A stacked scintillator neutron spectrometer for measuring the fluence of mono-energetic

neutron beams up to 150 MeV

Andy Buffler, Frank Brooks, Saalih Allie, Mark Herbert, Siphiwo Makupula,

Department of Physics, University of Cape Town, South Africa

Ricky SmitiThemba LABS, Cape Town, South Africa

Volker Dangendorf, Ralf NoltePhysikalisch-Technische Bundesanstalt, Braunschweig, Germany

SAIP conference, Stellenbosch 2003

For neutron spectrometry up to ∼150 MeV, liquid scintillators(e.g.NE213 or BC501A) offer:

• high neutron detection efficiency, and • good n-γ discrimination.

Response functions can be simulated by Monte Carlo (e.g. NRESP, SCINFUL, MCNPX, etc.) to determine:

• neutron detection efficiencies; and• neutron energy spectra by unfolding measured pulse height spectra.

Two main difficulties for En > 20 MeV:

• contributions from n-12C interactions cannot be simulated reliably; and • proportion of escaping protons increases with energy.

Solution:• Measure response functions• Use stacked scintillators to control escapes

The Stacked Scintillator Neutron Spectrometer

Two event types accepted:1. A only (“singles”, LB = 0); and2. A + B (“coincidences”)

Summed pulse height spectrum for event types 1 and 2 gives an escape-free response function.

p

p

n

n n

n

A

B

1 2

V

Principle:• Select all events for

which a charged particle is detected in A.

• Veto proton escapes into V.

• (LA / MeVee) = (LB / MeVee)

• Response L = LA + LB

Response function measurements

• Experiments at the neutron time-of-flight facility of iThemba LABS• ns-pulsed proton beam energies of 66, 80, 100, 120 and 160 MeV• 5 mm natLi target• Quasi-monoenergetic

(time-of-flight selected) neutron beam energies selected using 7Li(p,n)7Be(gs+1) at 0o

over a flight path of 6.00 m.

40

30

20

10

0

10k

20k

62.5 MeV

Time-of-flight T (ns)

160120

8040

Pulse height L (MeVee )

66 MeV protons on natLi (5 mm)Singles in detector A

140 120 100 80 60 400

50k

100k

150k

200k

250k

W

10 20 30 40 50 70En (MeV)

Cou

nts

per c

hann

elTime-of-flight T (ns)

62.5 MeV

0 10 20 30 40 50 60 70 80 90 100 1100

1

2

3

4

5

6

7

natLi(p,n): 0° 16°

Φ E /

Nm

on (a

rb. s

cale

)

En / MeV0 20 40 60 80 100

0

1

2

3

4

5

6

natLi(p,n): difference beam

ΦE /

Nm

on (a

rb. s

cale

)

En / MeV

100 MeV protons on a natLi target:Spectral neutron fluence, normalised to the same number of protons

C2

C1

0

4k

2k

4030

2010

0

e

Pulse height L (MeV ee

)Pulse shape

S

d

α

p

Cut C1 excludes γ-rays and escaping protons

En = 62.5 MeVSingles in Detector A

0 10 20 30 40 500

1000

2000

3000

4000

5000

1H(n,n)1H12C(n,x)

LT

Cou

nts

per c

hann

el

L (MeVee)

En = 62.5 MeVSingles in detector A

Experiment

SCINFUL

C2

C1

80

6040

200

e

Pulse height L A (M

eV ee)

Pulse shapeS

A

d

α

p

C4

C3

80

6040

200

Pulse height L B (M

eV ee) Pulse shape

SB

p

A singles

AB coincidences

En = 97.5 MeV

A

BV

Ep = 100 MeV

A

BV

Ep = 100 MeV

A

B

V

Pulse heightcalibration

LBLA

0 1000 2000 3000 40000

500

1000

1500

1320

1795

Cou

nts

per c

hann

el

Pulse height L (A or B) (ADC channel)

(a) ABV coincidence (b) BAV coincidence

(a) (b)

0 20 40 60 800

2000

Cou

nts

per c

hann

el

L = LA + LB (MeVee)

0

2000

0

2000

4000

LT

En = 97.5 MeV

L = LA + LB (MeVee)

SCINFUL

Experiment (a) A singles

(LB = 0)

(b) ABcoincidences

(a) + (b)

60 62 64 66 68 700

50

100

150

200

∆D/D = 2.1/64.2 = 3.3 %

- dN

(D) /

dD

Pulse height D (MeVee)

-dN/dL

L (MeVee)

Pulse height resolution(∆L/L) ≈ 3%

∆L

0 20 40 60 800

2000

4000

6000

Nnp

Dnp

12C(n,x)

1H(n,n)1H

Cou

nts

per c

hann

el

Pulse height D ( MeVee )

Lnp

L

Fluence calculation

( ) { }0 np H H np dead ABN N n σ φ φ φ=

Total counts recorded above pulse height threshold Lnp , where Lnp is set so as to select events associated with n-pelastic scattering only.

Total number of neutrons incident on the spectrometer.

where:

nH : number of hydrogen atoms per unit cross sectional area presented to the beam by the scintillator

σH : total cross section for n-p elastic scattering

φnp : fraction of neutrons detected above pulse height threshold Lnpφdead : correction factor for dead time of counting systemφAB : other factors associated with corrections for threshold, reaction tail

and escape effects

Fluence calculation cont.

The neutron fluence Φ is then

where A is the cross section area of the beam.

1

np

H H np dead AB

Nn Aσ φ φ φ

⎧ ⎫⎪ ⎪Φ = ⎨ ⎬⎪ ⎪⎩ ⎭

For experiments requiring high flux neutron beam irradiationsat iThemba LABS, the general approach used for neutron beam monitoring is to use the stacked spectrometer to calibrate a smaller (less efficient) detector (typically positioned at 8o) using a low intensity beam, which is then used to monitor the high intensity neutron beams used during the irradiation experiments.

Typical fluences measured in this way at the neutron beam facility at iThemba LABS are of the order of

Φ = (1.5 ± 0.1) × 1010 neutrons cm-2

for about 40 hours of running with a 10 µA proton beam (pulse selector off) incident on a 10 mm Be target, with the detector at 6.00 m from the target at 0˚.

150 MeV n

A B C

0 100 200 3000

1000

2000

3000

4000

5000

6000

7000

Cou

nts

per c

hann

el

Pulse height L (ADC channel)

LnpA

ABABC

Full response function

Triple scintillator system

Summary

• The effect of escaping charged particles on response function measurements has been minimized using the stacked scintillator system.

• Full response functions have been measured for quasi-mono-energetic neutron beams of energies between 63 and 150 MeV.

• A method for measuring fluences for these beams has been developed, using the total cross section for n-p scattering as a reference.

• Response function measurements have also been used for determining energy spectra via unfolding analyses (MSc of Siphiwo Makupula … next talk).

For more information see …

“Measurement of neutron energy spectra from 15-150 MeV using stacked liquid scintillators”A. Buffler, F.D. Brooks, M.S. Allie, P.J. Binns, V. Dangendorf, K.M. Langen, R. Nolte and H. SchuhmacherNuclear Instruments and Methods A 476 (2002) 181-185

“High energy neutron reference fields for the calibration of detectors used in neutron spectrometry”R. Nolte, M.S. Allie, P.J. Binns, F.D. Brooks, A. Buffler, V. Dangendorf, J.P. Meulders, H. Schuhmacher, B. WiegelNuclear Instruments and Methods A 476 (2002) 369-373

Measurement of neutron fluence spectra up to 100 MeV using a stacked scintillator

neutron spectrometer

Siphiwo Makupula and Andy BufflerDepartment of Physics, University of Cape Town, South Africa

SAIP conference, Stellenbosch 2003

Introduction

Neutron time-of-flight is the technique most often used to determine the energy spectra of ns-pulsed neutron beams.

For continuous (non-pulsed) beams, alternative methods need to be employed, such as those based on unfolding analyses.

The pulse height spectrum that is recorded from any radiation detector is the convolution of its inherent response function and the energy distribution of the incident radiation.

If the purpose of making the pulse height measurements is to obtain information about the energy spectrum of the incident radiation, as is the present case, then this process involves solving the basic system of linear integral equations:

∫∞

φ=0

)( )( dEEERz ii for i = 1 to m

which represents a model of the measurement.

∫∞

φ=0

)( )( dEEERz ii

zi: the input measurands (the recorded pulse height spectrum). Ri(E): the response functions of the measuring system which include the effects

of limited energy resolution of the spectrometer. φ(E): the average fluence values in the intervals between Ei and Ei+1.

The subscript i is related to the channel number of the measuring system havingm channels in total.

Reliable knowledge of Ri(E) is needed to solve for φ(E).

Problem: Monte Carlo codes (eg. NRESP, SCINFUL, MCNPX) which may be used to simulate response functions Ri(E) for NE213 liquid scintillators, are limited to En below ~20 MeV, since the cross-sections for n-12C reactions above these energies are not well known.

Solution: Measure the response functions Ri(E) required for unfolding analyses.

The previous talk demonstrated how a stacked scintillator neutron spectrometer (S3N) may be used to measure response functions for mono-energetic neutron beams up to about 150 MeV, which not affected by charged particle escape.

Measurements were with the S3N at the neutron time-of-flight facility of iThemba LABS, for neutron beams at 0o and 16o, using targets of natural lithium (3 mm and 5 mm) and graphite (10 mm)

window number

( j )

1412

108

7654

32

1

16

80

120160

40T (ns)

DA

Time-of-flight T versus pulse height DA measured by the S3N positioned at 0o for the neutron beam produced by a 100 MeV proton beam on a 3 mm natLi target.

Time-of-flight windows used to select 16 quasi-monoenergetic response functions between 20 and 100 MeV

En = 97 MeV

Response functions measured by the S3N

(for protons only, selected by pulse shape discrimination)

0 10 20 30 40 50 60 70 800

600

j = 8 (55 MeV)

D (MeVee)

0

600

j = 5 (40 MeV)

j = 6 (45 MeV)

j = 7 (50 MeV)

0

600

j = 1 (20 MeV)

0

600

j = 2 (25 MeV)

0

600

Cou

nts

per c

hann

el

j = 3 (30 MeV)

0

600

j = 4 (35 MeV)

0

600

0

600

1200

0 10 20 30 40 50 6

0

600

16 (97 MeV)

0 70 80

j =

D (MeVee)

0

600

j =

j =

j = 15 (

0

600

13 (80 MeV)

14 (85 MeV)

90 MeV)

= 9 (60 MeV)j

0

600

10 (65 MeV)j =

0

600

Cou

nts

per c

hann

el

j =

0

600

11 (70 MeV)

12 (75 MeV)j =

0

600

0

3000

6000

Response function measured by the S3N for 97 MeV neutrons

0 20 40 60 800

2000

4000

6000

Nnp

Dnp

12C(n,x)

1H(n,n)1H

Cou

nts

per c

hann

el

Pulse height D ( MeVee )

The neutron fluence for each response function was determined using the total cross section of n-p scattering as a reference (previous talk).

Region of pulse height spectrum associated with n-p scattering only

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

5000

10000

15000

20000

25000

30000

35000

Fl

uenc

e Φ

(ne

utro

ns c

m-2)

Energy bin j

x 10

Fluence Φ measured for each quasi-monoenergetic response function between 20 and 97 MeV (j = 1 to 16)

The response matrix

0 10 20 30 40 50 60 70 800.00

0.25

j = 8 (55 MeV)

D (MeVee)

0.00

0.25

j = 5 (40 MeV)

j = 6 (45 MeV)

j = 7 (50 MeV)

0.00

0.05

j = 1 (20 MeV)

0.00

0.05

j = 2 (25 MeV)

0.00

0.05

Cou

nts

per c

hann

el (n

orm

aliz

ed to

a fl

uenc

e of

1 n

eutro

n cm

-2)

j = 3 (30 MeV)

0.00

0.05 j = 4 (35 MeV)

0.00

0.05

0.00

0.05

0.10

0 10 20 30 40 50 60 70 800.000

0.025

j = 16 (97 MeV)

D (MeVee)

0.000

0.025

j = 13 (80 MeV)

j = 14 (85 MeV)

j = 15 (90 MeV)

0.000

0.025

j = 9 (60 MeV)

0.000

0.025

j = 10 (65 MeV)

0.000

0.025

Cou

nts

per c

hann

el (n

orm

aliz

ed to

a fl

uenc

e of

1 n

eutro

n cm

-2)

j = 11 (70 MeV)

0.000

0.025

j = 12 (75 MeV)

0.000

0.025

0.000

0.025

0.050

Each response function was smoothed, and scaled to the same incident fluence of 1 neutron cm-2 by multiplying each channel by the factor: -21 neutron cm .

(measured)Φ

Unfolding

The equation

∫∞

φ=0

)( )( dEEERz ii

may be transformed (in a number of ways) into the matrix equation

z = R Φ.

which represents a system of linear equations which must be solved for the column matrix Φ = (φ1 … φn)T

where the superscript T indicates transposition.

If data values are known for both:the column matrix z = (z1 … zm)T of the measured pulse height spectrum; and the response matrix R = Rij (i = 1 … m ; j = 1 … n), then z = R Φ may be unfolded to determine Φ.

The unfolding code used for this work was MIEKE (based on a Monte Carlo algorithm) and is available from the PTB, Braunschweig, Germany.

Unfolding ... 2

Tests of the unfolding code were performed by unfolding pulse height spectra “manufactured” from different combinations of the (unsmoothed) response functions making up the response matrix.

The results are not presented here, but were very satisfactory.

It is more interesting to look at the analyses of different neutron beams ...

0

50000

100000

0

50000

100000

0

50000

100000

500 1000 1500 20000

50000

100000

0

250000

500000

Cou

nts

per c

hann

el

(a) 3 mm Li, 0o (G)

160 140 120 100 80 60 40 20 0 Tn (ns)

En (MeV) 10 20 30 40 60 100

(b) 5 mm Li, 0o (H1)

(c) 10 mm C, 0o (H2)

(d) 3 mm Li, 16o (H3)

(e) 5 mm Li, 16o (H4)

T (ADC channel)

ToF spectrum measured at 0˚for 3 mm Li target (data used to construct the response matrix)

Four other measurements made at 0o and 16o with Li and C targets

0

5000

10000

15000

20000

0 20 40 60 800

5000

10000

15000

(d) H4 (5 mm Li, 16o)

Cou

nts

per c

hann

el

(c) H3 (3 mm Li, 16o)

Pulse height D (MeVee)

0

2000

4000

6000

8000

0 20 40 60 800

15000

30000

45000

(b) H2 (10 mm C, 0o)

Cou

nts

per c

hann

el

(a) H1 (5 mm Li, 0o)

Pulse height D (MeVee)

Pulse height spectra measured (histograms) for 4 neutron beams, together with refolded fits (red curves) resulting from the unfolding analyses.

Spectral fluence measured from unfolding analyses (points) for 4 neutron beams, together with the “expected” spectra (histograms - rebinned ToF spectra).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

10000

20000

30000

40000

50000

60000

0

20000

40000

60000

80000

100000

(b) H2 (10 mm C, 0o)

Flue

nce

Φ (n

eutro

ns c

m-2)

Energy bin j

(a) H1 (5 mm Li, 0o)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160

5000

10000

15000

20000

25000

0

10000

20000

30000

40000

(d) H4 (5 mm Li, 16o)

Flue

nce

Φ (n

eutro

ns c

m-2)

Energy bin j

(c) H3 (3 mm Li, 16o)

Summary

The stacked spectrometer allows the measurement of response functions for (quasi-) monoenergetic neutrons which are minimally affected by chargedparticle escape.

These response functions have been used to form a response matrix for the stacked spectrometer which has allowed reliable unfolding of measured pulse height spectra.

The spectrometer has thus been shown to be able to measure energy spectrafor continuous (unpulsed) neutron beams.

Future work is aiming at increasing the energy resolution of the unfolding analyses and extending the capability of the spectrometer up to 150 MeV.