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T0 chopper developed at KEK
Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
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T0 chopper developed at KEK Shinichi Itoh a,n, Kenji Ueno b, Ryuji Ohkubo b, Hidenori Sagehashi a, Yoshisato Funahashi b, Tetsuya Yokoo a a b
Neutron Science Division, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan Mechanical Engineering Center, Applied Research Laboratory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2011 Accepted 19 September 2011 Available online 1 October 2011
We developed a T0 chopper rotating at 100 Hz at the High Energy Accelerator Research Organization (KEK) for the reduction of background noise in neutron scattering experiments at the Japan Proton Accelerator Research Complex (J-PARC). The T0 chopper consists of a rotor of 120 kg made from Inconel X750, supported by mechanical bearings in vacuum. The motor is located outside the vacuum and the rotation is transmitted into vacuum through magnetic seals. The motor should rotate in synchronization with the production timing of pulsed neutrons. The rotational fluctuations and running time were in good agreement with the specifications, i.e., phase control accuracy of less than 5 ms and running time of more than 4000 h without changing any component. A semi-auto installation mechanism was developed for installing under the shielding and for maintenance purposes. Based on the result of the development, actual machines were made for the neutron beamlines at J-PARC. We successfully reduced the background noise to 1/30 at neutron energies near 500 meV. & 2011 Elsevier B.V. All rights reserved.
Keywords: Pulsed neutron Neutron scattering Neutron chopper Synchronization control High precision machining
1. Introduction High-energy neutrons emitted from a pulsed neutron source in a very short time after the injection of the proton beam into the neutron production target are scattered and moderated within neutron spectrometers, resulting in a loud background noise. A T0 chopper reduces such noise by blocking the incident neutron beamline of the spectrometer at around time zero. It consists of a rotor with blades for blocking the beamline, and the rotor rotates in synchronization with the repeated production of pulsed neutrons. Here, we consider the case that the rotational axis is parallel to and under the beamline, and the length of the blade along the beamline is l. When the blade center is initially positioned on the beamline, it takes t 0 ¼ ðwþ DwÞ=ð2pRf Þ for the blade to be removed from the beam cross-section, where w is the width of the neutron beamline, w þ2Dw is the width of the chopper blade, R is the rotational radius of the chopper blade, and f is the rotational frequency. If this T0 chopper is located at a distance L from the neutron source, neutrons having energies less than E0 ¼ ð1=2ÞmN ðL=t 0 Þ2 are transmitted through the T0 chopper and utilized on the spectrometer, and the intensities of high energy neutrons, i.e., neutrons having energies larger than E0, are reduced, where mN is the neutron mass. The requirement for the T0 chopper varies with the spectrometer and it depends on the beam size as well as the neutron energy
n
Corresponding author. Tel.: þ81 298 64 5616; fax: þ 81 298 64 3202. E-mail address: [email protected] (S. Itoh).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.09.037
range. The largest beam size is about 80 mm, which corresponds to that of the chopper spectrometers utilizing high energy neutrons up to the eV region. To utilize eV neutrons for the neutron beam width of about 80 mm, a T0 chopper rotating at f¼100 Hz is required, although the conditions depend on L. In 2002, at the High Energy Accelerator Research Organization (KEK), we began the development of T0 choppers for use in the pulsed neutron source at the Japan Proton Accelerator Research Complex (J-PARC). In those days, T0 choppers rotating at 50–60 Hz were in practical use at the ISIS Facility at the Rutherford Appleton Laboratory [1], IPNS (Intense Pulsed Neutron Source) at the Argonne National Laboratory [2], and LANSCE (Los Alamos Neutron Science Center) at the Los Alamos National Laboratory [3]. In this article, we describe the design and performance of the T0 choppers rotating up to 100 Hz developed at KEK.
2. Developments 2.1. Specifications of development We developed the T0 choppers by choosing the following parameters: w¼80 mm, Dw ¼ 1 mm, l¼300 mm, R¼300 mm and f¼100 Hz, which correspond to the parameter values for the chopper spectrometers utilizing high energy neutrons up to the eV region. There are rectangular chopper blades (shielding parts) of 82 mm 82 mm 300 mm (Dw ¼ 1 mm and l¼300 mm) on both ends of the rotor diameter. The blade has margins of 1 mm to the
S. Itoh et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
beam cross-section. The rotational axis is parallel to and under the beamline. Since t 0 ¼ 430 ms for these parameters, neutrons having energies up to 2 eV can be utilized if this T0 chopper is installed at L¼8.5 m. The margin of 71 mm (Dw ¼ 1 mm) corresponds to the phase control accuracy of 7 5 ms (Dt ¼ Dw=ð2pRf Þ) with respect to the timing of the production of pulsed neutrons. With respect to maintenance, a continuous running time of more than 1000 h corresponding to the length of the machine cycle and a total running time of more than 4000 h corresponding to the annual beam time are required without changing any component of the T0 chopper. To realize these requirements, we designed a test machine for the T0 chopper, as shown in Fig. 1. Almost all of the parts were designed considering the Japanese Industrial Standards (JIS). However, there is no standard product for the bearings for the rotor. Therefore, ball bearings of 45 mm diameter were selected for supporting the rotor; this diameter was the maximum for f¼100 Hz among the commercially available products. The bearings are located in a vacuum chamber made of steel. However, since the pressure is around 1 Pa, the bearings for the atmosphere were selected, but the grease was changed to that used for vacuum. In order to investigate the heating in the bearings, cooling water pipes and thermocouples were mounted near them. The heat exchanger was made of stainless steel of the ferrite series having good thermal conductivity as well as good corrosion resistance. The rotational axis in the vacuum is connected to the motor outside the vacuum through a magnetic seal. The T0 chopper has a simple structure where the rotor, with a mass of m¼ 120 kg (as determined below) as shown in Fig. 2, is supported by ball bearings at the ends of the shaft. Circular holes are machined for mounting the bearings, magnetic seals, and the stabilizer (described below) on the shaft. The centers of these circular holes should be located on a line with proper coaxiality for stable rotation. Very precise machining is required for the rotor and its supporting parts; the coaxiality of the components on the shaft of the rotor as well as that of the shaft itself should be within 10 mm, and the balance quality of the rotor should be G r1 mm=s (i.e., G1 [4]). Since G ¼ ou=m with the unbalance u, where o ¼ 2pf is the angular velocity, G1 corresponds to u r 0:19 kg mm for the present case. Considering that u results from the displacement of the center of gravity of the rotor with mass m, u corresponds to a displacement of less than 1:6 mm. The coaxiality of 10 mm is the highest precision of the present machining for the present size, and the present balance quality does not affect the coaxiality in the machining and the assembly of the T0 chopper. The power of the motor can be determined from the ratio of the moment of inertia of the chopper rotor (I¼4.7 kg m2, as determined
Fig. 1. Cross-sectional view (side view) of the test machine of the T0 chopper. Units: mm.
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Fig. 2. Structure of the rotor, which is machined to maintain the coaxiality within 10 mm for the shaft and with the balance quality of G1. The shaded parts are the blades. Units: mm.
below) to that of the motor rotor. Normally, the ratio should be less than 10 to realize a precise phase control, and therefore, a motor power of more than 30 kW is required [1]. Alternatively, assuming the phase control of Dt ¼ 5 ms within the repetition of the production of pulsed neutrons (tR ¼ 40 ms for J-PARC), the power required for the torque T ¼ Ido=dt ¼ 2pI½f 1=ð1=f þ DtÞ=t R is P ¼ T o. For the present parameters, P¼23 kW is obtained, which is comparable to the reported value. The chopper rotor rotates at a constant frequency, and if the balance in the mechanical system is appropriate, rotational fluctuations and vibrations with a short period need not be controlled and it is enough to control just the longperiod fluctuations. Based on this concept, the mechanical parts were machined with an appropriately high precision, and then, we chose a servo motor of 10 kW and tried to develop the control system. This power corresponds to the phase control for up to two repetitions. The maximum rotational frequency in commercially available 10 kW servo motors is 50 Hz. The 100 Hz rotation is realized by doubling the frequency with a belt drive. 2.2. Determination of material and shape of rotor The chemical composition and mechanical properties of the Nibased superalloys at room temperature are listed in Table 1 [5]. Nimonic 75 is used for chopper blades for f¼50 Hz at ISIS [1], and Inconel X750 is used for f¼60 Hz at IPNS [2] and LANSCE [3]. We aimed to choose such a material whose maximum stress will be less than half of the proof stress to avoid fatigue breaking. From the radiation activation properties, the material should not include cobalt. Consequently, Inconel X750 was chosen as the chopper blade material for f¼100 Hz. In order to determine the shape of the rotor, we performed stress analysis calculations. First, we assumed the ISIS-type rotor structure [1], even for f¼100 Hz. The chopper blade (Inconel X750) is supported by light metals (aluminum) around the shaft, and the blade and the support are connected with a pin (titanium). In the calculation, a maximum stress more than the tensile strength occurred at the joint section between the materials around the pin. Next, in order to reduce the maximum stress, the material of the support was changed to titanium and the blade and the support were connected using the so-called Christmas-tree shaped structure, which is similar to the structure used in a gas turbine, instead of a
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5
Table 1 Ni-based superalloys used for chopper blades [5]. Chemical composition and mechanical strength. The balance in the composition is Ni. Chemical composition (%)
Cr
Nimonic 75 Nimonic 90 Inconel 625 Inconel X 750
Fe
19.5 – 19.5 – 21.5 – 15 7
Co
Ti
Al
– 17 – –
0.4 2.4 – 2.5
– 1.4 – 0.7
Mechanical strength (MPa) Nb þTa
– – – 1.0
0
Tensile 0.2% proof strength stress 820 1230 900 1220
450 810 810 830
-5
fluctuations (μs)
Material
0
10
20
30
40
10
5
0 50
100
150
200
150
200
10
5
0 50
100
time (s) Fig. 3. Calculated stress distribution in the chopper rotor at f¼ 100 Hz. Gray scale represents the value of the von Mises stress (units: MPa). The maximum stress is 332 MPa.
pin. We calculated the stress distribution in the rotor for some patterns of the Christmas-tree shaped structure on the connecting part; however, the maximum stress could not be reduced to less than 700 MPa. Finally, we found that we can successfully reduce the maximum stress if the rotor, including the chopper blade, the support, and the shaft are forged as a single body without any connection. In this case, the maximum stress was calculated to be 332 MPa, as shown in Fig. 3. For this shape, the mass of the rotor is m¼120 kg and the moment of inertia is I¼4.7 kg m2. In order to machine the rotor in a single body, 530 mm 700 mm 95 mm of forged material is required. Such a large-sized forged material of Inconel X750 was not known until the start of the present design. However, during the present design, it was found that such a largesized forged material can be manufactured and machined with an appropriately high precision. Therefore, we decided to make the rotor as a single body of Inconel X750. Also, the frequencies of the proper vibration of the rotor were calculated to be 138.0 Hz, 380.7 Hz, and 827.6 Hz. These are beyond the range of the rotational frequencies used in the operation (f r 100 Hz). 2.3. Mechanical properties and running test For the rotation, the servo motor was driven by pulses generated from a function generator. The motor makes one rotation by applying 8192 pulses from the function generator to the servo amplifier, and the motor rotates at 50 Hz by using a pulse repetition of 409.6 kHz. The motor frequency of 50 Hz was doubled using a timing belt, and a rotation sensor for counting 100 Hz was mounted on the shaft of the rotor. The base timing of 100 Hz was generated by the same function generator used to drive the motor. The difference between the rotational pulse from the encoder and the base timing was observed. Peak-to-peak fluctuations of 3 ms were initially observed without any control
Fig. 4. Rotational fluctuations at f ¼100 Hz with respect to the base timing: (a) a typical example of the original fluctuations in the best case; (b) and (c) are irregular fluctuations.
device, as shown in Fig. 4(a). To reduce the fluctuations, we mounted a stabilizer, which is a dynamic damper used as a device for absorbing the change of the rotational frequency, on the shaft. By using the stabilizer, the peak-to-peak fluctuations were successfully reduced to 1:5 ms. This satisfied the specification for the phase control accuracy of 75 ms. The vibration levels were evaluated using the vibration amplitudes. For 100 Hz, the excellent vibration level is defined for amplitudes less than 7:95 mm [6]. The amplitudes for the stable operation at 100 Hz were measured to be 2:0 mm, 2:7 mm, and 1:9 mm at the bearing housings on the motor side, the rotation sensor side, and at the rotation sensor base, respectively. The vibrations were in the excellent level. Since the excellent mechanical properties were confirmed, as described above, we started a continuous running test to investigate the lifetime of this T0 chopper. An unmanned system was also designed to monitor the vibrations, temperatures, water flows, vacuum, electric currents, and so on. We defined a threshold value for each monitoring item, and the system will be stopped automatically in case of an emergency. The monitoring items were selected for practical operation. In the running test, a total running time of 4648 h was recorded; also, continuous operation time of 1529 h was recorded. However, the lifetime of the magnetic seal was limited to approximately 2500 h in the early stage of our development, because of evaporation of solvents in the magnetic fluids. By changing the magnetic fluid to that which has a higher vapor pressure, optimizing the filling quantity of the magnetic fluid, and changing the magnetic circuit, the performance of the magnetic seals was greatly improved. As the result, we confirmed that the lifetime of the magnetic seals was extended to 4400 h. During the running test, we observed two types of irregular fluctuations, as shown in Fig. 4(b) and (c). In the first type, the
S. Itoh et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
1
ON OFF
number of events
rotational phase recovered after sudden fluctuations. In the other type, the rotational phase was shifted to the delay side after irregular fluctuations. The origin of these fluctuations is unknown; however, it might be related to the condition of the bearings. We recorded the number of events for these types of fluctuations. The fluctuation amplitude was found to be almost less than 5 ms, which is satisfied by the specification for the control accuracy of 75 ms. The number of such fluctuations gradually decreased during the running test and the system became stable after running for about 200 h at f¼100 Hz.
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2.4. Control system We took measures to reduce the rotational fluctuations mechanically as much as possible as described above, but the rotational fluctuations are influenced by factors such as heat transformation and change in the grease viscosity with changing temperature. It is not easy to maintain the rotational frequency at 100 Hz within the fluctuations of 7 5 ms. Also, the rotational phase should be synchronized with the production time of the pulsed neutrons. Therefore, a control system was developed for the T0 chopper, and the rotational frequency and phase were automatically controlled by the pulses given to the servo motor in the case of frequencies beyond a designated range of the expected frequency, as shown in Fig. 5. The control system consists of the acceleration, pulse generation, and phase comparison blocks. The J-PARC provides a 12 MHz timing signal from the master clock (MC) and the 25 Hz timing signal is synchronized with the MC. The accelerator tries to operate in synchronization with the 25 Hz timing (so-called scheduled timing), and pulsed neutrons are produced at 25 Hz with some timing fluctuations reflecting the fluctuations in the accelerator [7]. The T0 chopper is operated based on the MC signal and the scheduled timing; both timings are synchronized. In the control system, the acceleration block has a direct digital synthesizer (DDS) that generates frequencies up to 12 MHz independent of the MC. First, the acceleration of the T0 chopper is controlled by the acceleration of the DDS. When the rotor frequency of the T0 chopper is close to f¼100 Hz and the frequency of DDS is close to that of the MC, the control source is switched to MC. When the MC signal is lost or the T0 chopper ramps down for stopping, the control source is switched to DDS. The motor makes one rotation by applying 8192 pulses to the servo amplifier, and the T0 chopper runs at f¼100 Hz by rotating the motor at 50 Hz by applying 409.6 kHz pulses generated by the MC or DDS to the servo amplifier. Normally, the MC timing is used, but the DDS timing is used in the case where the MC signal
Fig. 5. Block diagram of the control system.
0
-5
0
5
deviation from target timing (μs) Fig. 6. Rotational fluctuations in the T0 chopper with (ON) and without (OFF) the control system. Number of events normalized by the total number within a constant measuring time is plotted as a function of the deviation from the target timing.
is lost. During the pulse interval of the scheduled timing of 40 ms, 480 000 pulses are issued by the MC and 8192 2 pulses are applied to the servo amplifier for f¼ 100 Hz. Adding one pulse, during 480 001 pulses issued by the MC, by applying 8192 2 pulses to the servo amplifier, the phase of the rotation of the T0 chopper is delayed. In the pulse generation block, by choosing the number of pulses to be added to the 480 000 pulses while applying the pulses to the servo amplifier corresponding to f, the phase delay of the T0 chopper rotor is controlled. The addition of one pulse corresponds to the phase delay of 0:083 ms and 0.0441. In the phase comparison block, by comparing the timing of the rotor phase of the T0 chopper with the scheduled timing, the number of pulses to be added in the pulse generation block is controlled. Fig. 6 shows the rotational fluctuations observed with and without the control system. The reduction of the rotational fluctuations was greatly improved to approximately 1 ms at the full-width at the half-maximum (FWHM).
2.5. Other developments toward actual machines The test machine for f¼100 Hz shown in Fig. 1 has a length of approximate 1.6 m along the beamline and occupies a large space. By considering the rearrangement of the components of the T0 chopper, the length was reduced. By placing the motor under the vacuum chamber, the length was reduced to 1.3 m. For the case of low frequencies such as f¼50 Hz and 25 Hz, it was found by measuring the rotational fluctuations as well as the vibration levels at some parts of the T0 chopper that the base plate can be shortened and that the stabilizer is not effective. Also, the connecting region of the water and electricity at the base was redesigned. The length was successfully reduced to less than 1 m for low frequency machines. The location of the T0 chopper on the neutron beamlines at J-PARC is at the bottom of the shielding whose height is 4 m. During maintenance, we should stand on a part of the shielding to reduce radiation exposure, and mount and remove the T0 chopper by employing few procedures using a crane after removing the shields above the T0 chopper. The main body sits on the base and the cooling water, electricity, and vacuum are connected through the base. In our design, these are connected/disconnected just by sitting on/removing them from the base by a crane. Fig. 7 shows the design of the model of the semi-auto installation mechanism
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Fig. 7. A model of the semi-auto installation mechanism for cooling water.
for the cooling water. We performed a repeating test for the connection/disconnection mechanism. The radiation damage of the main components such as bearings, magnetic seals, timing belts, and rotation sensors is one of the factors that limits the lifetime of the T0 chopper. The important components are located on the rotor shaft, 300 mm from the neutron beamline. At this position, g-rays are dominant and the absorption dose can be calculated to be in the order of 1 kGy/yr at 1 MW of the accelerator beam power [8]. We investigated the radiation properties of some components of the T0 chopper by irradiating high-energy g-rays emitted from Co-60. The irradiation was performed at the Cobalt 60 Irradiation Facility, Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. The properties of mechanical parts such as bearings, magnetic seals, and timing belts were almost unchanged up to the irradiation of 100 kGy, but the rotation sensors were damaged even at 1 kGy. To avoid radiation damage to the sensors, the marking of the rotation on the rotor shaft was transferred through an optical fiber to the control system located out of the beamline shielding. The details will be described in a separate paper [9].
3. Actual machine production for neutron spectrometers at J-PARC Based on the above developments, we designed the actual machines and installed them in the neutron beamlines at J-PARC. Fig. 8 shows the T0 chopper installed on the High Resolution Chopper Spectrometer (HRC) [10]. The HRC installed at the 12th neutron beamline (BL12) is an inelastic neutron scattering instrument that uses neutrons monochromatized by a Fermi chopper, and the dynamics in matter can be investigated using high resolutions by utilizing neutrons with energies up to the eV region. The T0 chopper was installed at L¼9 m. At this position, the beam cross-section is 76 mm 76 mm (w¼ 76 mm) and the size of the T0 chopper blade is 78 mm 78 mm (Dw ¼ 1 mm). Since t 0 ¼ 408 ms for f¼100 Hz, the upper boundary of the energy of the neutrons utilized on the HRC is estimated as E0 ¼2.5 eV. In this high frequency model, the rotor, which has a symmetric shape with respect to the rotational shaft shown in Fig. 2, was originally made of Inconel X 750 as a single body, including the rotational axis as mentioned above, to reduce the maximum stress to the material strength. Recently, since the material price was drastically increased, the structure was changed. The rotor body, including the rotational axis, was made of cobalt-free stainless steel and a cylindrical Inconel part was inserted at the beam hitting position. These two materials were bonded by the hot isostatic press method, and then, the stress was successfully reduced. This high frequency model was also installed on the NeutronNucleolus Reaction Instrument (BL04). The cross-sections of nuclear reactions are measured on BL04 and eV neutrons are required. By installing the T0 chopper at L¼13 m, neutrons up to 18 eV are
Fig. 8. High frequency model (f¼ 100 Hz) of the T0 chopper mounting the semiauto installation mechanism. In the figure on the right, the upper part of the vacuum chamber is removed to show the rotor. Normally, the upper part is fixed at the body. The motor is mounted under the vacuum chamber.
Fig. 9. Variations of models of the T0 chopper. A short model (f¼50 Hz) for the total scattering instrument (a), and a two-hole model (f¼25 Hz) for the reflectometer (b).
utilized at 100 Hz. The High-Intensity Total Diffractometer (BL21) utilizes neutrons with a wide wavelength band. To increase the wavelength band, the rotor of the T0 chopper has an asymmetric shape and the beamline is blocked once for one rotation; also, a counter-rotating set-up of two T0 choppers is proposed. For this purpose, the length along the neutron beam should be minimized. The short model can be realized for a lower rotational frequency of f¼50 Hz, as mentioned above. The total length of the high frequency model is 1.3 m and that of the short model is 1 m as shown in Fig. 9(a). One of the two counter-rotating T0 choppers was installed. The surface structure of the materials on the High-Performance Neutron Reflectometer with a Horizontal Sample Geometry (BL16) can be investigated through the momentum dependence of the neutron reflection on the sample surface mounted horizontally. BL16 has two beamlines, and the angle between the horizontal line and the beamline can be chosen to be 2.21 or 5.71. Therefore, the T0 chopper for BL16 should have two beam holes. The rotational axis is located between the two beamlines and the rotor shape is asymmetric, as shown in Fig. 9(b), and f¼25 Hz.
4. Performance The performance of the T0 chopper was investigated on the HRC. The fluctuations in the rotational period of the actual T0
S. Itoh et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
2.5eV
0.63eV
0.16eV
Transmission
1
100Hz 50Hz 25Hz 0 0
1000
2000
3000
4000
TOF (μs) Fig. 10. TOF dependence of the T0 chopper transmission measured with the white neutron beam. The designed values of E0 are indicated at the corresponding TOF.
105 OFF 100Hz 50Hz 25Hz
104
Intensity (arb.units)
chopper were measured to be approximately 1 ms (FWHM) in the range f¼25–100 Hz, i.e., they were almost independent of f. This result reproduces the performance shown in Fig. 6. Although these fluctuations increase to 3 ms (FWHM) during the beam time probably due to the electrical noise generated in the experimental hall, these fluctuations still satisfy the requirement demanded by the phase control accuracy within 75 ms. Transmission through the T0 chopper was measured with the neutron beam. The white beam of pulsed neutrons is incident on the vanadium standard sample located at 15 m; then, it is scattered and detected by the detector system located at 4 m from the sample [10]. Fig. 10 shows the time-of-flight (TOF) dependence of the observed transmission, which is the ratio of the spectrum measured with the T0 chopper being operated at the appropriate f to that with the T0 chopper off and the blade removed from the beamline. We confirmed that the transmission recovers below 2.5 eV of neutron energy at f¼100 Hz, 0.63 eV at f¼50 Hz, and 0.16 eV at f¼25 Hz, as designed. Inelastic neutron scattering experiments were performed on the HRC by using the T0 chopper. The pulsed neutron beam is monochromatized by a Fermi chopper located at 14 m from the neutron source and made incident on the vanadium standard sample located at 15 m; it is then scattered and detected by the detector system located at 4 m from the sample [10]. The Fermi chopper consists of a slit package rotating in synchronization with the production timing of pulsed neutrons to monochromatize the neutrons. The Fermi chopper opens and the neutrons pass through the slit when it rotates to a position where the slit is parallel to the incident neutron beam, and the slit-open time determines the energy resolution of the HRC. Fig. 11 shows the TOF spectrum at the detector system, with the T0 chopper at f¼25 Hz, 50 Hz, 100 Hz, and with the T0 chopper off and the blade removed from the beamline. The Fermi chopper was operated at 600 Hz. Since the slit width of the Fermi chopper is coarse, the neutron beam passes through the Fermi chopper at every half turn, then many peaks appear. The neutron energies of the peaks at TOF ¼ 1940 ms and 4190 ms correspond to 505 meV and 108 meV, respectively. We successfully reduced the background noise at neutron energies of around 500 meV by two orders of magnitude, as shown in Fig. 11. This indicates that inelastic neutron scattering experiments that require the detection of very small signals can be conducted on the HRC. In the very short TOF region, the background noise increases with
91
103
102
101
100
0
2000
4000
6000
8000
TOF (μs) Fig. 11. Effect of T0 chopper on background-noise reduction for monochromatic neutron beam. The TOF spectra for the T0 chopper operation at 100 Hz, 50 Hz, 25 Hz, and in no operation (OFF) condition are indicated.
increasing f. This suggests that the beam size for high-energy neutrons, which should be reduced by the T0 chopper, might be slightly wider than the blade width of the T0 chopper. At present, a boron tertacarbide (B4C) collimator defining the beam crosssection for thermal neutrons is installed in the region of the incident flight path between the neutron source and the sample [10]. By replacing the collimator by a supermirror guide tube, a larger flux gain is expected; also, the beamline will be defined by a steel wall and the collimation for high-energy neutrons will be improved. Further background-noise reduction will therefore be expected. Inelastic neutron scattering occurs around each peak in Fig. 11. The energy transfer from the neutron to the sample is defined as E¼Ei Ef, where Ei is the incident neutron energy, which is the energy at the peak, and Ef is the energy of the scattered neutron determined from the TOF. Fig. 12 shows the inelastic spectra at Ei ¼108 meV and Ei ¼505 meV as a function of E converted from the spectra shown in Fig. 11. The central peak at E¼0 corresponds to the peak at TOF ¼ 4190 ms (Ei ¼108 meV) or TOF ¼ 1940 ms (Ei ¼ 505 meV), and the width (FWHM) of the central peak gives the energy resolution DE ¼ 1:7 meV for Ei ¼108 meV or DE ¼ 32 meV for Ei ¼505 meV. In Fig. 12, ION is the spectrum observed with the T0 chopper rotating at f¼100 Hz and IOFF is that with the T0 chopper off and the blade removed from the beamline; also, the ratio IOFF/ION is indicated. The spectrum at f¼50 Hz was identical to that at f¼100 Hz. As shown in Fig. 12(a), phonons are clearly observed for ION around the central peak because of the good energy resolution (DE ¼ 1:7 meV) as well as the low background noise for Ei ¼108 meV. Since the measurement was performed at the room temperature, the scattering intensity is significantly reduced at E o 30 meV, corresponding to the thermal energy at the room temperature. At present, on the HRC, there exists another source of background noise that can be attributed to the scattering at the beam windows of the devices in the incident light path just upstream of the sample [10]. This noise cannot be removed by the T0 chopper and is rapidly increased at large E due to the conversion of TOF to E. This noise should be removed by further development of the HRC. However, large background noise arises without the T0 chopper and the phonons can be seen in the background noise for IOFF. The ratio IOFF/ION indicates the background noise without the T0 chopper. Since the ratio decreases around the central peak and the phonons and also decreases at large E due to the existence of another
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Ei = 108 meV IOFF ION 0.1 IOFF / ION
0.0 -100
-50
50
0
50
IOFF / ION
ION , IOFF (n/s/meV)
0.2
0 100
Ei = 505 meV 50
0.1
0.0
IOFF / ION
ION , IOFF (n/s/meV)
0.2
bearings in vacuum. The motor is located outside the vacuum and the rotation is transmitted to the vacuum through magnetic seals. The motor should be rotated in synchronization with the production timing of pulsed neutrons. The rotational fluctuations and the running time were well in agreement with the specifications. A semi-auto installation mechanism was also developed for installation under the shielding and for maintenance. Based on the result of the development, actual machines were manufactured for the neutron beamlines at J-PARC. We successfully reduced the background noise at neutron energies of around 500 meV to 1/30 in inelastic neutron scattering experiments.
0 -400
-200
0
200
400
E (meV) Fig. 12. Effect of T0 chopper on background-noise reduction on inelastic spectra. The inelastic spectra for the T0 chopper operation at 100 Hz (ION) and in the no operation (IOFF) condition are indicated for Ei ¼108 meV (a) and Ei ¼ 505 meV (b). Also, the ratio IOFF/ION is plotted.
background noise mentioned above, the ratio at E5 0 indicates the background noise in the absence of the T0 chopper. The background noise is reduced to 1/10 for Ei ¼108 meV. Similar features are also seen for Ei ¼505 meV in Fig. 12(b), although phonons are smeared out due to the coarse resolution of DE ¼ 32 meV. The background noise is reduced to 1/30 for Ei ¼505 meV. 5. Summary We developed a T0 chopper rotating at 100 Hz with a rotor of 120 kg, made from Inconel X750, and supported by mechanical
Acknowledgments We are grateful to Prof. N. Hitomi for initiating the collaboration project for these developments, Profs. M. Ishihama and M. Kawai for insightful discussions, and Metal Technology Co. Ltd., Techno AP Co. Ltd., Rigaku Corporation, Toshiba Machine Co. Ltd. for realization of the T0 chopper. References [1] T.J.L. Jones, I. Davidson, B.C. Boland, Z.A. Bowden, A.D. Taylor, in: Proceedings of the 9th Meeting of the International Collaboration on Advanced Neutron Sources, Swiss Institute for Nuclear Research, 1987, p. 529 (ISBN 3-90799801-4). [2] C. Loong, private communications, 2002. [3] R.J. McQueeney, R.A. Robinson, Neutron News 14 (2003) 36. [4] Japanese Industrial Standards, JIS B 0905, 1992. [5] For instance, Heat Resistance Alloys, MMC Superalloy Corporation, 2010 /http://group.mmc.co.jp/superalloy/global/S. [6] Japanese Industrial Standards, JIS B 8330, 2000. [7] F. Tamura, M. Yoshii, A. Schnase, C. Ohmori, M. Yamamoto, M. Nomura, M. Toda, T. Shimada, K. Hara, K. Hasegawa, Nuclear Instruments and Methods in Physics Research Section A 647 (2011) 25. [8] M. Tamura, unpublished, 2004. [9] S. Itoh, K. Ueno, R. Ohkubo, H. Sagehashi, Y. Funahashi, T. Yokoo, Nuclear Instruments and Methods in Physics Research Section A 654 (2011) 527. [10] S. Itoh, T. Yokoo, S. Satoh, S. Yano, D. Kawana, J. Suzuki, T.J. Sato, Nuclear Instruments and Methods in Physics Research Section A 631 (2011) 90.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
T0 chopper developed at KEK Shinichi Itoh a,n, Kenji Ueno b, Ryuji Ohkubo b, Hidenori Sagehashi a, Yoshisato Funahashi b, Tetsuya Yokoo a a b
Neutron Science Division, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan Mechanical Engineering Center, Applied Research Laboratory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2011 Accepted 19 September 2011 Available online 1 October 2011
We developed a T0 chopper rotating at 100 Hz at the High Energy Accelerator Research Organization (KEK) for the reduction of background noise in neutron scattering experiments at the Japan Proton Accelerator Research Complex (J-PARC). The T0 chopper consists of a rotor of 120 kg made from Inconel X750, supported by mechanical bearings in vacuum. The motor is located outside the vacuum and the rotation is transmitted into vacuum through magnetic seals. The motor should rotate in synchronization with the production timing of pulsed neutrons. The rotational fluctuations and running time were in good agreement with the specifications, i.e., phase control accuracy of less than 5 ms and running time of more than 4000 h without changing any component. A semi-auto installation mechanism was developed for installing under the shielding and for maintenance purposes. Based on the result of the development, actual machines were made for the neutron beamlines at J-PARC. We successfully reduced the background noise to 1/30 at neutron energies near 500 meV. & 2011 Elsevier B.V. All rights reserved.
Keywords: Pulsed neutron Neutron scattering Neutron chopper Synchronization control High precision machining
1. Introduction High-energy neutrons emitted from a pulsed neutron source in a very short time after the injection of the proton beam into the neutron production target are scattered and moderated within neutron spectrometers, resulting in a loud background noise. A T0 chopper reduces such noise by blocking the incident neutron beamline of the spectrometer at around time zero. It consists of a rotor with blades for blocking the beamline, and the rotor rotates in synchronization with the repeated production of pulsed neutrons. Here, we consider the case that the rotational axis is parallel to and under the beamline, and the length of the blade along the beamline is l. When the blade center is initially positioned on the beamline, it takes t 0 ¼ ðwþ DwÞ=ð2pRf Þ for the blade to be removed from the beam cross-section, where w is the width of the neutron beamline, w þ2Dw is the width of the chopper blade, R is the rotational radius of the chopper blade, and f is the rotational frequency. If this T0 chopper is located at a distance L from the neutron source, neutrons having energies less than E0 ¼ ð1=2ÞmN ðL=t 0 Þ2 are transmitted through the T0 chopper and utilized on the spectrometer, and the intensities of high energy neutrons, i.e., neutrons having energies larger than E0, are reduced, where mN is the neutron mass. The requirement for the T0 chopper varies with the spectrometer and it depends on the beam size as well as the neutron energy
n
Corresponding author. Tel.: þ81 298 64 5616; fax: þ 81 298 64 3202. E-mail address: [email protected] (S. Itoh).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.09.037
range. The largest beam size is about 80 mm, which corresponds to that of the chopper spectrometers utilizing high energy neutrons up to the eV region. To utilize eV neutrons for the neutron beam width of about 80 mm, a T0 chopper rotating at f¼100 Hz is required, although the conditions depend on L. In 2002, at the High Energy Accelerator Research Organization (KEK), we began the development of T0 choppers for use in the pulsed neutron source at the Japan Proton Accelerator Research Complex (J-PARC). In those days, T0 choppers rotating at 50–60 Hz were in practical use at the ISIS Facility at the Rutherford Appleton Laboratory [1], IPNS (Intense Pulsed Neutron Source) at the Argonne National Laboratory [2], and LANSCE (Los Alamos Neutron Science Center) at the Los Alamos National Laboratory [3]. In this article, we describe the design and performance of the T0 choppers rotating up to 100 Hz developed at KEK.
2. Developments 2.1. Specifications of development We developed the T0 choppers by choosing the following parameters: w¼80 mm, Dw ¼ 1 mm, l¼300 mm, R¼300 mm and f¼100 Hz, which correspond to the parameter values for the chopper spectrometers utilizing high energy neutrons up to the eV region. There are rectangular chopper blades (shielding parts) of 82 mm 82 mm 300 mm (Dw ¼ 1 mm and l¼300 mm) on both ends of the rotor diameter. The blade has margins of 1 mm to the
S. Itoh et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
beam cross-section. The rotational axis is parallel to and under the beamline. Since t 0 ¼ 430 ms for these parameters, neutrons having energies up to 2 eV can be utilized if this T0 chopper is installed at L¼8.5 m. The margin of 71 mm (Dw ¼ 1 mm) corresponds to the phase control accuracy of 7 5 ms (Dt ¼ Dw=ð2pRf Þ) with respect to the timing of the production of pulsed neutrons. With respect to maintenance, a continuous running time of more than 1000 h corresponding to the length of the machine cycle and a total running time of more than 4000 h corresponding to the annual beam time are required without changing any component of the T0 chopper. To realize these requirements, we designed a test machine for the T0 chopper, as shown in Fig. 1. Almost all of the parts were designed considering the Japanese Industrial Standards (JIS). However, there is no standard product for the bearings for the rotor. Therefore, ball bearings of 45 mm diameter were selected for supporting the rotor; this diameter was the maximum for f¼100 Hz among the commercially available products. The bearings are located in a vacuum chamber made of steel. However, since the pressure is around 1 Pa, the bearings for the atmosphere were selected, but the grease was changed to that used for vacuum. In order to investigate the heating in the bearings, cooling water pipes and thermocouples were mounted near them. The heat exchanger was made of stainless steel of the ferrite series having good thermal conductivity as well as good corrosion resistance. The rotational axis in the vacuum is connected to the motor outside the vacuum through a magnetic seal. The T0 chopper has a simple structure where the rotor, with a mass of m¼ 120 kg (as determined below) as shown in Fig. 2, is supported by ball bearings at the ends of the shaft. Circular holes are machined for mounting the bearings, magnetic seals, and the stabilizer (described below) on the shaft. The centers of these circular holes should be located on a line with proper coaxiality for stable rotation. Very precise machining is required for the rotor and its supporting parts; the coaxiality of the components on the shaft of the rotor as well as that of the shaft itself should be within 10 mm, and the balance quality of the rotor should be G r1 mm=s (i.e., G1 [4]). Since G ¼ ou=m with the unbalance u, where o ¼ 2pf is the angular velocity, G1 corresponds to u r 0:19 kg mm for the present case. Considering that u results from the displacement of the center of gravity of the rotor with mass m, u corresponds to a displacement of less than 1:6 mm. The coaxiality of 10 mm is the highest precision of the present machining for the present size, and the present balance quality does not affect the coaxiality in the machining and the assembly of the T0 chopper. The power of the motor can be determined from the ratio of the moment of inertia of the chopper rotor (I¼4.7 kg m2, as determined
Fig. 1. Cross-sectional view (side view) of the test machine of the T0 chopper. Units: mm.
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Fig. 2. Structure of the rotor, which is machined to maintain the coaxiality within 10 mm for the shaft and with the balance quality of G1. The shaded parts are the blades. Units: mm.
below) to that of the motor rotor. Normally, the ratio should be less than 10 to realize a precise phase control, and therefore, a motor power of more than 30 kW is required [1]. Alternatively, assuming the phase control of Dt ¼ 5 ms within the repetition of the production of pulsed neutrons (tR ¼ 40 ms for J-PARC), the power required for the torque T ¼ Ido=dt ¼ 2pI½f 1=ð1=f þ DtÞ=t R is P ¼ T o. For the present parameters, P¼23 kW is obtained, which is comparable to the reported value. The chopper rotor rotates at a constant frequency, and if the balance in the mechanical system is appropriate, rotational fluctuations and vibrations with a short period need not be controlled and it is enough to control just the longperiod fluctuations. Based on this concept, the mechanical parts were machined with an appropriately high precision, and then, we chose a servo motor of 10 kW and tried to develop the control system. This power corresponds to the phase control for up to two repetitions. The maximum rotational frequency in commercially available 10 kW servo motors is 50 Hz. The 100 Hz rotation is realized by doubling the frequency with a belt drive. 2.2. Determination of material and shape of rotor The chemical composition and mechanical properties of the Nibased superalloys at room temperature are listed in Table 1 [5]. Nimonic 75 is used for chopper blades for f¼50 Hz at ISIS [1], and Inconel X750 is used for f¼60 Hz at IPNS [2] and LANSCE [3]. We aimed to choose such a material whose maximum stress will be less than half of the proof stress to avoid fatigue breaking. From the radiation activation properties, the material should not include cobalt. Consequently, Inconel X750 was chosen as the chopper blade material for f¼100 Hz. In order to determine the shape of the rotor, we performed stress analysis calculations. First, we assumed the ISIS-type rotor structure [1], even for f¼100 Hz. The chopper blade (Inconel X750) is supported by light metals (aluminum) around the shaft, and the blade and the support are connected with a pin (titanium). In the calculation, a maximum stress more than the tensile strength occurred at the joint section between the materials around the pin. Next, in order to reduce the maximum stress, the material of the support was changed to titanium and the blade and the support were connected using the so-called Christmas-tree shaped structure, which is similar to the structure used in a gas turbine, instead of a
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5
Table 1 Ni-based superalloys used for chopper blades [5]. Chemical composition and mechanical strength. The balance in the composition is Ni. Chemical composition (%)
Cr
Nimonic 75 Nimonic 90 Inconel 625 Inconel X 750
Fe
19.5 – 19.5 – 21.5 – 15 7
Co
Ti
Al
– 17 – –
0.4 2.4 – 2.5
– 1.4 – 0.7
Mechanical strength (MPa) Nb þTa
– – – 1.0
0
Tensile 0.2% proof strength stress 820 1230 900 1220
450 810 810 830
-5
fluctuations (μs)
Material
0
10
20
30
40
10
5
0 50
100
150
200
150
200
10
5
0 50
100
time (s) Fig. 3. Calculated stress distribution in the chopper rotor at f¼ 100 Hz. Gray scale represents the value of the von Mises stress (units: MPa). The maximum stress is 332 MPa.
pin. We calculated the stress distribution in the rotor for some patterns of the Christmas-tree shaped structure on the connecting part; however, the maximum stress could not be reduced to less than 700 MPa. Finally, we found that we can successfully reduce the maximum stress if the rotor, including the chopper blade, the support, and the shaft are forged as a single body without any connection. In this case, the maximum stress was calculated to be 332 MPa, as shown in Fig. 3. For this shape, the mass of the rotor is m¼120 kg and the moment of inertia is I¼4.7 kg m2. In order to machine the rotor in a single body, 530 mm 700 mm 95 mm of forged material is required. Such a large-sized forged material of Inconel X750 was not known until the start of the present design. However, during the present design, it was found that such a largesized forged material can be manufactured and machined with an appropriately high precision. Therefore, we decided to make the rotor as a single body of Inconel X750. Also, the frequencies of the proper vibration of the rotor were calculated to be 138.0 Hz, 380.7 Hz, and 827.6 Hz. These are beyond the range of the rotational frequencies used in the operation (f r 100 Hz). 2.3. Mechanical properties and running test For the rotation, the servo motor was driven by pulses generated from a function generator. The motor makes one rotation by applying 8192 pulses from the function generator to the servo amplifier, and the motor rotates at 50 Hz by using a pulse repetition of 409.6 kHz. The motor frequency of 50 Hz was doubled using a timing belt, and a rotation sensor for counting 100 Hz was mounted on the shaft of the rotor. The base timing of 100 Hz was generated by the same function generator used to drive the motor. The difference between the rotational pulse from the encoder and the base timing was observed. Peak-to-peak fluctuations of 3 ms were initially observed without any control
Fig. 4. Rotational fluctuations at f ¼100 Hz with respect to the base timing: (a) a typical example of the original fluctuations in the best case; (b) and (c) are irregular fluctuations.
device, as shown in Fig. 4(a). To reduce the fluctuations, we mounted a stabilizer, which is a dynamic damper used as a device for absorbing the change of the rotational frequency, on the shaft. By using the stabilizer, the peak-to-peak fluctuations were successfully reduced to 1:5 ms. This satisfied the specification for the phase control accuracy of 75 ms. The vibration levels were evaluated using the vibration amplitudes. For 100 Hz, the excellent vibration level is defined for amplitudes less than 7:95 mm [6]. The amplitudes for the stable operation at 100 Hz were measured to be 2:0 mm, 2:7 mm, and 1:9 mm at the bearing housings on the motor side, the rotation sensor side, and at the rotation sensor base, respectively. The vibrations were in the excellent level. Since the excellent mechanical properties were confirmed, as described above, we started a continuous running test to investigate the lifetime of this T0 chopper. An unmanned system was also designed to monitor the vibrations, temperatures, water flows, vacuum, electric currents, and so on. We defined a threshold value for each monitoring item, and the system will be stopped automatically in case of an emergency. The monitoring items were selected for practical operation. In the running test, a total running time of 4648 h was recorded; also, continuous operation time of 1529 h was recorded. However, the lifetime of the magnetic seal was limited to approximately 2500 h in the early stage of our development, because of evaporation of solvents in the magnetic fluids. By changing the magnetic fluid to that which has a higher vapor pressure, optimizing the filling quantity of the magnetic fluid, and changing the magnetic circuit, the performance of the magnetic seals was greatly improved. As the result, we confirmed that the lifetime of the magnetic seals was extended to 4400 h. During the running test, we observed two types of irregular fluctuations, as shown in Fig. 4(b) and (c). In the first type, the
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1
ON OFF
number of events
rotational phase recovered after sudden fluctuations. In the other type, the rotational phase was shifted to the delay side after irregular fluctuations. The origin of these fluctuations is unknown; however, it might be related to the condition of the bearings. We recorded the number of events for these types of fluctuations. The fluctuation amplitude was found to be almost less than 5 ms, which is satisfied by the specification for the control accuracy of 75 ms. The number of such fluctuations gradually decreased during the running test and the system became stable after running for about 200 h at f¼100 Hz.
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2.4. Control system We took measures to reduce the rotational fluctuations mechanically as much as possible as described above, but the rotational fluctuations are influenced by factors such as heat transformation and change in the grease viscosity with changing temperature. It is not easy to maintain the rotational frequency at 100 Hz within the fluctuations of 7 5 ms. Also, the rotational phase should be synchronized with the production time of the pulsed neutrons. Therefore, a control system was developed for the T0 chopper, and the rotational frequency and phase were automatically controlled by the pulses given to the servo motor in the case of frequencies beyond a designated range of the expected frequency, as shown in Fig. 5. The control system consists of the acceleration, pulse generation, and phase comparison blocks. The J-PARC provides a 12 MHz timing signal from the master clock (MC) and the 25 Hz timing signal is synchronized with the MC. The accelerator tries to operate in synchronization with the 25 Hz timing (so-called scheduled timing), and pulsed neutrons are produced at 25 Hz with some timing fluctuations reflecting the fluctuations in the accelerator [7]. The T0 chopper is operated based on the MC signal and the scheduled timing; both timings are synchronized. In the control system, the acceleration block has a direct digital synthesizer (DDS) that generates frequencies up to 12 MHz independent of the MC. First, the acceleration of the T0 chopper is controlled by the acceleration of the DDS. When the rotor frequency of the T0 chopper is close to f¼100 Hz and the frequency of DDS is close to that of the MC, the control source is switched to MC. When the MC signal is lost or the T0 chopper ramps down for stopping, the control source is switched to DDS. The motor makes one rotation by applying 8192 pulses to the servo amplifier, and the T0 chopper runs at f¼100 Hz by rotating the motor at 50 Hz by applying 409.6 kHz pulses generated by the MC or DDS to the servo amplifier. Normally, the MC timing is used, but the DDS timing is used in the case where the MC signal
Fig. 5. Block diagram of the control system.
0
-5
0
5
deviation from target timing (μs) Fig. 6. Rotational fluctuations in the T0 chopper with (ON) and without (OFF) the control system. Number of events normalized by the total number within a constant measuring time is plotted as a function of the deviation from the target timing.
is lost. During the pulse interval of the scheduled timing of 40 ms, 480 000 pulses are issued by the MC and 8192 2 pulses are applied to the servo amplifier for f¼ 100 Hz. Adding one pulse, during 480 001 pulses issued by the MC, by applying 8192 2 pulses to the servo amplifier, the phase of the rotation of the T0 chopper is delayed. In the pulse generation block, by choosing the number of pulses to be added to the 480 000 pulses while applying the pulses to the servo amplifier corresponding to f, the phase delay of the T0 chopper rotor is controlled. The addition of one pulse corresponds to the phase delay of 0:083 ms and 0.0441. In the phase comparison block, by comparing the timing of the rotor phase of the T0 chopper with the scheduled timing, the number of pulses to be added in the pulse generation block is controlled. Fig. 6 shows the rotational fluctuations observed with and without the control system. The reduction of the rotational fluctuations was greatly improved to approximately 1 ms at the full-width at the half-maximum (FWHM).
2.5. Other developments toward actual machines The test machine for f¼100 Hz shown in Fig. 1 has a length of approximate 1.6 m along the beamline and occupies a large space. By considering the rearrangement of the components of the T0 chopper, the length was reduced. By placing the motor under the vacuum chamber, the length was reduced to 1.3 m. For the case of low frequencies such as f¼50 Hz and 25 Hz, it was found by measuring the rotational fluctuations as well as the vibration levels at some parts of the T0 chopper that the base plate can be shortened and that the stabilizer is not effective. Also, the connecting region of the water and electricity at the base was redesigned. The length was successfully reduced to less than 1 m for low frequency machines. The location of the T0 chopper on the neutron beamlines at J-PARC is at the bottom of the shielding whose height is 4 m. During maintenance, we should stand on a part of the shielding to reduce radiation exposure, and mount and remove the T0 chopper by employing few procedures using a crane after removing the shields above the T0 chopper. The main body sits on the base and the cooling water, electricity, and vacuum are connected through the base. In our design, these are connected/disconnected just by sitting on/removing them from the base by a crane. Fig. 7 shows the design of the model of the semi-auto installation mechanism
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Fig. 7. A model of the semi-auto installation mechanism for cooling water.
for the cooling water. We performed a repeating test for the connection/disconnection mechanism. The radiation damage of the main components such as bearings, magnetic seals, timing belts, and rotation sensors is one of the factors that limits the lifetime of the T0 chopper. The important components are located on the rotor shaft, 300 mm from the neutron beamline. At this position, g-rays are dominant and the absorption dose can be calculated to be in the order of 1 kGy/yr at 1 MW of the accelerator beam power [8]. We investigated the radiation properties of some components of the T0 chopper by irradiating high-energy g-rays emitted from Co-60. The irradiation was performed at the Cobalt 60 Irradiation Facility, Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. The properties of mechanical parts such as bearings, magnetic seals, and timing belts were almost unchanged up to the irradiation of 100 kGy, but the rotation sensors were damaged even at 1 kGy. To avoid radiation damage to the sensors, the marking of the rotation on the rotor shaft was transferred through an optical fiber to the control system located out of the beamline shielding. The details will be described in a separate paper [9].
3. Actual machine production for neutron spectrometers at J-PARC Based on the above developments, we designed the actual machines and installed them in the neutron beamlines at J-PARC. Fig. 8 shows the T0 chopper installed on the High Resolution Chopper Spectrometer (HRC) [10]. The HRC installed at the 12th neutron beamline (BL12) is an inelastic neutron scattering instrument that uses neutrons monochromatized by a Fermi chopper, and the dynamics in matter can be investigated using high resolutions by utilizing neutrons with energies up to the eV region. The T0 chopper was installed at L¼9 m. At this position, the beam cross-section is 76 mm 76 mm (w¼ 76 mm) and the size of the T0 chopper blade is 78 mm 78 mm (Dw ¼ 1 mm). Since t 0 ¼ 408 ms for f¼100 Hz, the upper boundary of the energy of the neutrons utilized on the HRC is estimated as E0 ¼2.5 eV. In this high frequency model, the rotor, which has a symmetric shape with respect to the rotational shaft shown in Fig. 2, was originally made of Inconel X 750 as a single body, including the rotational axis as mentioned above, to reduce the maximum stress to the material strength. Recently, since the material price was drastically increased, the structure was changed. The rotor body, including the rotational axis, was made of cobalt-free stainless steel and a cylindrical Inconel part was inserted at the beam hitting position. These two materials were bonded by the hot isostatic press method, and then, the stress was successfully reduced. This high frequency model was also installed on the NeutronNucleolus Reaction Instrument (BL04). The cross-sections of nuclear reactions are measured on BL04 and eV neutrons are required. By installing the T0 chopper at L¼13 m, neutrons up to 18 eV are
Fig. 8. High frequency model (f¼ 100 Hz) of the T0 chopper mounting the semiauto installation mechanism. In the figure on the right, the upper part of the vacuum chamber is removed to show the rotor. Normally, the upper part is fixed at the body. The motor is mounted under the vacuum chamber.
Fig. 9. Variations of models of the T0 chopper. A short model (f¼50 Hz) for the total scattering instrument (a), and a two-hole model (f¼25 Hz) for the reflectometer (b).
utilized at 100 Hz. The High-Intensity Total Diffractometer (BL21) utilizes neutrons with a wide wavelength band. To increase the wavelength band, the rotor of the T0 chopper has an asymmetric shape and the beamline is blocked once for one rotation; also, a counter-rotating set-up of two T0 choppers is proposed. For this purpose, the length along the neutron beam should be minimized. The short model can be realized for a lower rotational frequency of f¼50 Hz, as mentioned above. The total length of the high frequency model is 1.3 m and that of the short model is 1 m as shown in Fig. 9(a). One of the two counter-rotating T0 choppers was installed. The surface structure of the materials on the High-Performance Neutron Reflectometer with a Horizontal Sample Geometry (BL16) can be investigated through the momentum dependence of the neutron reflection on the sample surface mounted horizontally. BL16 has two beamlines, and the angle between the horizontal line and the beamline can be chosen to be 2.21 or 5.71. Therefore, the T0 chopper for BL16 should have two beam holes. The rotational axis is located between the two beamlines and the rotor shape is asymmetric, as shown in Fig. 9(b), and f¼25 Hz.
4. Performance The performance of the T0 chopper was investigated on the HRC. The fluctuations in the rotational period of the actual T0
S. Itoh et al. / Nuclear Instruments and Methods in Physics Research A 661 (2012) 86–92
2.5eV
0.63eV
0.16eV
Transmission
1
100Hz 50Hz 25Hz 0 0
1000
2000
3000
4000
TOF (μs) Fig. 10. TOF dependence of the T0 chopper transmission measured with the white neutron beam. The designed values of E0 are indicated at the corresponding TOF.
105 OFF 100Hz 50Hz 25Hz
104
Intensity (arb.units)
chopper were measured to be approximately 1 ms (FWHM) in the range f¼25–100 Hz, i.e., they were almost independent of f. This result reproduces the performance shown in Fig. 6. Although these fluctuations increase to 3 ms (FWHM) during the beam time probably due to the electrical noise generated in the experimental hall, these fluctuations still satisfy the requirement demanded by the phase control accuracy within 75 ms. Transmission through the T0 chopper was measured with the neutron beam. The white beam of pulsed neutrons is incident on the vanadium standard sample located at 15 m; then, it is scattered and detected by the detector system located at 4 m from the sample [10]. Fig. 10 shows the time-of-flight (TOF) dependence of the observed transmission, which is the ratio of the spectrum measured with the T0 chopper being operated at the appropriate f to that with the T0 chopper off and the blade removed from the beamline. We confirmed that the transmission recovers below 2.5 eV of neutron energy at f¼100 Hz, 0.63 eV at f¼50 Hz, and 0.16 eV at f¼25 Hz, as designed. Inelastic neutron scattering experiments were performed on the HRC by using the T0 chopper. The pulsed neutron beam is monochromatized by a Fermi chopper located at 14 m from the neutron source and made incident on the vanadium standard sample located at 15 m; it is then scattered and detected by the detector system located at 4 m from the sample [10]. The Fermi chopper consists of a slit package rotating in synchronization with the production timing of pulsed neutrons to monochromatize the neutrons. The Fermi chopper opens and the neutrons pass through the slit when it rotates to a position where the slit is parallel to the incident neutron beam, and the slit-open time determines the energy resolution of the HRC. Fig. 11 shows the TOF spectrum at the detector system, with the T0 chopper at f¼25 Hz, 50 Hz, 100 Hz, and with the T0 chopper off and the blade removed from the beamline. The Fermi chopper was operated at 600 Hz. Since the slit width of the Fermi chopper is coarse, the neutron beam passes through the Fermi chopper at every half turn, then many peaks appear. The neutron energies of the peaks at TOF ¼ 1940 ms and 4190 ms correspond to 505 meV and 108 meV, respectively. We successfully reduced the background noise at neutron energies of around 500 meV by two orders of magnitude, as shown in Fig. 11. This indicates that inelastic neutron scattering experiments that require the detection of very small signals can be conducted on the HRC. In the very short TOF region, the background noise increases with
91
103
102
101
100
0
2000
4000
6000
8000
TOF (μs) Fig. 11. Effect of T0 chopper on background-noise reduction for monochromatic neutron beam. The TOF spectra for the T0 chopper operation at 100 Hz, 50 Hz, 25 Hz, and in no operation (OFF) condition are indicated.
increasing f. This suggests that the beam size for high-energy neutrons, which should be reduced by the T0 chopper, might be slightly wider than the blade width of the T0 chopper. At present, a boron tertacarbide (B4C) collimator defining the beam crosssection for thermal neutrons is installed in the region of the incident flight path between the neutron source and the sample [10]. By replacing the collimator by a supermirror guide tube, a larger flux gain is expected; also, the beamline will be defined by a steel wall and the collimation for high-energy neutrons will be improved. Further background-noise reduction will therefore be expected. Inelastic neutron scattering occurs around each peak in Fig. 11. The energy transfer from the neutron to the sample is defined as E¼Ei Ef, where Ei is the incident neutron energy, which is the energy at the peak, and Ef is the energy of the scattered neutron determined from the TOF. Fig. 12 shows the inelastic spectra at Ei ¼108 meV and Ei ¼505 meV as a function of E converted from the spectra shown in Fig. 11. The central peak at E¼0 corresponds to the peak at TOF ¼ 4190 ms (Ei ¼108 meV) or TOF ¼ 1940 ms (Ei ¼ 505 meV), and the width (FWHM) of the central peak gives the energy resolution DE ¼ 1:7 meV for Ei ¼108 meV or DE ¼ 32 meV for Ei ¼505 meV. In Fig. 12, ION is the spectrum observed with the T0 chopper rotating at f¼100 Hz and IOFF is that with the T0 chopper off and the blade removed from the beamline; also, the ratio IOFF/ION is indicated. The spectrum at f¼50 Hz was identical to that at f¼100 Hz. As shown in Fig. 12(a), phonons are clearly observed for ION around the central peak because of the good energy resolution (DE ¼ 1:7 meV) as well as the low background noise for Ei ¼108 meV. Since the measurement was performed at the room temperature, the scattering intensity is significantly reduced at E o 30 meV, corresponding to the thermal energy at the room temperature. At present, on the HRC, there exists another source of background noise that can be attributed to the scattering at the beam windows of the devices in the incident light path just upstream of the sample [10]. This noise cannot be removed by the T0 chopper and is rapidly increased at large E due to the conversion of TOF to E. This noise should be removed by further development of the HRC. However, large background noise arises without the T0 chopper and the phonons can be seen in the background noise for IOFF. The ratio IOFF/ION indicates the background noise without the T0 chopper. Since the ratio decreases around the central peak and the phonons and also decreases at large E due to the existence of another
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bearings in vacuum. The motor is located outside the vacuum and the rotation is transmitted to the vacuum through magnetic seals. The motor should be rotated in synchronization with the production timing of pulsed neutrons. The rotational fluctuations and the running time were well in agreement with the specifications. A semi-auto installation mechanism was also developed for installation under the shielding and for maintenance. Based on the result of the development, actual machines were manufactured for the neutron beamlines at J-PARC. We successfully reduced the background noise at neutron energies of around 500 meV to 1/30 in inelastic neutron scattering experiments.
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E (meV) Fig. 12. Effect of T0 chopper on background-noise reduction on inelastic spectra. The inelastic spectra for the T0 chopper operation at 100 Hz (ION) and in the no operation (IOFF) condition are indicated for Ei ¼108 meV (a) and Ei ¼ 505 meV (b). Also, the ratio IOFF/ION is plotted.
background noise mentioned above, the ratio at E5 0 indicates the background noise in the absence of the T0 chopper. The background noise is reduced to 1/10 for Ei ¼108 meV. Similar features are also seen for Ei ¼505 meV in Fig. 12(b), although phonons are smeared out due to the coarse resolution of DE ¼ 32 meV. The background noise is reduced to 1/30 for Ei ¼505 meV. 5. Summary We developed a T0 chopper rotating at 100 Hz with a rotor of 120 kg, made from Inconel X750, and supported by mechanical
Acknowledgments We are grateful to Prof. N. Hitomi for initiating the collaboration project for these developments, Profs. M. Ishihama and M. Kawai for insightful discussions, and Metal Technology Co. Ltd., Techno AP Co. Ltd., Rigaku Corporation, Toshiba Machine Co. Ltd. for realization of the T0 chopper. References [1] T.J.L. Jones, I. Davidson, B.C. Boland, Z.A. Bowden, A.D. Taylor, in: Proceedings of the 9th Meeting of the International Collaboration on Advanced Neutron Sources, Swiss Institute for Nuclear Research, 1987, p. 529 (ISBN 3-90799801-4). [2] C. Loong, private communications, 2002. [3] R.J. McQueeney, R.A. Robinson, Neutron News 14 (2003) 36. [4] Japanese Industrial Standards, JIS B 0905, 1992. [5] For instance, Heat Resistance Alloys, MMC Superalloy Corporation, 2010 /http://group.mmc.co.jp/superalloy/global/S. [6] Japanese Industrial Standards, JIS B 8330, 2000. [7] F. Tamura, M. Yoshii, A. Schnase, C. Ohmori, M. Yamamoto, M. Nomura, M. Toda, T. Shimada, K. Hara, K. Hasegawa, Nuclear Instruments and Methods in Physics Research Section A 647 (2011) 25. [8] M. Tamura, unpublished, 2004. [9] S. Itoh, K. Ueno, R. Ohkubo, H. Sagehashi, Y. Funahashi, T. Yokoo, Nuclear Instruments and Methods in Physics Research Section A 654 (2011) 527. [10] S. Itoh, T. Yokoo, S. Satoh, S. Yano, D. Kawana, J. Suzuki, T.J. Sato, Nuclear Instruments and Methods in Physics Research Section A 631 (2011) 90.