Preliminary design of TMR for CPHS and proposal of installation of UCNS

Takeshi KAWAI,Qixi Feng, Guo Xiaohong

Quanke Feng, School of Energy and Power Engineering,

Xi’an Jiaotong University

Outline

1. CPHS2. Neutron3. CNS 1) Preliminary design of TMR 2) Application of CN: (1) Neutron Optics (2) Supermirror for controlling the neutron beam and its spin states4. Proposal of UCNS 1) Preliminary design of UCNS 2) Application of UCN to Material Science (1) Investigation of the surface: 5. Conclusion

1. CPHS: 1) What is China Pulsed Hadron Source (CPHS)?: (Prof. Wei Jie, Private communications) (1) Beam power: 16 kW at proton energy of 13 MeV, (2) Repetition rate is 50 Hz, (3) Pulse length: 0.5 ms (Long pulse), (4) Peak beam current: 50 mA. (5) Average beam current: 1.25 mA.

2) Peculiarities of CPHS: 1) Low proton energy: 13MeV, resulting in, maybe, no need of the remote

handling system available for simulating TMR (Target, Moderator and Reflector) to check its designs of TMR for the China Spallation Neutron Source (CSNS) which is now planned to construct in Guangdong Province （广东） ,

2) For acceleration of Proton: a linear accelerator consisting of a succession of accelerating elements (called drift tubes) in line for the particles to traverse a radio-frequency-voltage passage repeatedly, 3) Long pulsed neutron beam (0.5ms):

For acceleration of Proton: Linear accelerator (linacs):

　 It consists of an evacuated tube containing a set of metal drift tubes with alternate tubes attached to either side of a radio-frequency voltage. The proton (hydrogen ion) source is continuous, but only those protons inside a certain time bunch will be accelerated. Such protons traverse the gap between successive tubes (therefore in a field free region) from left to right and inside a tube where the voltage changes sign.

Ion source

+

Fig.1 Scheme of a proton linear accelerator. Protons from the ion source traverse the line of drift tubes (evacuated pipe). The successive lengths are chosen so that as the proton velocity increases, the transit time from tube to tube remains constant.

Neutron Yield from 9Be (p,n) reaction

InterdisciplinaryScience

Engineering:Purpose

Science:Mental

Curiosity

Harmonious Society & Comfortable Life

Amenities

Protecting Environment,Purification of Contamination,

Recycling TechnologyHarmonious Energy Society

Saving EnergyGreen Energy

SafetyLife & Material Science

Social Science

2. Neutron: Many countries carry the following banner for Scientific Challenges as their strategy in 21st century:

1. Life Science: Creation of New Medicine: Protein & Enzyme,2. Material Science: Nano-scale technology: Composite Material functioning in the extreme conditions3. Environmental Science: Earth, atmosphere, land, water, climate Compatible Concept to live together, 4. Energy and Power Technology: ITER to Green Energy 5.Communication and Computer Vision Technology: Smart Sensor with the perception functions beyond the ability of human brain

Why? Of course, these subjects determine not only their fortune, but also mankind’s destiny. Neutrons can be utilized as their probes in these fields.

Cold neutron and Cold Neutron Source:Neutron is discovered in 1933 by ChadwickNeutron is quantum particle (quanta): 1) Duality: Particle property and Wave Property: verified by Takeshi Kawai et al. in 1997. de Broglie relation: λ = h/p Wave length of thermal neutrons is comparable with mean separation of atoms in solids.

Why the neutron is so useful in the study of condensed matter?

2) Light elements are transparent and heavy elements are relatively opaque to X-rays. But light elements such as hydrogen, lithium, boron are highly absorbent for neutrons, whereas heavy elements, such as lead, bismuth, transmit neutrons freely. 3) No charge: no accompanying electric field and thus this property enables the neutron to penetrate most materials, in situ experiments and in a variety of environments. 4) The neutron interacts with the atomic nuclei. The strength of interaction (scattering length) depends not only on the kind of atom but also on its isotopic mass. By substituting isotopes in defined way, a useful contrast variation can be achieved. Hydrogen is hardly detectable with x-rays, but neutrons has a rather large scattering length for it.

de Brogliewavelength(Ǻ)

Velocity(m s-1)

Temperature (K)

Energy(eV)

Hot neutrons 0.69 5,730 2,000 0.172

Thermal neutrons

1.8 2,200 300 0.025

Cold neutrons

4 1,000 58 0.005

Very-cold neutrons 28.6 138 1.16 10-4

Ultra-coldneutrons 1,000 <7 10-3

10-7

Table 1 Approximate neutron classifications

　Advanced Utilization of Neutrons Life Science, Frontier Science, Innovation, Discovery 20

139

11

8

32

11

　

Construction 　

　 Heavy Industry 　 Stress-strain

Measurement

Chemistry 　Multi-layer of

polymerNew Medicine

　 MaterialHigh performance

MagnetStress-strain analysis

　 AutomobileFuel-Battery

　Medicine 　New

medicine

Atomic Energy

1 　other 　

Application of Neutron to Industry

Development of New Material

Total78

Comp.

　 Electro-material Comp. 　

Lithium battery, Smart SensorHigh Temp. Superconductor

3. CNS 1) Preliminary design of TMR

For getting the intense cold neutron beam from the CPHS we need to design the Target, Moderators and Reflectors (TMR) appropriately and optimize them taking also account of safety of the facility.

We are now starting the design of theTMR at the Xi’an Jiaotong University in Xi’an under the collaboration with Prof. Wei Jie group.

Preliminary simulation on the TMR:

The relative neutron flux for six kinds of geometries of TMR including the CNS to predict the thermal and cold neutron fluxes. 1)Geometry A: water as a pre-moderator,2)Geometry B: water as reflector,3)Geometry C: water is used as reflector and pre-moderator, 4)Geometry D, E, F: Liquid para-hydrogen as a CNS is attached.

BeWater

Detecting SurfaceBe

Water

Detecting Surface

BeWater

Detecting Surface

Water

BeWater

Detecting Surface

Liquid H2

Geometry A

Geometry D

Geometry B Geometry C

BeWater

Detecting Surface

Liquid H2

Geometry E

BeWater

Water

Detecting SurfaceLiquid H2

Geometry F

hahb

hb

hc

hb

hchf

hb

hehahd

Maxwell distribution:

Normalized constant:

( ) exp( / )p E C E E a 1 3

2 2(2 )C a

Spallation Neutron: Beryllium film (1mm) ； Isotropic Neutron flux with Maxwell distribution

Result of simulation:1.Geometry for getting the Optimum Thermal Neutron Flux: 1) Reflector: 10 cm, 2) Pre-moderator: 3.5 cm,

Geometry for getting the optimum cold neutron flux: 1) Reflector: 10 cm, 2) Pre-moderator: 3.5 cm, 3) Liquid para-Hydrogen: 4 cm

LENS case: Reflector: 25 cm, Be: 1mm, Pre-moderator: 3.8 cm,SCH4: 1 cm in the Geometry F Low energy neutron flux is much higher than the case of LH2. Ratio of the cold neutron flux to the incident neutron flux is about 10-3. However the control of LH2 is much easier than the solid CH4.

Cold Neutron Flux:

Fig. 8 Relative cold neutron flux depending on the thickness of LH2 in the geometry F. Optimum thickness is about 4 cm. Conclusion: Ratio of cold neutron flux with an energy range of 4 to 6 meV to incident spallation neutron flux is about 7x10-3. However high energy neutron is also ejected from the surface of the LH2. This ratio is about 0.17 which must be removed using the neutron guide tube and shielding.

Example: Neutron optics: Supermirror:Application of cold neutron to the fundamental physics:

1)Delayed Choice Experiment: In 1978, J. A. Wheeler presented the delayed choice thought-experiment to verify Bohr’s view concerning the propagation behavior of a photon after being split through a semi-transparent mirror. In this experiment, it is required to establish the condition of “whether the second wave splitter is introduced or not at the point of intersection of the two partial waves split by the first wave splitter” after the neutron is split through a semi-transparent mirror (wave splitter). Cold neutron pulser (not Pulsar): a multilayer polarizing neutron-mirror placed in the pulsed magnetic field.

Jamin-type Neutron Spin Interferometer using Polarized Neutrons

Fig. 6 Schematic diagram of arrangement of Composite Neutron Mirrors for Delayed Choice Experiment

Multi-layers Neutron Mirror (Supermirror):

We are preparing to fabricate the Supermirror at the Electronics Material Laboratory of Xi’an Jiaotong University (XJTU) under collaboration of Prof. Wei Ren.

Example of Magnetic Supermirror:1. Pulser (not Pulsar) to produce the pulsed polarized neutrons:

1) Low field magnetic mirror: The magnetic devices for a cold neutron optics have to satisfy the following requirements; (1) as small as possible to install it in a restricted space, (2) the stray fields from the magnetic devices should be low enough so as to give no magnetic effects to nearby devices. (3) the magnetic devices must control neutron spin states easily and precisely.For satisfying these requirements, the neutron magnetic mirror should be functioned under as low magnetic field as possible. (4) Refractive index n± of a material for a neutron is given by

2

218 2 2

12

1 ,

( ) / ,

22.56 10 [ ] [ ],

6.03 10 [ / ] [ ],

n m

Nn N

m

n

V V E

bV x eV cm b cm

m

V B x eV gauss B gauss

where is the magnetic moment of the neutron, E neutron energy in vacuum, the density of the mirror material, bN a coherent scattering length of the nuclei of the material, and B a saturation induction of a ferromagnetic material.

2 2 / 2xk m

nV B

nV B

Vn= 220 neV

Ge

Permalloy

Fig. 1 Schematic one-dimensional potential of Permalloy/Ge multilayer. Vn = 220 and 93 [neV] for Permalloy and germanium.The Permalloy is composed of Fe 55% and Ni 45%.

=316 neV

=123 neV

If the magnetic field B is inhomogeneous, the force F exerts on the neutron as;

( ).F B r

This holds only in the case where the neutron’s motion is so slow that the magnetic moment of the neutron follows the same orientation with B (adiabatic condition). Therefore the field must be changed so enough slowly than the precession frequency of the neutron magnetic moment.

1 1 2L

B B

B t

If / 0B t the Hamiltonian of neutron does not explicitly

depend on time and thus the total energy of the neutron is conserved, whatever the spatial distribution of the field along the neutron trajectory may be.

The change of the potential energy induced by the spatial variation of a static magnetic field brings about an equivalent change of the kinetic energy of the neutron while its total energy is to remain constant.

Fig.2 Schematic diagram illustrating the reflection of neutrons from the neutron magnetic mirror. Neutrons sense different optical potential depending on its spin state.

Fig. 3 Reflectivity of the Fe-Co/V polarizing monochromator. Polarization is 85% for 6.5 Å neutrons of 5 mm in beam width.

Permalloy/Ge multilayer neutron polarizer (PGM) consisting of parallel layers of Permalloy and germanium alternatingly stacked was developed. This magnetic mirror works under a very low magnetic field around zero gauss. It is fabricated by the vacuum evaporation of material onto a Si-wafer under a magnetic field of about 100 gauss. The refractive index of Ge for neutron is approximately equal to that of permalloy for neutrons with spin aligned anti-parallel to the magnetization of the permalloy layer. The bilayer consisting of a permalloy and Ge layers function as a single layer. The pulsed monochromatic beam can be generated if the magnetic induction orientation in a ferromagnetic layer can be changed by an external pulsing magnetic field as shown in Fig. 5. We call “ a low field magnetic mirror placed in the pulsed magnetic field ” a cold neutron pulser.

Neutron Reflectivity by Neutron Pulser

Fig. 4 Variation of the neutron reflectivity from the Permalloy/Ge polarizing monochromator with the magnetic field in which the reflectivity is saturated around 10 gausses.

Figure 4 shows also the magnetization history of this material where the saturation induction of the Permalloy 45 is 1.6 Tesla. This is demonstrated from the neutron reflectivity from the magnetic mirror.

Cold Neutron Pulser placed in the pulsed magnetic field

Permalloy/Ge multilayer

Fig. 5 Time spectra of the pulsed magnetic field and neutron pulses

Jamin-type Neutron Spin Interferometer using Polarized Neutrons

Fig. 6 Schematic diagram of arrangement of Composite Neutron Mirrors for Delayed Choice Experiment

Composite mirror 2, CM2

Composite mirror 1,CM 1

Jamin-type Neutron Interferometer

Fig. 8 Interference fringes observed by changing the angle of the second composite mirror relative to the first one in the Jamin type interferometer.

Fig. 7 Jamin type neutron interferometer

Delayed Choice Experiments

Fig. 9 Schematic diagram of the delayed choice experiment.The pair of identical composite neutron mirrors composes a Jamin type interferometer.

Fig.10 The space time relationship for the delayed choice experiment. The trigger pulse frequencies of a -flipper and the pulsing magnetic coil of the second composite mirror (CM2) are the same, i.e. 234 Hz, of which half period corresponds to the neutron flight time from the -flipper to CM2.

(a) The PGM of CM2 is not placed when the neutron reached at CM1, and it is introduced after the neutron passed through CM1.

(b) The PGM of CM2 is placed when the neutron reached CM1, and it is removed after the neutron passed through CM1.

Fig.

Fig. 11 Interference fringes obtained from the delayed choice

Experiment.

Conclusion of the delayed choice experiment:

It was shown that whether the interference fringe is obtained or not depends only on whether the second wave splitter is introduced or not, after the neutron passed through the first wave splitter. Delaying choice has no effect on the interference. From this result, we could deduce that the neutron wave propagates Both ways with equal probability after being split by the first wave splitter and does not choose one of the two ways which is explained by the superposition principle of quantum mechanics.

Let’s challenge to measure the longitudinal coherence length Using a time-resolved measuring method;

Method: The direction of the magnetic induction of the pulsed magnetic mirror (PMM) is changed alternately by applying the alternating high-frequency external field to the PMM (high-frequency cold neutron pulser), and thus the up- and down-spin components are reflected alternately through the PMM. By this method, we could generate the state consisting of two coherent spin eigen-states staggered in time. The longitudinal coherence length could be measured in combination of the high-frequency cold neutron pulser with the cold neutron spin-interferometer.

Thus if we install the CNS and the neutron spin-echo instrument at the CPHS facility, they contribute understanding of Quantum Mechanics toward students.

4. Proposal of UCNS:

UCN Energy Scales:1.Energy of UCN moving 8 m/sec (58Ni): 500Å, 340 neV≈5 mK,2.Energy of UCN under 1 T magnetic field: ±60 neV 3.Energy change associated with a 1 m rise: 104 neV,4.Effective potential barrier (Ueff) at a diamond film: 260 neV. They can undergo total external reflection at all angles from the surfaces of a variety of materials, which leads to the possibility that ultra-cold neutrons can be totally confined within a bottle for periods in excess of 100 seconds, making a compact source of stored neutrons for use in measurements of material and fundamental physics. It is also possible to 100% polarize UCN by passage through a magnetic field gradient with a maximum field of 6 Tesla, due to the fact that the kinetic energy of an UCN is so small that UCN of one spin state cannot overcome the potential barrier of the magnetic field, B.

Principle of UCNS using SD2:

Super-thermal UCN source exploits a large number of phonon modes for neutron down-scattering by the low temperature substances with a small absorption cross-section where neutrons do not reach thermal equilibrium with the moderator due to the negligible up-scattering

by the Boltzmann factor compared with the down-scattering.

Z-Ch. Yu S. S. Malik and R. Golub, Z. Physik B (1986)137

1) Preliminary design of UCNS What neutron flux we can get? 1A=1[C/s]=1/1.6x10-19 [protons/s]1mA=0.6x1016[protons/s]=0.6x{n/p}x 1016[n/s]

　 (1) Reference data: {n/p} 10-2, for Be target and proton beam with

13MeV, (C.M. Lavelle, D.V. Baxter, et. al., LENS report, March, 2008) Then, 1mA=0.6x1014[n/s] 1.23mA=7x 1013[n/s], for CPHS(Prof. Wei Jie, Private communications)

When we use LH2 for CNS and LD2 for UCNS, relative cold neutron flux is (7x10-3)x(7x 1013)=5x1011

For solid deuterium, assuming Tn=100Ku=6.4x10-100 [cm-3] =320 UCN/cm3 30 UCN/cm3, for 0=5x1011 cold neutron fluxR. Golub and K. Boning, Z. Phys. B Condensed Matter 51(1983)

95-98,

VR

T0=300KUCN

SD2(5K) VH

Al

L-He

Gaseous D2 storage tank

SD2 moderator cell

1) Para-Ortho Converter2) SD2 volume: 150 mℓ3) Flow rate of LHe : 2.56 ℓ/hr for QH=1W and QC=2W

UCNS source:

P-T Diagram of Normal Deuterium

0 5 10 15 20 25 301.8

1.9

2.0

2.1

2.2

2.3

2.4

pre

ssure

of D

2 g

as

stora

ge tank

(atm

)

time (hr)

Fig. 2 Variation of pressure in the storage tank in case of QC=2W, QH=1W

20 hr

0 5 10 15 20 25 301

10

100te

mpera

ture

at D

2 m

odera

tor ce

ll (K

)

time (hr)

Fig. 3 Variation of temperature of D2 moderator cell in case of QC=2W, QH=1W

2) Application of UCN for the Material Science:

Ultra-cold neutrons (UCN) have energies low enough that 　 they are totally reflected by suitable materials and so can be 　confined in bottles. 　 The UCN flux is not so high for the CPHS, and also the CSNS is now constructing, and thus I recommend also to utilize the CPHS for developing the CSNS and the new type of source to be installed at the CSNS. However the CPHS facility has its peculiarity. So, I recommend to install the UCNS., because the UCN have the unique property that they penetrate surfaces to depths of a few hundred Angstroms, and very curious property including the absorption by the surface substances, not being made clear. So, I propose the following study, (Ref: , R. Golub, TUM, Private communication).

The depth of penetration is a function of neutron energy as well as the density and composition of the material on the surface. The loss rate of UCN on a surface is also a function of these parameters. The loss rate depends on the surface layers or film covering the surface. And thus UCN will provide a unique method for studying such a surface.

(1) Depth of penetration of ions into the surface: After ion bombardment, the surface will exposed to the UCN by which the depth penetrated by the implanted-ions are characterized through the measurement of the characteristic -raysfrom (n, ) reaction. When the UCNS is installed at the CSNS and the UCN densities is expected to be much higher, then the implantation profile is also made clear at even low concentration of the implanted ions.

Conclusion:

1. When TMR is selected adequately the Cold Neutrons are able to be utilized for investigatigating materials and fundamental physics.2. Installation of UCNS at the CPHS facility is also compatible with the TMR of the CPHS.

Thank you for your attention!