGEN in SNS [9]. However, it can be arranged discontinuously in a low frequency source, such as J-PARC, since wide wavelength band in a frame can give a wide Q range for each detector bank as shown as red lines in Fig. 3 and a real instrument becomes as seen in Fig. 4 [10]. 4.3

Total scattering diffractometer Another kind of diffractometer has a quite different concept for its design. This is so called a total scattering diffractometer, which is used to analyze structure of liquids and amorphous, whose diffraction does not show any sharp Bragg peaks, but a smooth structure, containing dynamical intensity by energy integration through a scattering locus, giving an instantaneous atomic correlation. Those materials have not regular atomic periodicity, hence, it is important to have a very wide dynamical range in the momentum transfer, Q, so that Fourier transformation truncation error is minimized. However, the energy integration gives serious difficulty to give a precise structural information especially for light elements. Those have a recoiling effect [11] on scattering, and emphasize inelasticity effect in the diffraction data [12]. Therefore, a design work on detector arrangement should take both of the resolution effect and the inelasticity effect at the same time [13]. Here an extreme case for a scattering from hydrogen (H2 ) was examined by a free gas model [11], 4

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Fig. 6. detector arrangement for a total scattering instrument TAIKAN. Detector banks are declined so as to optimize the resolution effect and inelasticity effect.

Fig. 5. S (Q, E) for a free gas model and scattering locus for a time of flight measurement. Two lines are scanning locus at the nearest edges of the small (7 deg, red) and medium angle (14 deg, blue) bank for the same Qe .

Eq. (4), which gives the dynamical structure factor as followings, and shown in Fig. 5. (

β S (Q, ω) = 4πEr

) 12

[ ] β 2 exp − (ω − Er ) , 4πEr

(4)

where β = 1/kB T and Er = 2 Q2 /2mp . Elastic component is not elastically scattered anymore because of recoiling, and the dynamical structure factor has a quadratic shape in the Q-E space. On the other hand, scattering locus is described as combination of Eqs. (5) and (6), whose scanning route is strongly depend on both of scattering angle and neutron energy. [ ] ( Q )2 2 f cos 2θ f2 e 2 2 Q = (1 + x) 1 + − , (5) 2 sin θ ( f + x) ( f + x)2 ( ] )2 [ 1+x f2 1− . (6) ω = E Q 2 sin θ ( f + x)2 Here, the parameters are:

Qe = 2ke sin θ, E Q = 2 Q2e /2m, x = k0 /ke − 1, f = L2 /(L1 + L2 ). Those are momentum transfer Qe for the elastic position, corresponding recoiling energy, and ratios between flight paths etc. Scattering process at smaller angle with higher energy makes the locus more straight along the energy axis and reduces the inelasticity effect, so called static approximation. The detector arrangement is chosen by taking these effects, so that the difference of the integrated intensity at an edge of a detector bank is minimized from the next detector bank, resulting a detector arrangement as shown in Fig. 6, which shows TAIKAN, a wide and small angle instrument. A similar consideration was also taken for NOVA, a total scattering instrument. 5

Flight Distance

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Scattering Angle 2θ θ

Scattered Signals Detector

d2

d2

Sample

Detector

d3 

d1

d1

Time of Flight

5.

Fig. 7.

Source

Layout of a chopper instrument.

TOF Inelastic Scattering

5.1

Chopper instruments (direct geometry instrument) A chopper inelastic scattering instrument, so called direct geometry instrument, is one of the most typical and popular instrument for inelastic neutron scattering measurement in spallation neutron sources. It equips with a Fermi chopper just before the sample position, which synchronously rotates in phase with the source and chops out monochromatic neutrons. Figure 7 illustrates the principle of a chopper instrument. Since chopper instruments are installed in most of spallation neutron sources and frequently used for variety of experiments, hence, some details are described here [14]. 5.1.1 Energy resolution Energy resolution is composed of two terms. One is a contribution from the chopper opening time as shown in Fig. 8. Another comes from the pulse width of moderator, Fig. 9. Energy derivative is translated into the time derivative as 3 1 −2 ∂E1 ∆E1 = ∆t1 = ( ) 1 (E1 ) 2 ∆t1 . (7) ∂t1 L1 2 1 m 2 The same is for the scattered neutrons by a sample ∆E2 =

∂E2 ∆t2 . ∂t2

(8)

The time width appears at the detector is the sum of ∆t1 and ∆t2 . Here we note that the energy ambiguity of E2 should be the same as E1 and maintains even after scattering. Hence ∆E1 = ∆E2 . It gives from Eqs. (7) and (8), ( )3 L2 E 1 2 ∆t1 . (9) ∆t2 = L1 E 2 Hence, the time ambiguity detected at the detector is

 ( ) 32   L E1  2  . ∆td = ∆t1 + ∆t2 = ∆t1 1 + L1 E 2 

(10)

Since the energy resolution detected at detector is calculated with ∆td as an ambiguity in E2 ∆ =

∂E2 E2 ∆td = 2 ∆td . ∂t2 t2 6

(11)

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∆

∆

∆ ∆ 

∆ ∆



 



  

∆



 ∆



 











  ∆

 

 

Fig. 8. Ray diagram of energy resolution from chopper opening time.

Fig. 9. Ray diagram of energy resolution due to pulse time width in moderator.

With Eq. (10) this leads E2 ∆td ∆t1 ∆ =2 =2 E1 E 1 t2 t1

 ( ) 32   L ∆t E2  1 1 +  = 2 1  L2 E 1  t1

 ( ) 32    L  1 1 +  . 1−  L2 E1 

(12)

Actual chopper instruments have three important distances, d1 , d2 and d3 as shown in Fig. 8. Therefore, Eq. (12) is rewritten as ∆tch ∆ch =2 E1 tch

 ( ) 32    d + d  3 1 + 1  . 1−  d2 E1 

(13)

The another contribution to the energy resolution is created from the time width of moderation time in the moderator, ∆tm . In Fig. 9 the effect of ∆tm is illustrated by assuming chopper opening time is decoupled and treated as a pin-hole in time. Now it is easily deliberated and leads the following contribution as an analogy of Eq. (13) by neglecting the unaffected process before the chopper, ∆m ∆tm =2 E1 tch

 ( ) 32    d  1 + 3 1 −  .  d2 E1 

(14)

Therefore, the energy resolution is the root mean square of sum of each squared term of Eqs. (13) and (14), and is,   ∆   ∆tch 2 =   tch E 1 

  2 ( ) 32      d + d     ∆tm 3 1 + 1  + 2 1−    tch d2 E1  

1  2 2 ( ) 32       . 1 + d3 1 −       d2 E1 

(15)

In Fig. 10 a typical performance of Eq. (13), (14) and (15) is illustrated. In the early stage of chopper instrument development in 1970’s and 1980’s, spectrometer was installed at a bare beam port without any neutron optical component aiming at utilizing up to epithermal neutrons. In this situation a long d1 loses flux squared-inversely, hence, short d1 with sharp chopper time width and moderation time width were chosen under a matching condition, ∆tch = ∆tm , in terms of an intensity optimization. This is the reason why a decoupled moderator was taken for a 7

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Fig. 10. Energy resolution of Eqs. (13), (14), and (15). Here, E1 = 100 meV, d1 = 20 m, d2 = 2.5 m, d3 = 1.5 m, ∆tch = ∆tm = 10 µs are taken. Obviously dominant term is the chopper opening term Eq. (13).

Fig. 11. Layout of 4SEASONS at J-PARC. Long position sensitive detectors are installed in the vacuum tank.

Fig. 12. AMATERAS, a cold neutron disk chopper instrument with a pulse shaping chopper.

Fermi chopper instruments as a conventional concept in the past. However, importance of cold neutron has been gradually recognized even in spallation neutron sources in 1990’s. Hence, flux gain has been improved by optical component such as a suppermirror guide, and innovative design concept has emerged and proposed by taking an advantage of a high flux coupled moderator. This modern concept leads fairly long d1 and tunable ∆tch to match with a broad ∆tm from a coupled moderator to realize high intensity by keeping reasonable resolution. As shown in Fig. 10 the dominant term in the energy resolution is Eq. (13) for a conventional concept ∆tch = ∆tm . In terms of resolution matching, ∆tm can be made much more relaxed and realize a high intensity option. A contribution of Eq. (14) can be increased, however, it can keep the whole resolution reasonably well. 4SEASONS is the first instrument of this class, providing very intensive ability in J-PARC [15], Fig. 11. The same concept and formulation are valid even for a cold neutron disk shopper instrument viewing a coupled moderator with a pulse shaping chopper, whose chopped time width can be treated as a moderator time width. Hence the first disk chopper close to the actual neutron source has a role of a tunable alternative neutron source. Hence instrument can have a high flexibility/tunability in intensity and resolution as has been achieved/demonstrated by AMATERAS [16], Fig. 12. 5.2

Inverted geometry instrument There is another type of inelastic neutron scattering instrument, so called inverted geometry instrument, which analyzes scattered neutron energy by a crystal analyzer, Fig. 13. The final energy is fixed instead of the monochromatic incident energy. Therefore, white beam reaches at the sample, are scattered by a sample, and are scattered back to the detector by a Bragg reflection at the crystal analyzer, which is normally a very large crystal mirror to enhance intensity but with sacrificing the 8

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∆ 















∆

  ∆





Fig. 13. Illustration of an inverted geometry instrument with a shaping chopper.

Fig. 14. Contribution from the peak time width of moderator.

momentum resolution. Therefore, this kind of instrument is very intensive with a wide dynamical range, but at the expense of high back ground caused by neutrons unmatched with analyzing energy. 5.2.1 Energy resolution The energy resolution is also composed of two terms. The first term is attributed to the angular ambiguity of the analyzer crystal, whose process gives a similar diagram of the chopper instrument, Fig. 8, but the dispersive character is created by the analyzer crystal. Instead of time width contribution in Eq. (12), an angular contribution comes out, and this term is described as  ( ) 23   L ∆  E2 E1  2  . = (cot θ · ∆θ) 1 + E1 E1 L1 E 2 

(16)

The second contribution comes from a time ambiguity of the pulse width. Here, we assume the analyzer is a perfect crystal and the time width at moderator propagates without any change as shown in Fig. 14. Hence, the contribution from the pulse time width becomes, ∆  ∆tm δm =2 =2 . E1 t1 L1

(17)

Here, the peak time width ∆tm is converted to a length δm , typically this is about 2.5 cm for a decoupled moderator according to Eq. (1), independent from moderator materials. The energy resolution is a squared sum of Eqs. (16) and (17), and the total resolution can be described as 1   2 2 ( ) 23  ( )2         L2 E1   ∆  E2  δm    . = 2  + (cot θ · ∆θ) 1 + (18)    E1  L1 E1 L1 E2 

Here, the E2 is fixed by the analyzer. Therefore, in order to have a good energy resolution, it is important to have a small δm , a long L1 , a large θ and short L2 . Because of these reasons this kind of instrument with high energy resolution option was traditionally installed at a beam port viewing decoupled moderator with a long flight-path L1 . There is, however, an innovative concept to realize a required energy resolution by putting a pulse shaping chopper near the source as illustrated in Fig. 13 with a much shorter flight pass L1 . In an extreme case with a back scattering geometry, Eq. (18) has an additional term, Darwin width, due to a dynamical scattering process in a perfect crystal and it is described as follows at the 9

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Fig. 15. Energy resolution of a backscattering instrument with L1 = 100 m at a typical decoupled moderator, δm = 2.5 cm satisfying Eq. (1).

Fig. 16. Tunneling spectrum from γ-Picoline. The energy resolution is about 3 µeV with a single pulse shaping chopper as designed (blue line), compared with a spectrum without operating the pulse shaping chopper, having 14 µeV in resolution.

elastic position,  (  ( )2 ( )2 ) ( )2   ∆tm 2  ∆d ∆d ∆  δm 2  2   = 2  + + (cot θ · ∆θ)  = 2  + + (cot θ · ∆θ)  . E1 L1 d t d

(19)

Estimated energy resolution of a typical backscattering instrument with L1 = 100 m at a decoupled moderator can reach 1.5 µeV in resolution as a natural consequence with a Si(111) backscattering, E2 = 2 meV, as shown in Fig. 15. 5.2.2 DNA instrument in J-PARC A nearly backscattering instrument DNA has adopted pulsed shaping chopper with a coupled moderator for the first time [17]. By shaping a broad peak of 200 µs at 2 meV to 10 µs, even with a short flight path of 40 m, three terms in Eq. (19) become comparable to be an order of 10−4 and can achieve 1 µeV level resolution with a Si(111) crystal. Figure 16 shows a tunneling spectrum of γ-Picoline [18], where the energy resolution of 3 µeV is achieved with a single shaping chopper compared to 15 µeV for a natural pulse width without the chopper. A counter rotating double shaping chopper will realize a resolution 1.5 µeV. Although the dynamical range becomes narrower because of the pulse shaping window, and several rephasing of chopper is necessary to scan the whole spectrum in ±300 µeV range, very high statistics can be obtained within a reasonable measuring time, in a half day at 1 MW power of accelerator. Asynchronous phasing of the chopper is also planned to automatically scan the entire spectrum in a wide energy band.

6.

Multi-Ei Method

6.1

Multi-Ei on 4SEASONS Traditionally a chopper instrument monochromates neutron energy by a Fermi chopper once in one frame, which has curved slits and optimized to a target incident energy in terms of resolution and transmission efficiency. However, adaption of either a disk chopper or a straight-slit Fermi chopper with a flexible eventrecording data acquisition method makes it possible that several incident energies can be monochromated and can be used in one frame at the same time. Since the chopper frequency is much faster 10

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Energy Transfer (meV)

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40

Ei =45.6 meV

20

Ei =21.6 meV

10

120 100 80 60 40

30

15

20

10

10

5

0

0

8 6 4 2

20 0 -20

-2

-1

0

1

2

3

-1.5 -1

-0.5 0

0.5

1

1.5

2

-1

Ei =12.6 meV

0 -0.5

0

0.5

1

-0.5

0

0.5

1

Momentum Transfer (r.l.u.)

Fig. 17. Four sets of phase spaces observed at once by the Multi-Ei method. Spin excitation of an onedimensional antiferromagnet CuGeO3 . The measurement created four sets of data ranging from a wide phase space, where the continuous spin excitation and spin wave dispersion are shown, to a low energy region, where the details are clearly shown.

than the source frequency, hence a proper design of a rotor slit system realizes this innovative method fairly easily. This method, here we call “the Multi-Ei method” gives a very high efficiency for scattering experiment with simultaneous observation in the hierarchical phase spaces. A similar idea was proposed by F. Mezei group [19]. Figure 17 shows an example of the Multi-Ei method obtained on 4SEASONS [20]. Here the sample was a one-dimensional antiferromagnet CuGeO3 , which has a spin continuous excitation in the high energy region in addition to a conventional spin wave dispersion at low energy. Figure 17 shows four sets of data in hierarchical phase spaces, showing entire spectrum of the spin excitation in the wide phase space and simultaneously shows details of the energy gap state in the low energy region. As it is easily imagined, this method has a great benefit not only in its efficiency but a dramatic promotion of new findings at an unexpected phase space. 6.2

Magic chopper In order to improve and enhance performance of the Multi-Ei method, continuous effort has been made to develop a new type of chopper slit package by stacking suppermirror layers. The reflection angle of the suppermirror is proportional to the wavelength of neutrons. Hence with this chopper, Magic Chopper, the opening time can be also proportional to the wavelength, and it automatically optimizes the energy resolution as shown in Eq. (15) in the wide energy region, where the pulse time width is proportional to the wavelength, Eq. (1) [21]. A feasibility study of a Magic chopper successfully demonstrated the expected performance as shown in Fig. 18 [22]. The pulse width through the Magic chopper is almost proportional to the wavelength (energy) in comparison to a constant nature of the pulse width for a conventional chopper system without suppermirror slits. This result promises that resolution and intensity of transmitted neutrons through the Magic chopper for the Multi-Ei method can be automatically optimized in a wide energy range and enhance the performance of this method. 6.3

Prospects of Multi-Ei in the future Once we can admit the performance of the Magic chopper, we can adapt it and extend its possibility of a flexible use in multi-frames. Magic chopper can allow neutrons passing though it in an optimized condition. By choosing a proper frequency of a source and the chopper at a proper position/distance in a beam line, we can apply the Multi-Ei in multi-frames (Multi-frame multi-Ei ). This is an active use of frame overlapped neutrons and an instrument will have another dimension of efficiency/flexibility and may have a new concept of methodology. 11

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Fig. 18. A test performance of a Magic chopper. Transmitted pulse widths through a Magic chopper with a supermirror (SM) slit are almost proportional to the neutron energy compared with a constant nature of a conventional chopper without SM.

7.

Background Suppression

7.1

To chopper It is a quite important issue for any instrument to reduce a background as low as possible. Especially this is a golden rule for inelastic instrument design. An aiming signal can be as weak as 5 orders of magnitude smaller than an elastic signal. Low background at epithermal neutron region is also important for a total scattering diffractometer, whose analyzed intensity at high Q region has to be unity in the absolute scale. One of obvious background killers is a background suppression chopper, so called To-chopper [23]. This apparatus has a thick blade, 30 cm long bulk Inconel alloy, along beam line, and dynamically close the beam line on proton bombardment on a target to suppress high energy neutrons and also gammas, and otherwise those can easily come into the secondary spectrometer, can be thermalized in instrument shield, and comes out at later time, making very high background. Figure 19 is an example of background suppression at various rotation speed of a To chopper. Lower speed of rotation can suppress more background because of a slower opening time. 7.2

Instrument shield It is also important to have shields inside and outside a vacuum tank surrounding sample. Essentially a chopper instrument can have less background because unnecessary neutron cannot reach sample. However inverted geometry instrument has intrinsically high background, since white beam reaches at sample and is scattered around. The DNA instrument has taken a special effort to reduce background by having a thin absorber layer just behind the analyzer crystal [24]. It is not an easy task to achieve by keeping very fine precision of the analyzer crystal. We, however, have overcome a technical difficulty, and we realized a very low background as low as 5 order of magnitude lower than incoherent elastic peak as seen in Fig. 16. It is comparable to a background level of a chopper instrument AMATERAS, which has a intrinsically low background, at a similar energy resolution as seen in Fig. 20.

8.

Conclusion

The power, 1 MW, of accelerator only gives an improvement of factor 6 in comparison with that of ISIS [25], which was built in the 1980’s. Such a brute-force in improvement has a technical and budgetary limit even in the future. New idea, innovative concept and developments in instrumentation extend other dimension in improvement, and easily give a performance of a factor of two orders of magnitude higher and may create a paradigm shift in neutron experimental technique. In this paper 12

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Fig. 20. Tunneling spectrum from γ-Picoline at 14 µeV resolution of DNA (black line) in comparison with AMATERAS, 15 µeV resolution (red line). Background is comparable to each other. Here, pulse shaping chopper is not operated for DNA to degrade the energy resolution.

we have demonstrated some of recent developments of instrumentation in J-PARC, which may help to imagine a trend of instrumentation in the future. References [1] http://j-parc.jp/index-e.html. [2] M. Arai et al.: Proc. 16th Meet. Int. Collaboration on Advanced Neutron Sources (ICANS-XVI), 2003, p. 157. [3] F. Tamura, M. Yoshii, A. Schnase, C. Ohmori, M. Yamamoto, M. Nomura, M. Toda, T. Shimada, K. Hara, and K. Hasegawa: Nucl. Instrum. Methods Phys. Res., Sect. A 647 (2011) 25. [4] M. Arai: in Neutrons in Soft Matter, ed. T. Imae, T. Kanaya, M. Furusaka, and N. Torikai (John Wiley & Sons Publication, 2011) p. 601. [5] M. Harada, N. Watanabe, M. Teshigawara, T. Kai, F. Maekawa, T. Kato, and Y. Ikeda: Proc. 17th Meet. Int. Collaboration on Advanced Neutron Sources (ICANS-XVII), 2005, p. 700. [6] S. Ikeda and J. M. Carpenter: Nucl. Instrum. Methods Phys. Res., Sect. A 239 (1985) 536. [7] D. F. R. Mildner and R. N. Sinclair: J. Nucl. Energy 6 (1979) 225. [8] K. Oikawa et al.: unpublished. [9] http://neutrons.ornl.gov/powgen/. [10] S. Torii, M. Yonemura, T. Y. Surya Panca Putra, J. Zhang, P. Miao, T. Muroya, R. Tomiyasu, T. Morishima, S. Sato, H. Sagehashi, Y. Noda, and T. Kamiyama: J. Phys. Soc. Jpn. 80 (2011) SB020. [11] W. Marshall and S. W. Lovesey: Theory of Thermal Neutron Scattering (Oxford University Press, 1971) p. 440. [12] C. G. Windsor: Pulsed Neutron Scattering (Taylor and Francis LTD, 1981) p. 278. [13] S. Takata, J. Suzuki, and M. Arai: to be published. [14] C. J. Carlile, A. D. Taylor, and W. G. Williams: Neutron Scattering in the ’Nineties (IAEA, Vienna, 1985) p. 421; M. Arai et al.: in Recent Developments in the Physics of Fluids, ed. W. S. Howells and A. K. Soper (Adam Hilger, 1992) p. F321. [15] R. Kajimoto, M. Nakamura, Y. Inamura, F. Mizuno, K. Nakajima, S. Ohira-Kawamura, T. Yokoo, T. Nakatani, R. Maruyama, K. Soyama, K. Shibata, K. Suzuya, S. Sato, K. Aizawa, M. Arai, S. Wakimoto, M. Ishikado, S. Shamoto, M. Fujita, H. Hiraka, K. Ohoyama, K. Yamada, and C-H. Lee: J. Phys. Soc. Jpn. 80 (2011) SB025. 13

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[16] K. Nakajima, S. Ohira-Kawamura, T. Kikuchi, M. Nakamura, R. Kajimoto, Y. Inamura, N. Takahashi, K. Aizawa, K. Suzuya, K. Shibata, T. Nakatani, K. Soyama, R. Maruyama, H. Tanaka, W. Kambara, T. Iwahashi, Y. Itoh, T. Osakabe, S. Wakimoto, K. Kakurai, F. Maekawa, M. Harada, K. Oikawa, R. E. Lechner, F. Mezei, and M. Arai: J. Phys. Soc. Jpn. 80 (2011) SB028. [17] N. Takahashi, K. Shibata, Y. Kawakita, K. Nakajima, Y. Inamura, T. Nakatani, H. Nakagawa, S. Fujiwara, T. J. Sato, I. Tsukushi, F. Mezei, D. A. Neumann, H. Mutka, and M. Arai: J. Phys. Soc. Jpn. 80 (2011) SB007. [18] S. Ikeda, N. Watanabe, K. Inoue, Y. Kiyanagi, A. Inaba, S. Takeda, T. Kanaya, K. Shibata, T. Kamiyama, Y. Izumi, Y. Ozaki, and C. J. Carlile: J. Phys. Soc. Jpn. 60 (1991) 3340. [19] F. Mezei: J. Neutron Res. 6 (1997) 3. [20] M. Nakamura, R. Kajimoto, Y. Inamura, F. Mizuno, M. Fujita, T. Yokoo, and M. Arai: J. Phys. Soc. Jpn. 78 (2009) 093002. [21] M. Nakamura, M. Arai, R. Kajimoto, T. Yokoo, K. Nakajima, and Th. Krist: J. Neutron Res. 16 (2008) 87. [22] M. Nakamura et al.: to be published. [23] S. Itoh, K. Ueno, R. Ohkubo, H. Sagehashi, Y. Funahashi, and T. Yokoo: Nucl. Instrum. Methods Phys. Res., Sect. A 661 (2012) 86. [24] M. Kambara, K. Shibata, M. Arai et al.: to be published. [25] http://www.isis.stfc.ac.uk/.

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