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HYSPECHYSPEC

Instrument design, shielding/background simulations for Hyspec

Vinita J. Ghosh

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HYSPECHYSPEC

• Shielding simulations performed using MCNPX. I would like to thank Erik Iverson and Franz Gallmeier for their help.

• Instrument design simulations were performed using MCSTAS. I would like to thank Garrett Granroth.

• The polarizations simulations were performed using the NISP (neutron instrument simulation package) code developed at LANL.

I would like to acknowledge the help of P. A. Seeger and Luke Daemen.

• At BNL Kim Mohanty who takes care of all our computers and work stations

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HYSPECHYSPEC

Part I

Shielding simulations – results that influence the instrument design

Part II

Comparison of instrument performance (monochromatic flux on sample and energy resolution) inside and outside the target hall.

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HYSPECHYSPEC

Hyspec design goals:

1. Largest possible monochromatic flux on small (2cm x 2cm) single crystal samples for moderate energy resolution.

2. Lowest possible background. In optimizing the details of the instrument design we are simulating both signal and background to ensure that we have the largest possible signal to background ratio.

3. Ability to do polarization analysis Polarization analysis will be performed for Ei ~ 3.6 – 20meV Since less than 50% of the neutrons will be used in a given

polarization experiment it is important to have the highest possible flux on sample to ensure that we will have reasonable data collection rates.

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HYSPECHYSPEC

Goals for Hyspec shielding design.

1. Meet the biological dose rate criteria for the SNS facility:

Biological dose rate less than 0.25 mrem/hr.

2. Keep the background down to 1 neutron/detector/minute.

10-10 10-8 10-6 1x10-4 10-2 100 102 1040.1

1

10

100

1000

Neutron, photon flux (n/s*cm2) required for a dose rate of 0.25 mrem/hr

Ne

utr

on

, ph

oto

n flu

x (n

/s*c

m2 )

req

uir

ed

fo

r a

do

se r

ate

of

0.2

5 m

rem

/hr

Neutron energy(MeV)

neutron fluxphoton flux

meV neutrons: flux of 100n/cm2/s will meet the dose rate requirement

MeV neutrons: only a flux of 1n/cm2/s can be allowed

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HYSPECHYSPEC

E. Iverson’s simulation results for different SNS moderators

BL9 Water moderatorBL5 Cpl-H2BL2 Decoupled H2Generic average

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HYSPECHYSPEC

10-9 10-6 10-3 100 103

0.0

0.2

0.4

0.6

0.8

1.0

He3 detector response function

Fra

ctio

n of

neu

tron

s ab

sorb

ed

Energy(MeV)

Absorption in He3 cell2.5cm thick, T = 300K

3atm. pressure 6atm. pressure

Shielding Goal #2.

Keep background down to 1 neutron per detector per minute.

Neutrons of low energies can be stopped easily in the drum shield

Neutrons with energy 1keV or more will not contribute to background.

High energy neutrons will moderate in the drum shield to produce neutrons that will add to background.

‘zhip’ zero hydrogen matrix for B4C?

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HYSPECHYSPEC

T1A chopper

T1B chopper

Drum shield

Beam stop

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HYSPECHYSPEC

T0 chopper • Counter-rotating pair of rotors, rotating at 60Hz• 20cm of inconel (or tungsten or steel) Its main function is to stop MeV and keV neutrons and the initial gamma burst.

Attenuation due to 2 rotors ~10-5 – 10-6

T1A (frame overlap)chopper • boron-loaded disc chopper at 60Hz• attenuation of 10-3-10-4

T1B (order suppressor) chopper • Same as T1A• stops eV and some keV neutrons as well as low energy neutrons

T2 (wavelength selector) chopper • Pair of counter-rotating discs, maimum rotation rate of 300Hz• Coated with gadolinium – an effective absorber of meV neutrons.

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HYSPECHYSPEC

Drum shield simulations

0 2 4 6 8 10 1210-5

10-4

10-3

Ratio of intensities for a shield thickness of 80cm

I(w

ith d

rum

shi

eld)

/I(w

ithou

t dr

um s

hiel

d)

thickness of material 2 (cm)

Material 1= borated poly5% boron by weightMaterial 2

stainless steel tungsten Steel_shot_mix

IR=0.2m OR=1.0m, height 1.5m, surface area=9m2 =105cm2

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HYSPECHYSPEC

Hyspec shielding inside the target hall

1/R2 estimate for LMM=25m, straight 4cm(w) 12cm(h) guide, no choppers Total # of neutrons (all energies) at end of guide = 2.4x1010 n/s

MCNPX resultTotal # of neutrons (all energies) at end of guide = 1.2x1010 n/s = 7.2x1011neutrons/minute.

If we want 1n/min/detector we need a total attenuation of 10-11 to 10-12

Attenuation due to T0 chopper ~ 10-5 – 10-6

Attenuation due to drum shield ~ 10-3 – 10-4

For isotropic (nonBragg) scattering by the monochromator neutron flux at drum shield face < 1n/cm2/s

Since scattering is not isotropic there may be a ‘hot spot’ in the line of sight of the moderator

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HYSPECHYSPEC

Shielding outside the target hallLMM ~ 35-40m

Guide options1. 4cm wide straight2. 4cm wide, curved offset at mono. 16cm

Attenuation due to curvature ~10-6

This will allow us to make the Drum shield thinner

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HYSPECHYSPEC

Comparison of instrument performance (monochromatic flux on sample and energy resolution) inside and outside the target hall.

20 25 30 35 40105

106

107

108

Energy 15meV Energy 60 meV

Mo

no

chro

ma

tic fl

ux

on

sa

mp

le(n

/cm

2 /s)

Moderator-monochromator distance (m)

Guide width 4cm, height 15cm3 theta-c supermirror coatingChopper slot width 4cmRotation rate 300 Hz

Ei(meV) LMM=25 LMM=35 LMM=40

3.6 2.2% 2.2% 2.1%

15 4.5% 4.3% 4.2%

30 6.4% 6.0% 5.9%

60 9.0% 8.2% 8.2%

90 10.4% 10.1% 9.8%

Energy resolution for moderator-monochromator distance LMM=25,35,40m

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HYSPECHYSPEC

0 20 40 60 80 100105

106

107

Monochromatic flux on sample as a function of energy

Flu

x o

n (

2cm

x2cm

) sa

mp

le(n

/cm

2 /s)

Energy(meV)

MM distance 25mMM distance 35mMM distance 40m

coupled-H2 moderator

Ei(meV) LMM=25 LMM=35 LMM=40

3.6 7.2e6 5.2e6 4.7e6

15 8.0e6 5.9e6 5.3e6

30 2.9e6 2.0e6 1.7e6

60 1.1e6 8.2e6 7.1e6

90 1.1e6 10.1e6 6.6e5

Monochromatic flux on sample for LMM = 25,35, and 40m

Monochromatic flux on sample for some representative energies

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HYSPECHYSPEC

1. Use a tapered guide that converges from 10 to 4 cm in the horizontal plane.

Does not look promising.

2. Increase the burst width at the sample

a. by slowing rotation rate of wavelength-defining (T2) chopper, or

b. by increasing the slot width of the T2 chopper.

60 120 180 240 300105

106

107

108

MM distance 40 mGuide width 4cm, straight3 theta-c supermirrorGuide reflectivity 98%Chopper slot width 4cm

Energy 15 meV Energy 60 meV

Mo

no

chro

ma

tic fl

ux

on

sa

mp

le(n

/cm

2 /s)

T2 chopper rotation rate (Hz)

Can we increase signal outside the target hall?

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HYSPECHYSPEC

Energy resolution δE/E ~ 2 δt/tAs the burst width increases the energy resolution deterioratesEnergy resolution can be improved by increasing the sample-detector

distance from 4.5 to 6 or 7.5m.

150 180 210 240 270 300 3300

1

2

3

4

5

6

7

8

9

10L1 = 40m, E=15meV, T2 slot width 4cm

dE

/E (

%)

T2 Chopper rotation rate (Hz)

Fixed dE/E=4.5% Lsd = 4.5 m Lsd = 6.0 m Lsd = 7.5 m

LMM(m) T2 slot width

Lsd(m) T2 rotation rate(Hz)

Flux on sample (n/cm2/s)

25 4cm 4.5 300 8.0e6

40 4cm 4.5 300 5.3e6

40 4cm 6.0 240 6.5e6

40 4.5cm 6.0 240 7.0e6

40 4cm 7.5 180 8.7e6

Ei = 15meV

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HYSPECHYSPEC

All results so far were for straight guides

Attenuation of background due to curvature ~10-6

How much loss of signal due to curvature?

-10 0 10 20 30 40 50 60 70 80 90 100

0.6

0.8

1.0

1.2

1.44cm wide straight guide4cm wide curved guide

offset at monochromator = 16cm

Rat

io o

f Flu

x at

sam

ple

for

curv

ed g

uide

wrt

flu

x at

sam

ple

for

4cm

-wid

e st

raig

ht g

uide

Energy(meV)

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HYSPECHYSPEC

Comparison of instrument performance inside and

outside the target hall

Inside LMM=25m

Lsd=4.5m

Outside LMM=35-40

Lsd=4.5m

Outside LMM=35-40

Lsd=6.0m

Signal (15meV) 8.0e6 5.3e6 7.0e6

Energy resolution 4.5% 4.5% 4.5%

Shielding/background Difficult with straight guide

Much easier with curved guide

Much easier with curved guide

Advantages Shorter primary guide

Drum shield is simpler

Drum shield is simpler

Extra Cost Building Bldg + larger detector bank

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HYSPECHYSPEC

Future plans

1. Shielding sims with BL5 source instead of generic source

Using BNL multinode computer cluster

Drum shield design

2. Instrument optimization

Guide configuration, coating

Chopper placement

Variable monochromator-sample distance

3. Q resolution

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HYSPECHYSPEC

Questions?