ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

�Corr

E-m

(A.S. Cr1 Pr2 Pr

journal homepage: www.elsevier.com/locate/nima

Neutron detection efficiency determinations for the TUNL neutron–neutronand neutron–proton scattering-length measurements

D.E. Gonzalez Trotter a,1 , F. Salinas Meneses a,2, W. Tornow a, A.S. Crowell a,�, C.R. Howell a,D. Schmidt b, R.L. Walter a

a Department of Physics, Duke University and Triangle Universities Nuclear Laboratory, Durham, NC 27708-0308, USAb Physikalisch-Technische Bundesanstalt, D-38116, Braunschweig, Germany

a r t i c l e i n f o

Article history:

Received 17 January 2008

Received in revised form

7 October 2008

Accepted 29 October 2008Available online 14 November 2008

Keywords:

Liquid organic scintillator

Neutron detection efficiency

Scattering length

02/$ - see front matter & 2008 Elsevier B.V. A

016/j.nima.2008.10.036

esponding author. Tel.: +1919 660 2639; fax:

ail addresses: tornow@tunl.duke.edu (W. Tor

owell).

esent address: Merck & Co., Inc., West Point,

esent address: Neighborhood America, Naple

a b s t r a c t

The methods employed and the results obtained from measurements and calculations of the detection

efficiency for the neutron detectors used at Triangle Universities Nuclear Laboratory (TUNL) in the

simultaneous determination of the 1S0 neutron–neutron and neutron–proton scattering lengths ann and

anp , respectively, are described. Typical values for the detector efficiency were 0.3. Very good agreement

between the different experimental methods and between data and calculation has been obtained in

the neutron energy range below En ¼ 13 MeV.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

In order to obtain absolute (and relative) cross-sectioninformation for reactions with free neutrons in the final state,the neutron detection efficiency has to be known. The onlyexceptions are reactions where the so-called associated charged-particle method [1] can be applied. However, this is not the casefor the experimental setup used at the Triangle UniversitiesNuclear Laboratory (TUNL) for the simultaneous measurement ofthe neutron–neutron and neutron–proton 1S0 scattering lengthsann and anp, respectively [2]. Therefore, considerable effort hasbeen devoted to measuring and calculating the efficiency of theneutron detectors used in this experiment. Most of these activitiestook place prior to the actual ann=anp final-state-interaction (FSI)cross-section measurements.

Measurements of the pulse-height (PH) characteristics ofneutron detectors were performed at the Physikalisch-TechnischeBundesanstalt (PTB) in Braunschweig, Germany, from whichdetection efficiencies were deduced. Direct measurements ofneutron detector efficiencies were carried out at TUNL. Thesestudies will be described in this article. They will be preceded by a

ll rights reserved.

+1919 660 2634.

now), crowell@tunl.duke.edu

PA, USA.

s, FL, USA.

short review of the basic features of neutron detection in the fewMeV energy range using liquid organic scintillators.

Because neutrons have no charge it is necessary to rely onreaction products to detect them. For neutron energies below En ¼

15 MeV a neutron may interact with the organic liquid scintillatormaterial used in the present work in the following ways:

(a)

Single and multiple elastic scattering on hydrogen nuclei. (b) Single and multiple elastic and inelastic scattering on carbon

nuclei.

(c) Scattering on hydrogen followed by a subsequent scattering

process on carbon (elastic or inelastic) and vice versa.

(d) Breakup of 12C into 8Be and an a-particle. (e) Breakup of 12C into three a-particles.

A charged particle resulting from these processes transfers atleast part of its kinetic energy to the scintillator molecules, whichupon de-excitation, emit photons with an intensity proportionalto the charged particle’s kinetic energy. However, different energydependent quenching factors are associated with charged parti-cles, ranging from 1.0 for electrons and increasing with thestopping power dE=dx of the particles in the scintillator material.The light output refers to the amount of light produced in ascintillator for a given charged-particle energy [3]. The scintillatormedium is coupled to a photomultiplier tube (PMT). The PH of itsoutput signal is proportional to the intensity of the light emittedand therefore also proportional to the kinetic energy of thecharged particle(s) produced by the interaction of the incoming

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Fig. 1. Layout of neutron detectors at TUNL for the simultaneous measurement of the 1S0 neutron–neutron and neutron–proton scattering lengths ann and anp , respectively.

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242 235

neutron with the organic liquid scintillator. The probability fordetecting a neutron depends on the following factors [4]:

(a)

3

was

[2].

Neutron energy.

(b) Detection threshold (i.e., lower PH cutoff). (c) Volume and shape of the scintillator medium, which deter-

mines also the probability of occurrence and detection ofmultiple scattering events.

(d)

Chemical composition and purity of the scintillator material. (e) Physical construction and support of the detector resulting in

in-scattering of neutrons from housing and structural materi-als into the scintillator volume.

(f)

Direction of the incoming neutron flux with respect to thedetector orientation.

2. Measurements at PTB

In a 10-day experiment the neutron detectors #9 (Bicron type,B2), #8 (ring type, R1), and #1 (transmission type, T13) used in theTUNL setup [2] (see Fig. 1) and shown in Figs. 2 and 3 werecarefully studied at the Cyclotron Fast Neutron Facility [5,6] of PTBin a manner similar to that described in the PTB report of Tichyet al. [7]. The detectors’ PH characteristics were incorporated inMonte-Carlo (MC) programs (NRESP7 [4] and NEFF7 [4]) that

About halfway through the course of the ann=anp experiment [2] detector T1

replaced by a spare ring detector, as shown in Fig. 1 The reason is given in Ref.

reproduce the PH response and calculate the absolute detectionefficiency of organic scintillator detectors.

2.1. Experimental setup

Pulsed neutron beams were produced by bombarding adeuterium gas cell with deuteron beams at six different energiesto produce monoenergetic neutrons from the 2H(d,n)3He reactionat 0� at En ¼ 8;9;10;11;12, and 14 MeV, in addition to asignificant continuum of low-energy ‘‘gas-cell breakup’’ neutrons.The neutron production yield from the 2H(d,n)3He reaction wasmonitored at all times by an NE213 liquid scintillator detector(3:81 cm� 3:81 cm) placed in a collimator at an angle of 60:4�

relative to the 0� direction. This monitor yield was later usedfor relative fluence normalization among the detectors studied.The neutron beam at 0� was collimated to minimize the effectof neutrons scattered from air into the detectors. All detectorswere placed successively at 0� and 12 m from the deuteriumgas cell. An already fully characterized PTB reference detector(D1, 10:16 cm� 2:54 cm in dimensions and filled with NE-213scintillator fluid) was also irradiated. This detector was calibratedby means of a proton recoil telescope [8] and was used for fluencecomparisons with the three TUNL detectors.

2.2. Proton light-output function determination

MC techniques have been used for decades for simulating theinteraction of neutrons with materials. The MC code NRESP7models the PH response for a scintillator irradiated by fastneutrons with energies between 0.2 and 20 MeV. Scattering

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6.96

13.447.3 7.6 14.04

7.6NE-213

Glass

Lucite

Aluminum

Light guide

Active volume

PMT

Fig. 2. Details of ring- and transmission-type neutron detectors designed and built at TUNL for neutron–neutron scattering-length measurements (units in cm).

5.08

NE−213/BC−501

Glass

Lucite

Aluminum

13.0

812

.68

5.28

2.3

2.3

13.4

812

.63

6.70

6.08

0.7

Units in cm.

Fig. 3. Details of Bicron-type (top) and Argonne-type (bottom) neutron detectors

used at TUNL for scattering-length measurements.

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242236

effects from structural materials, like the aluminum housing andlucite light pipes, are included by using comprehensive sets ofcross-section libraries and accounting for neutron transportthroughout the different materials that compose the detectorassembly. The NRESP7 code models cylindrically symmetricBicron-type detectors. Therefore, modifications had to be madeto account for the special geometry of the TUNL ring- andtransmission-type detectors. The associated new programs arereferred to as NRESPR1 and NRESPT1, respectively.

To model the response of a neutron detector realistically, manyingredients must be considered [4]. Here we concentrate on thelight production of electrons and protons. The light production ofthese particles is given by light-output functions, which weredetermined from the experimental PH response using the codes

GRESP [9] and SPEKT [10] developed at PTB. The standard steps forobtaining the proton light-output function LpðEpÞ are as follows:

�

The zero-intercept channel (corresponding to zero PH) wasdetermined for the complete electronics chain (pre-amp, main-amp, ADC) by means of an electronic pulser. The recoil-edgechannel of all spectra (see below) was then correctedaccordingly. � The PH spectra for various g-ray sources with recoil-electron

energies 0:341 MeVoEo4:197 were accumulated for alldetectors in order to obtain the electron light-output function.The Compton edges of these spectra were determined with theaid of the PTB code GRESP. The position Le (in channels) of theCompton edge should be directly proportional to E, with aslope Gg

LeðEÞ ¼ GgðE� E0Þ (1)

where E0 is a small offset of 5 keV [11]. For our detectors R1 andT1 we observed a small nonlinearity caused by PH saturationeffects for electron currents above a certain threshold in ourHamamatsu H1161 phototubes and base assemblies. Correc-tions were applied accordingly.

� The single-scattering parts of the PH response spectra obtained

for different neutron energies were fitted by rectangulardistributions whose rightmost edges were folded with aGaussian resolution function. The single-scattering edge-channel position LpðEpÞ (where Ep is the maximum proton-recoil energy) and the Gaussian folding parameter wereobtained from each individual neutron PH spectrum.

� The experimental PH resolution folding parameter dL=L was

established as a function of L (light-output units given byLpðEp=GgÞ using the expression [9]

dL

LðEpÞ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA2þ

B2

Lþ

C2

L2

s(2)

where the variables A, B, and C were determined by fitting theplot of ðdL=LÞðEpÞ against L using Eq. (2).

� The light-output function for protons is produced by plotting

Lp versus Ep and smoothly interpolating between points,creating a numerical table for Epp8:0 MeV. A linear fit wasused to represent LpðEpÞ for Ep48:0 MeV.

� The mean energy and energy spread of the incident neutrons

needed for the NRESP calculations were derived from the

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Cou

nts

Figtyp

3000

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242 237

experimental Time-of-Flight (TOF) spectra according to Ref.[6].

�

1000

1500

2000

2500 En = 14 MeV Experimental Data.

NRESPR1

Cou

nts

The table and function descriptions for LpðEpÞ, the parametersA, B, and C, the detector positions, dimensions, materialcompositions, and the incoming neutron energies are storedin input files to be used by the MC programs NRESP7, NRESPR1,and NRESPT1, which then generate PH responses with finiteGaussian resolutions determined by the parameters given inEq. (2). The PH spectra gains are adjusted according to the Ggfactors.

�

0

500

The resulting MC generated spectra were fitted to theexperimental ones with the same En, thus obtaining gaincorrection factors gx which were then applied to the protonlight-output functions. This process was iterated until the gaincorrection factors were close to unity (typically 1:00� 0:02).

0 100 200 300 400 500 600 700 800

� Pulse Height (arbitrary units)

Fig. 5. Comparison of simulated and measured pulse-height response for ring-

type detector R1 at En ¼ 14 MeV.

0

500

1000

1500

2000

2500

100

En = 14 MeV Experimental Data

En = 14 MeV NRESPT1

Pulse Height (arbitrary units)

0

Cou

nts

200 300 400 500 600 700 800

Fig. 6. Comparison of simulated and measured pulse-height response for

transmission-type detector T1 at En ¼ 14 MeV.

The PH spectra were corrected for gain shifts/drifts during themeasurement period, derived from g-source measurementsand zero-intercept calibration procedures before and after theneutron beam measurements. Typically, shifts were of order2–3%. These shifts could have been reduced if gain stabilizationof the PMTs were used. In fact, the PTB detector D1 wasstabilized with an LED, making its gain shifts less than 1%.

2.3. Comparison of experimental and simulated PH spectra and

fluences

Figs. 4 (Bicron-type detector, B2), 5 (ring-type detector, R1),and 6 (transmission-type detector, T1) show the comparisonbetween experimental and simulated pulse-shape responses fortwo different neutron energies. The agreement between data andsimulations is quite remarkable, except for the very low PHs. Thelight production by the process 12Cðn;3aÞn is important for thelower PH channels. The cross-section for this three-alpha breakupof 12C is not accurately known, introducing inaccuracies in theNRESP7, NRESPR1, and NRESPT1 calculations. For the a-particlelight-output function we used the relations given by Tichy et al.[7]

LaðEÞ ¼ 0:021 � E1:871 for Eo6:76 MeV

¼ 0:20755 � E� 0:65314 for EX6:76 MeV (3)

which are in close agreement with the expressions given in Ref.[4]. It should be noted that the agreement in shape betweensimulated and experimental responses is comparable to the resultof Tichy et al. [7].

500

1500

3000

3500

4000

2000

2500

Experimental Data En = 8.0 MeV

NRESP7 En = 8.0 MeVn

0

1000

Pulse Height (arbitrary units)

0 100 200 300 400 500

. 4. Comparison of simulated and measured pulse-height response for Bicron-

e detector B2 at En ¼ 8 MeV.

The simulated responses were normalized to a fluence of1 neutron=cm2. Fitting the experimental to the simulated re-sponse one obtains a ‘‘count rate’’ factor. This factor Cd withd ¼ B2, R1, T1 is by definition the neutron fluence at a particularneutron energy. For every detector we calculated the normalizedquantity

Fd ¼Cd

DdMd(4)

where Dd is an electronically set divide-down factor used toreduce the rate of events going to the computer (to keep its deadtime low), and Md is the yield obtained with the monitor detectorreferred to in Section 2.1 during the irradiation of a particulardetector d at a specific monoenergetic neutron energy. All detectorcount rates were corrected for dead time. The probability ofdouble-event detection within one neutron pulse from the2H(d,n)3He reaction was always less than 1% and therefore wasneglected.

The reference detector D1 had already been fully studied andcharacterized at PTB. If the fluences for our detectors B2, R1, andT1 were calculated correctly, then the ratios FB2

=FD1, FR1

=FD1, and

FT1=FD1

should be near unity for all neutron energies investigated.Except for small PHs, these factors were within 1:00� 0:04for almost all energies and are comparable to the ones obtainedby Tichy et al. [7] for a 5:08 cm� 5:08 cm NE-213 detector.(See Ref. [12] for details.)

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D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242238

The results of our studies at PTB give us confidence in thecorrect performance of the NRESP7, NRESPR1, and NRESPT1programs, and in the accuracy of the light-output functions forprotons found for our detectors B2, R1 and T1. Using these light-output functions in the PTB code NEFF7 and its ring-detector andtransmission-detector type modifications NEFFR1 and NEFFT1, theneutron detector efficiency of our detectors can be calculated forany given PH threshold.

3. Measurements at TUNL

In our simultaneous determination of the nd breakup nn andnp FSI cross-sections at En ¼ 13 MeV, a total of 20 neutrondetectors were employed (see Fig. 1). It is expected that all thesedetectors have efficiencies identical or very similar to theirhomologues characterized at PTB, and described in the previoussection. This assumption was tested by direct efficiency measure-ments at TUNL using two different experimental approaches. Oneis based on the 2H(d,n)3He reaction and the other employed a252Cf neutron source.

1000

800

600

400

200

Cou

nts

Detector 8Angle = 60

3.1. 2Hðd;nÞ3He approach

As at PTB, the neutrons were generated via the 2H(d,n)3Hereaction by bombarding a deuterium gas cell with a pulseddeuteron beam. Instead of the cyclotron at PTB, we used the TUNLtandem accelerator and its associated Neutron TOF Target Area.The deuterium gas-cell pressure was 1:000� 0:005 atm and theaverage pulsed deuteron beam current incident on the gas cellwas 30 nA. The deuteron beam current was purposely kept verylow to eliminate the effects of beam heating of the deuterium gasalong the deuteron path within the gas cell, which could result inan incorrect deuterium pressure determination. Details about thedeuterium gas cell are given in Ref. [2]. The efficiencies weremeasured simultaneously for neutron detector pairs positioned at0� and 60� with respect to the deuteron beam axis in order tocover a wider neutron energy range (see Fig. 7). Neutron energieswere varied from 3.93 up to 13.03 MeV (and then down again)in steps of roughly 0.5 MeV. The neutron detectors wereinterchanged after an energy sweep was completed, allowing

4.75 m

60.0°

4.15 m

neutrondetectors

deuteriumgas cell

beam pipe

z

x

Fig. 7. Top view of layout of detectors for neutron detector efficiency measure-

ments at TUNL.

both detectors to be exposed to the full range of available neutronenergies.

The PH thresholds of all detectors were determined using a137Cs g-ray source. In addition, 137Cs spectra were taken for eachdetector roughly every 8 h to check the gain stability of thephototubes and their associated electronics.

3.1.1. Data analysis

The efficiency of each detector was extracted from the yield/BCI (Beam Current Integration) from the monoenergetic neutronTOF peak. Here, BCI is a unit of charge deposited by the deuteronbeam in the deuterium gas cell. Fig. 8 shows sample TOF spectrafor detector #8 positioned at 0� (bottom panel) and 60� (toppanel). The efficiency is related to the neutron yield Yd by

�ðEnÞ ¼Zf Yd

TgcdsdoðEn; ylabÞ

(5)

where ðds=doÞðEn; ylabÞ is the cross-section for the 2H(d,n)3Hereaction as a function of resulting neutron energy En andlaboratory neutron production angle ylab [13]. The quantity Tgc isthe neutron attenuation factor due to the deuterium gas and theconstruction materials of the gas cell (0:980� 0:005) for allneutron energies and emission angles considered [14]. The netneutron yield Yd was corrected for dead time and background in

14000

12000

10000

8000

6000

4000

2000

0200 250 300 350 400 450 500 550 600

Time-of-Flight (channels)

Cou

nts

Detector 8Angle = 0

Fig. 8. Neutron Time-of-Flight spectrum at 0� (top) and 60� (bottom) using the2H(d,n)3He reaction at Ed ¼ 9:0 MeV. The number of neutrons detected is the

number of counts under the peak inside the gate shown, after background

subtraction. The background is the shaded region which is clearly seen in the

spectrum obtained at 60� .

ARTICLE IN PRESS

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10 12 14

det 8, 1/3xCs, 0° datadet 8, 1/3xCs, 60° dataNEFFR1

ε

NEFFR1 simulation threshold E = 0.185 MeV

En (MeV)

Fig. 10. Comparison of measured neutron detection efficiency and NEFFR1

simulations for ring-type detector R1. The simulations are normalized to the data.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10 12 14

det 1, 1/3xCs, 0° datadet 1, 1/3xCs, 60° dataNormalized NEFFT1

εNEFFT1 simulationthreshold E = 0.180 MeV Normalized by 0.989

En (MeV)

Fig. 11. Comparison of measured neutron detection efficiency and NEFFT1

simulation for transmission-type detector T1. The simulations are normalized to

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242 239

the TOF spectrum. Finally, the factor Zf is given by

Zf ¼ ðONtNiÞ�1 (6)

where O is the solid angle subtended by the neutron detector,and Ni is the number of deuterons/BCI incident on the gas cell.The quantity Nt is the density of deuterium atoms in the gas cell

Nt ¼lP

RT(7)

where l is the length of the gas cell (3:10� 0:05 cm), P is thedeuterium gas-cell pressure and T is the ambient temperatureduring the measurement (293:6� 0:5 K). The quantity R is theideal gas constant.

3.1.2. Results

Neutron detection efficiencies were determined for all neutrondetectors used in the ann=anp experiment with the exceptionof detectors #2, #3, #14, and #15 (see Fig. 1), because the countrates for nn-FSI events for these detectors were very low, andthe associated data were not taken into account in the finalanalysis.

The efficiency determinations of most importance are thosewith a 1

3� Cs PH threshold, because this PH threshold (set bysoftware) was used for all neutron detectors in the ann=anp

experiment. Figs. 9 (Bicron-type detector, B2), 10 (ring-typedetector, R1), and 11 (transmission-type detector, T1) show theefficiency data (dots with error bars) obtained at TUNL comparedwith the results of the NEFF7, NEFFR1, and NEFFT1 calculations,respectively. A normalization of 0.98 was applied to the calculatedefficiencies to bring them in better agreement with the measuredTUNL efficiencies. The estimated uncertainty of the neutrondetector efficiencies measured at TUNL is o� 2:5% for mostneutron energies studied. In the analysis of the nn and np

FSI cross-section data we used the normalized PTB efficiencycurves.

3.2. 252Cf approach

As can be seen from Fig. 12, three of the 12 neutron detectorsused for the measurement of the np FSI cross-section were of theArgonne type (detectors #17, #18, and #19). The proton light-output function of this detector type was not measured at PTB for

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 2 4 6 8 10 12 14

det9 1/3xCs, 0°

det9 1/3xCs, 60°

Normalized NEFF7

ε

NEFF7 simulationthreshold: E = 0.168 MeVNormalized by 0.974

En (MeV)

Fig. 9. Comparison of measured neutron detection efficiency and NEFF7 simula-

tions for Bicron-type detector B2. The simulations are normalized to the data.

the data.

an instrumental reason. The PTB code NEFF7 requires the totalnumber of atoms of the scintillator material present in thedetector. However, due to the lack of knowledge of the exact sizeof the nitrogen bubble in the scintillator volume of the Argonne-type detectors this number is not known very well and thereforeNEFF7 cannot calculate the absolute neutron detection efficiencyaccurately, even if the proton light-output function were known.

The efficiency measurements performed at TUNL and de-scribed in Section 3 were restricted to neutron energies aboveEn ¼ 3:93 MeV. This is not a concern for the Bicron-, ring-, andtransmission-type detectors, because the associated proton light-output functions were measured at PTB. Therefore, the efficiencycould be determined accurately also at the lower neutron energiesnot covered by the TUNL 2H(d,n)3He experiment.

As can be seen from Fig. 13, the efficiency of the Argonne-typedetectors is surprisingly well described in the energy range above3.93 MeV by the NEFF7 MC code using the Bicron-type protonlight-output function (measured at PTB for detector #9, B2). Inorder to check on the efficiencies of the Argonne-type detectorsfor neutron energies below 4 MeV, we used a third experimentalapproach which employed a well calibrated 252Cf neutron source.

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55.7°

SCALE

50 cm

CONCRETEPARAFFIN

HEAVY METALLEAD

GAS CELL

C6 D12

1.0 m1.

5 m

43.0°

35.5°28.0°

20.5°

69.0°83.5°100.5°

87

65

9

1011

12

17181920

Ring Detector

Bicron Detector

Argonne Detector

1.29

6 m

.1.

486

m.

1.821

m.

2.059 m.

Fig. 12. Layout of detectors used for the 1S0 neutron–proton scattering-length measurements. Detectors #17, #18, and #19 are Argonne-type detectors, while detectors #9

through #12, and #20 are of Bicron-type.

0.00

0.10

0.20

0.30

0.40

0

det 19 1/3xCs, 60°det 19 1/3xCs, 0°

Monte Carlo (PTB)

ε

Normalized by 0.975

Argonne

0.50

En (MeV)2 4 6 8 10 12 14

Fig. 13. Comparison of detection efficiencies obtained from the 2H(d,n)3He

measurements for the Argonne-type neutron detector and Monte-Carlo simula-

tions using the PTB code NEFF7. The simulations are normalized to the data.

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242240

3.2.1. Experimental setup and data analysis

This measurement was conducted in the Neutron TOF TargetArea at TUNL. The 252Cf was deposited (by the Kurchatov Institute,Moscow) on a circular and ultra-smooth platinum disk, positionedwith insulators inside of a low-mass parallel-plate ionization

chamber (25 mm in diameter) used to detect the 252Cf fissionfragments. The plates were 2 mm apart. During operation acontinuous flow of methane was used as counter gas. Theionization chamber was developed and built at PTB and purchasedby TUNL.

Two neutron detectors were placed at �15� with respect to thenormal of the 252Cf source disk (see Fig. 14). The neutron flightpath was adjusted to 300.0 cm. Shadow cones were used tomeasure the amount of neutrons that reach the detectorsindirectly through scattering from materials in the target roomand from air. To obtain a �1% statistical accuracy for neutronenergies between 3 and 6 MeV, data were accumulated for at least48 h without shadow cone (foreground measurements) and 24 hwith shadow cone (background measurements).

The fast signals from the 252Cf ionization chamber wereamplified and a hardware lower threshold was set to rejectcounts due to a-particles. The neutrons resulting from thespontaneous fission of 252Cf were detected via TOF measurements.After suitable delay the signals produced in the ionizationchamber by the fission fragments provided the stop signalfor the TOF measurements. The start signal was obtained fromthe neutron detectors. A 1

3� Cs software threshold was set on theneutron detectors. A measured TOF spectrum is shown in Fig. 15.The accidental background due to purely random coincidences inthe TOF spectra of the foreground and background measurementswas subtracted by creating a flat histogram of height equal to theaverage height of the background that appears before the g-raypeak. After the corrections for accidental gains and losses (due to

ARTICLE IN PRESS

Accidental Gains

neutrons

RandomAccidentalBackground

100

101

102

103

104

500

Cou

nts

Time−of−Flight

252Cf Neutron Spectrum

γ peak

1000 1500 2000

Fig. 15. Typical 252Cf Time-of-Flight spectrum. Time increases from right to left.

The narrow peak on the right side is due to g rays. The neutron distribution is

centered around channel 1450. The solid curve represents the calculated

accidental gains due to false start events.

0

0.1

0.2

0.3

0.4

0.5

Detector 19, 1/3xCsMonte Carlo (PTB)Normalized by 0.98

0

ε

Argonne

0.1

0.2

0.3

0.4

0.5

Monte Carlo (PTB)Normalized by 1.02

Detector 9, 1/3xCs

ε

Bicron

2 4 6 8 10En (MeV)

Fig. 16. Comparison of efficiency measurements using a 252Cf neutron source for

the two types of neutron detectors used in the neutron–proton scattering-length

measurements at TUNL in comparison to Monte-Carlo simulations using the PTB

code NEFF7. The simulations are normalized to the data.

30.0°

252Cf

ShadowBar

ShadowBar

Paraffin

Copper

Aluminum

Detector Detector

Fig. 14. Experimental setup for neutron detection efficiency measurements using a252Cf source embedded in a small ionization chamber.

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242 241

false starts) were applied, the background spectra werenormalized by the ratio of time spent in the foreground andbackground configurations. Then both spectra were correctedfor dead time and subsequently the background spectrum wassubtracted from the foreground spectrum. The resulting TOFspectrum was used to calculate the efficiency.

For this purpose the TOF spectrum was subdivided into bins ofequal energy using the known time calibration. The counts in eachbin were the number of neutrons detected at the mean energy ofthe bin. In order to extract the detection efficiency, this numberhas to be compared to the number of neutrons hitting the detectorfor each energy bin. To obtain this number we used the known

energy distribution of neutrons produced by 252Cf [15]. Thisdistribution was normalized by the measured fission rate andthen corrected for the attenuation of neutrons in air and thedetection efficiency of fission fragments correlated with neutronsemitted at �15� [3].

3.2.2. Results

Fig. 16 shows the measured efficiency (dots with error bars) forthe Bicron-type detector #9, B2 (top panel) and the Argonne-typedetector #19 (bottom panel) in comparison to the NEFF7 MCcalculation discussed in Section 2.3. For the Bicron-type detectorvery good agreement was found between the MC calculation andthe 252Cf-based neutron efficiency determination. We do not havea good explanation for the slightly different normalization factorsof 1.02 found here and the 0.974 required using the TUNL2H(d,n)3He based approach (see Fig. 9). However, there is clearlya disagreement between data and simulations for the Argonne-type detector (bottom panel of Fig. 16) between 1.5 and 3.5 MeVneutron energy. This indicates that the proton light-outputfunctions for the Bicron-type detector (filled with BC501A) andthe Argonne-type detector (filled with BC505) are not identical, asassumed in the MC calculation. In the analysis of the np FSI cross-section data involving Argonne-type detectors [16] we used themeasured efficiencies for Eno4 MeV rather than the MC basedefficiencies. Between En ¼ 3:5 and 7 MeV neutron energy, the252Cf based efficiency determination is in very good agreementwith both the PTB MC calculation and the 2H(d,n)3He based

ARTICLE IN PRESS

D.E.G. Trotter et al. / Nuclear Instruments and Methods in Physics Research A 599 (2009) 234–242242

determination at TUNL. For neutron energies above En ¼ 7 MeV,the accuracy of the 252Cf approach is limited due to the lowneutron yield and associated uncertainty in the neutron energyspectrum of 252Cf.

4. Conclusion

Results for the detection efficiency of various liquid organicscintillator-based neutron detectors have been obtained belowEn ¼ 13 MeV from measurements at PTB and TUNL and calcula-tions using the PTB code NEFF7. Typical values are 0.3. Consider-ing the fact that the neutron detector efficiencies determined atPTB and TUNL were obtained using greatly different experimentalapproaches, the close agreement between the results providesjustification for the estimated �3% overall uncertainty we haveassigned to the neutron detector efficiencies used in Ref. [2] forthe simultaneous determination of the 1S0 neutron–neutron andneutron–proton scattering lengths ann and anp, respectively.

Acknowledgments

The authors would like to acknowledge the many valuablecontributions to this work made by Prof. H. Klein. Furthermore,D. Gonzalez Trotter and W. Tornow would like to thank

Prof. H. Klein for his hospitality during their stays at PTB. Thiswork was partially supported by U.S. Department of Energy Grantno. DE-FG02-97ER41033.

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