feasibility of an ice based cold neutron source

S. Basu, Hokkaido, July07

Feasibility of An Ice based Cold Neutron Source

A suitably chosen cold moderator placed inside the reactor in a pre-decided location in a research reactor can shift the Maxwellian energy distribution of the neutrons to lower energies and can cause considerable enhancement in cold neutron flux. Broadly this is the underlying principle of a cold neutron source.

Saibal BasuSolid State Physics Division

Bhabha Atomic Research CentreMumbai 400085, INDIA

Typically, neutrons with energy less than 5 meV or wavelength more than 4 Å are termed as cold neutrons in neutron scattering parlance.


19601985


University-DAE

Neutron

Guides

Triple axis

Mag. Diffrac.

Single x-tal

Diffr.

Power Diffr

With PSD

FDS

Reactor Block

S. Basu, Hokkaido, July07MSANS PNR

SANS

Spin Echo+

POLSANS


SANS-I

Double Crystal SANS

Polarized NeutronReflectometer


SANS Facility at BARC (SANS-I)

BeO filteras monochromator

collimator: 2 m

1 m long PSD

guide

sample to detector: 2 m

Q range: 0.015 to 0.3 Å-1

* = 2.2 Å

mean= 5.2 Å [Aswal et al. Current Science 79, 947 (2000)]


Typical SANS Data at Dhruva Reactor

Transmission Data

ScatteringData

ScatteringData

DetectorBeam Stop

MeasuringTime

~ 10 Hrs

Most of the

samples are

measured in

time range of

8 to 24 Hrs.


Polarized Neutron Reflectometer at Dhruva

Monochromator : Si (113) Wavelength : 2.5 Å

Polarizer : Co-Fe/Ti-Zr SM

Non-Polarizer : Ni-Mo/Ti SM Detector : Linear He3 PSD

Flux at sample : 104 n/cm2/s D.C. Flipper efficiency : 92 %

Journal of Neutron Research, 14 , (2006) 109


Reflectivity geometry and measurements

Specular reflectivity

Off-Specular (diffuse) reflectivity

Θi = Θf Scattering length density profile along normal to surface

To study the lateral inhomogeneties and morphology of the interfaces

Height-height correlation functions


Refractive Index

For Ni: θc (in deg) = 0.1λ(Å) ≈ 6.0 arc minute per Å

2

2 2

2( ) 0

d mE V

dr

The Schrödinger equation for neutron in medium of potential V

22 2

2 2

20 ( )

d mq where q E V

dr

The refractive index is defines as2

220

qnq

2

12

n b

c

b

Reflection from smooth surface: matrix method

1 1 1exp( ) exp( )I RA iq z A iq z

2 2 2exp( ) exp( )t sA iq z A iq z

1; 0; ; tRI s

I I

AAA A r t

A A


Continuity condition: 1 2

' '1 2

( 0) ( 0)

( 0) ( 0)

z z

z z

1 2

1

(1 )

r t

q r q t

1 2

1 2

q qrq q

2R r

2

1 2

1 2

q qR

q q

22

4

16 ( )ziq z

z

d zR e dz

q dz

S. J. Blundell, et al, Phys. Rev. B 46, 3391 (1992).S. K. Sinha, et al, Phys. Rev. B 38, 2297 (1988).

When q to 10q, R becomes to 10-4R


Polarized Neutrons

• Diffraction from Heusler alloys

• Reflection from magnetized multilayers ( supermirrors)

• Polarized 3He

2

12nm nm nmn b

2

1 ( )2 m n mn b b

Nonmagnetic layer

Magnetic layer

Choose

nmn n


Neutron

X-ray

2

0

sin( / 2)

sin( / 2)

NQdI I

Qd

Fe/Ge Multilayer-Semicond./Mag. : Possible spintronics material

(A): Si /Ge(100Å)/[(Fe 20Å/Ge 20Å)]×5/Au 30Å

Appl. Surf. Sci., 240, (2005) 251

Fe layers are crystalline and textured along [110] direction normal to the surface of the substrate.

Ge layers are amorphous


Polarized neutron reflectometry

Reduction in magnetic moment of ultra thin Fe layer on Fe/Ge multilayers

Appl. Surf. Sci., 240, (2005) 251

Reduction in magnetic moment in ultrathin Fe (~17 Å) layer in this multilayer sample. 1.45 μB (bulk value is 2.22 μB)


(B) Si /Ge(100Å)/[(Fe 70Å/Ge 60Å)]×10/Au 30Å

Investigation of Interface magnetic moment

Unpolarized Neutron and X-ray reflectivity

Polarized neutron reflectometry

J. Appl. Phys., 101, ( 2007 ) 33913

Physica B, 385-386, (2006) 653


Asymmetric ratio

J. Appl. Phys., 101, ( 2007 ) 33913

Reduction in magnetic moment at interfaces

Asymmetric diffusion of Fe on Ge with respect to Ge on Fe.


Ni/Cu thin films and multilayer

Si (111)/[Ni 90 Å/ Cu 70 Å] × 5.

perpendicular magnetic anisotropy (PMA) and used in magneto-optical recording

magnetic moment: thickness dependent, reduction in magnetic moment alloying at interfaces reduce the magnetic moment of the Ni

(a) Ni/Cu Multilayer by sputtering

lattice mismatch of about 2.5 %

Absence of Ni peak

Solid State Comm., 136, (2005) 400


0 300 600

6.2µ

6.4µ

6.6µ

0 300 6000.0

300.0n

600.0n

Mag

netic

Nuc

lear

Sca

tterin

g Le

ngth

den

sity

pro

file

(No.

/Å

2 )

Structure of bilayer

Thickness (Å) Scattering length density (10-6 / Å2)

Magnetic moment (B)

Ni1, Ni2, Ni3 22.0, 45.0, 22.0 6.47, 6.61, 6.19 0.21, 0.34, 0.21

Cu1, Cu2, Cu3 14.0, 39.0, 15.0 6.19, 6.38, 6.23 0.10, 0.00, 0.09

Si Substrate - 1.92 -

Solid State Comm., 136, (2005) 400

PNR: Reduction in magnetic moment


(b) Ni/Cu Thin film by electrodeposition

Electro. and Solid-State Letters, 9, (2006) J5.

The XRD confirms that the film is of polycrystalline nature.

layers have not undergone any strain distortion

Magnetic moment of Ni atom increases gradually to the bulk value from the interface Ni layer to the deep-seated Ni layer.

XRD: Crystal Structure PNR: reduction in interface magnetic moment


This requires a suite of spectrometers:

**

*


There are several cold moderator materials used all over the world in major neutron sources (research reactors and spallation neutron sources).

These are liquid H2 or D2 at about 20 K, liquid CH4 at about 100 K, and solid CH4 at about 20 K.

Liquid CH4 undergoes rapid polymerization under irradiation in a reactor and at present it is not used in any research reactors due to this problem. Its use is limited to spallation neutron sources with powers ranging from tens of watts to few kilowatts.

The most commonly used cold moderator is liquid hydrogen around 20 K (e.g. Saclay, BENSC, NIST) or its more expensive variant liquid D2 (ILL Grenoble, FRM II Münich).

World Scenario


•The experiment consisted of starting with the Be-filtered neutron counts in the detector for the water moderator at room temperature and then collecting neutron counts after removing the Be filters from the beam

•Next the Be filters were placed in front of the detector and the moderator was cooled by flowing liquid nitrogen. While the moderator gets cold, the count in the Be-filtered beam goes up

The increase in the counts of Be-filtered neutron beam coming from the cold moderator with respect to the counts from room temperature moderator indicates the shift in the Maxwellian.

Data Collection


Other alternatives

Till recently a D2O ice-based source at 20 K was operational at NIST, Gaithersburg, USA

In the earlier days several D2O based sources were tested at MIT reactor

One was operated at now-decommissioned CP-5 reactor at Argonne

Experiments performed in 1960’s clearly demonstrated suitability of D2O ice as cold moderator

This moderator is not hazardous at all and may be an attractive option asan inexpensive, safe cold source with modest gain


With this background, we felt that a H2O ice based source maintained at 77K using liquid nitrogen as coolant is worth considering for implementation under the present circumstances at Dhruva.

At this temperature water becomes polycrystalline ice Ih.

This phase of ice is expected to give good moderation because it has six translational modes at 7.1mev, 13.2mev, 19.0mev, 24.2mev, 28.2mev and 37.7mev; while libration, bending and stretching modes exist at 89, 204 and 406 mev respectively.


S(α, β) for inelastic scattering from lightWater ice. Model by Nakahara et. al. inJournal of Nuclear Science and Technology, 5 ( 31), 1968

K. Nünighoff et. al., Investigation of the Neutronic Performance of Advanced Cold Moderators and Validation of New Evaluated S(,) Neutron Scattering kernels, 16th meeting of the International Collaboration on Advanced Neutron Sources (ICANS XVI), May 12-15 2003, Dusseldorf, Germany Energy Spectra for ice measured at

JESSICA


Experiment in APSARA reactor

A simple experiment was undertaken by us at APSARA reactor to reconfirm the moderating property of ice and to estimate the gain from a H2O ice source cooled by LN2 compared to the room temperature spectrum.

one may also estimate the spectrum by measuring the integrated intensity of a Be-filtered beam.

One may estimate the gain by measuring the integrated intensity of a Be-filtered ibeam.

0.00 0.07 0.140

20

40

60

Inte

nsity

Energy in eV

80 K

100 K

300 K

Be filtered part 0-5 meV


A moderator pot for efficiently producing ice in-situ was designed and fabricated. The moderator pot is made from pure aluminum in cylindrical geometry. Diameter of the cylindrical moderator pot was about 200 mm and the thickness of the pot was about 50 mm. It could hold approximately 600 cm3 of water in it. The pot was radially separated into two chambers: the central chamber was meant for holding water and the peripheral chamber is for circulating LN2 to cool the water chamber.


Experimental set up at APSARA

The present experiment was performed at the neutron radiography beam line at APSARA reactor. The schematic of the experimental set up is shown in the figure


Difference between counts with Be and without Be(H2O at 300K)

Estimated number of counts below Be cut-off 1.6% of Maxwellian at 300K (A)

Difference between counts below Be cut-off for water at 27oC and water at - 190oC (B)

Ratio B/A= Gain

Estimated neutron Temp

set 1 26044415 423 7 3080 378 7.3 1 ~ 100 K

Set 2 19687 444 320 7 2462 377 7.7 1.3 ~100 K

0.00 0.07 0.14 0.210

20

40

60

Inte

nsity

Energy in eV

300 K Maxwellian

100 K Maxwellianintegrated gain 0-5 meV~7.5

Results match with COSYresults


Proposed Cold Neutron Source for Dhruva

Moderator pot will be enclosed in a water cooled vacuum jacket. Liquid nitrogen will circulated within its outer annulus while water will be contained in its central region. Freezing of water will be facilitated by provision of fins projecting inwards from the wall. The tube carrying liquid nitrogen to and from the moderator pot will be thermally isolated by a spacer from the other parts. It will be made of aluminum towards the side of reactor core and made of SS304 outwards. This minimizes heat transfer as well as nuclear heating. The moderator pot will have separate ports for filling and draining water.


Proposed flowdiagram


Nitrogen Flow Loop

Liquid nitrogen circulation outside the in-pile assembly will occur through a number of double-walled transfer lines. These will be connected to the liquid nitrogen Dewars. When Dewar is pressurized liquid will flow to the moderator pot and will go to another Dewar which will be vented to atmosphere. When inlet Dewar is empty a liquid level sensor unit will do the flowing things to reverse the flow direaction

1. closes the pressurization valve of inlet Dewar2. opens vent valve of inlet Dewar3. opens pressurization valve of outlet Dewar4. closes vent of outlet Dewar


Safety

Commercial liquid nitrogen will be used as the coolant in this project. It contains certain amount of oxygen. Commercial liquid nitrogen will be used as the coolant in this project. It contains certain amount of oxygen.

Nitrogen will be removed continuously so that no oxygen enrichment may occur

Mock up test:

Presently we have made a loop to simulate the cooling of the moderator under an estimated nuclear heating of 900 W at the moderator pot location in Dhruva reactor and map the temperature profile of ice under heat load and LN2 flow rate


Thank You

feasibility of an ice based cold neutron source

Documents

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cold neutron sourcea

journal of neutron research

cold neutrons

cold moderator

research reactor

ge layers

fe layers