CHAPTER 17 -NEUTRON FOCUSSING LENSES

17:1. NEUTRON TRAJECTORY

17:2. THE PRISM DEFLECTION ANGLE

17:3. CONTRIBUTION TO THE Q RESOLUTION

17:4. CONTRIBUTION TO MINIMUM Q

17:5. MEASUREMENTS WITH GRAVITY CORRECTING PRISM

17:1. NEUTRON TRAJECTORY

sourceaperture

sampleaperture

2D area detector

L1 L2

prism

)zL(zB)z(y 12

2-192

2

Åcm10*073.32h

gmB

22122

21 L)LL(LB)LL(y

22 C2

tanb

)(

17:2. THE PRISM DEFLECTION ANGLE

Neutron trajectory:

Spot on detector:

Prism deflection angle:

2

2

y

2

42

2

3

2

2

1

21

2

1

2

1

2

2

2

2

Qy λ

Δλ

6

1Q

3

2A

2

y

3

1

4

R

L

LL

4

R

L

L

L

2

2

2

y

2

42

2

2

3

2

2

1

21

2

1

2

1

2

2

2

2

Qy λ

Δλ

6

1Q

3

2)CLA(

2

y

3

1

4

R

L

LL

4

R

L

L

L

2

Without Prisms

With Prisms

17:3. CONTRIBUTION TO THE Q RESOLUTION

geometry gravity wavelength spread

17:4. CONTRIBUTION TO MINIMUM Q

2

23

21

211

1

2

2min 2CLA

2

yR

L

LLR

L

L

L

2Q

geometry gravity

23

21

211

1

2

2min 2A

2

yR

L

LLR

L

L

L

2Q

With prisms

Without prisms

17:5. MEASUREMENTS WITH GRAVITY CORRECTING PRISMS

-5

-4

-3

-2

-1

0

1

5 10 15 20

With Prisms

measured y positionmeasured x positioncalculated y positionmeasured y position without prisms

Be

am S

po

t P

osi

tio

n (

cm

)

Neutron Wavelength (Å)

3 cm

3 cm

0.5 cm

COMMENTS

-- Long wavelength neutrons fall due to gravity.

-- Gravity drops the beam spot on the detector and distort its symmetry (from circular to elliptical shape).

-- Refractive prisms correct for the gravity effect by raising the beam spot on the detector and correcting its shape back to circular.