Nuclear Instruments and Methods in Physics Research A 621 (2010) 379–382

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

E-m

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

Multi-layered parallel plate ionization chamber for cross-sectionmeasurements of minor actinides

K. Hirose a,�, T. Ohtsuki a, Y. Shibasaki a, N. Iwasa b, J. Hori c, K. Takamiya c, H. Yashima c,K. Nishio d, Y. Kiyanagi e

a Research Center for Electron Photon Science, Tohoku University, Sendai 982-0826, Japanb Department of Physics, Tohoku University, Sendai 980-8578, Japanc Research Reactor Institute, Kyoto University, Kumatori-cho, Sennangun, Osaka 590-0494, Japand Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japane Graduate School of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan

a r t i c l e i n f o

Article history:

Received 8 April 2010

Accepted 3 May 2010Available online 10 May 2010

Keywords:

Parallel plate ionization chamber

Neutron-induced fission

Minor actinides

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.05.001

esponding author.

ail address: hirose@lns.tohoku.ac.jp (K. Hirose

a b s t r a c t

A multi-layered parallel plate ionization chamber (MLPPIC) has been developed for the measurement

of neutron-induced fission cross-sections using the lead slowing-down neutron spectrometer at the

Research Reactor Institute, Kyoto University. The MLPPIC consists of two sets of multi-layered

electrodes to detect fission fragments from two samples located back-to-back between them. The

performance of the MLPPIC was tested with a spontaneous fission of 248Cm. The cross-section for the

neutron-induced fission of 241Am was successfully obtained using that of 235U as a reference.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The management of high level radioactive wastes (HLW) is animportant issue for utilizing the nuclear power. Minor actinides(MAs) and long-lived fission products in HLW cause the environ-mental burden due to their long-term radiotoxicities extendingacross future generations. Since MAs are fissile nuclides, theycan be recycled as a fuel in fast reactors or accelerator-drivensubcritical systems. To evaluate the feasibility of the MArecycling, precise nuclear data for neutron-induced reactions,e.g., capture and/or fission cross-sections are required.

In order to obtain the neutron-induced fission cross-sectionsprecisely, an intense neutron source is needed when the cross-sections are small. A lead slowing-down neutron spectrometer isa powerful tool for the cross-section measurements of theneutron-induced fission because of its high neutron flux whichis three or more orders of magnitude higher than that in the time-of-flight method at the equivalent flight length of about 5 m [1].

In the present study, a multi-layered parallel plate ionizationchamber (MLPPIC) has been developed in order to measure thecross-sections for the neutron-induced fission of the MAs. Theperformance of the MLPPIC and the results of the cross-sectionmeasurement of 241Am(n,f) are described.

ll rights reserved.

).

2. Detector design and performance

A high capability of discrimination between fission fragmentsand a particles is required for the measurements of fission cross-sections. In the recent experiments using the lead slowing-downneutron spectrometer (KULS) at the Research Reactor Institute,Kyoto University (KURRI), a detector called the BTB chamber, asingle-layered ionization chamber, has been used for the detec-tion of fission fragments [2]. In this detector, the sample backingmade of a stainless steel was also used as an electrode in orderto cover a large solid angle. When fission fragments were emittedat small angles with respect to a sample surface, their signalsbecame smaller because of larger energy losses in the sample. Insuch cases, the discrimination between the fission fragments anda particles became unclear because of the large solid angle of theBTB chamber. Therefore, we need to develop a detector withhigher performance in particle discrimination.

The schematic layout of the MLPPIC developed in this work isshown in Fig. 1. It consists of two sets of four electrodes,two anodes and two cathodes, located on both sides of thesample holder. They are placed in an aluminum housing(90�90�132 mm3) filled with isobutane gas by flowing-through regime.

Each electrode is made of an aluminized polyester foil with athickness of 2mm and adhered to a stainless steel frame of 50mmin thickness and 40 mm in inner diameter. The aluminized side issandwiched by another stainless steel frame that is used for signalreadout. They are placed at 2 mm intervals using aluminum ring

Samples

AnodesCathodes

5.5mm

2mm2mm2mm

HV

4.7nF

Preamp

Fig. 1. Schematic layout of the MLPPIC. Two sets of identical electrodes are located

on both sides of the samples. Each electrode is made of 2mm aluminized polyester

foil.

10 0

10 1

10 2

10 3

10 4

10-16Torr

10Torr15Torr20Torr

Cou

nt ra

te (c

ps)

1st anode

300 400 500 600 700 800

10 0

10 1

10 2

10 3

10 4

10-16Torr

10Torr15Torr20Torr

2nd anode

Negative bias (V)

Fig. 2. The dependence of the count rates on the gas pressure and the detector bias

for the signals from the first (upper panel) and second anodes (lower panel). The

wide plateau regions of the fission fragments can be seen.

FF

FF

0

200

400

600

800

0

200

400

600

800

Cou

nts

/ 8ch

2000 4000 6000 80000ADC channel

1st anode

2nd anode

Fig. 3. The pulse-height spectrum obtained using a 248Cm spontaneous fission

source. The upper and lower panels show the spectra of the signals from the first

and second anodes, respectively. Fission fragments are clearly distinguished from

a particles.

Experimental hole PPAC

Bismuth

Lead

Tantalum target

Experimental holeBeam hole

e-

Fig. 4. Top view of the KULS. Photo-neutrons produced in the tantalum target are

slowed down in the lead. Fission-fragments are detected with MLPPIC placed in

the experimental hole provided to the KULS.

K. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 379–382380

spacers with an alumite surface as an insulator. Polyimide filmsare used to insulate the electrodes from the aluminum ringspacers more substantially.

Two samples are set in the sample holder. One is a sampleof interest and another is a reference one such as 235U for which thefission cross-section is well known. The fission cross-section for thesample of interest is obtained relative to that for the reference one.

A signal readout circuit for a pair of electrodes is depicted inFig. 1. A negative bias is supplied to each cathode through a loadresistor of 3 MO. The anode signals are fed to charge-sensitivepreamplifiers (CANBERRA Model2006). A 16-channel spectro-scopy amplifier (CAEN N568B) is used for further amplifying andshaping the signals. The pulse height is digitized and recordedusing a LIST&PHA module (IWATSU A3100).

In order to determine the best condition for operating theMLPPIC, the count rate was measured at various gas pressures anddetector biases using a spontaneous fission of 248Cm. As shown inFig. 2, the wide plateau regions, corresponding to detection of allfission fragments emitted to the sensitive area of the detector,were observed in each gas pressure. The lower gas pressure isfavorable to reduce the energy loss between the sample and thefirst anode. The electric discharge occurred when the detector biaswas �450 V at the pressure of 6 Torr. Therefore, the operatingcondition was determined to be 10 Torr for the gas pressure and�500 V for the detector bias, where the pulse heights due to thea particles are sufficiently lower than those of fission fragments.

The pulse-height spectra taken with the spontaneous fissionof 248Cm are shown in Fig. 3. As shown in the upper panel, a gooddiscrimination between fission fragments and a particles wasachieved by the signal from the first anode. The lower panel

shows the pulse-height spectrum for the signal from the secondanode. In the second layer, i.e., the region between the secondcathode and the second anode, the specific ionization of thefission fragments is larger than that in the first layer. Therefore,the double-humped structure corresponding to the light andheavy fragments is observed.

3. Cross-section measurement for 241Am(n,f)

The cross-section measurement for 241Am(n,f) was carried outusing the KULS [3] driven by the electron linear accelerator at theKURRI. The KULS is a lead cube of 1.5�1.5�1.5 m3 which is anassembly of 1600 lead blocks. As shown in Fig. 4, a beam hole isprovided for placing a neutron production target which is a stacktantalum plate with an effective thickness of 6 cm. The MLPPIC

241Am

0 250 500 750 1000

1000

800

600

400

200

0

ADC1 (ch)

AD

C2

(ch)

ADC1

ADC2

0 1000 2000

2000

1500

1000

500

0

ADC1+ADC2 (ch)

a

b d

c

Fig. 5. (a) A pulse-height correlation between the first and second anodes. The

solid lines in (b), (c) are the projections onto both axes. The dashed lines show

the spectra for all events. (d) The spectrum of the sum signal from the first and

second anodes.

K. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 379–382 381

was installed in one of the eight experimental holes (four holesare shown in the figure). In order to suppress backgrounds of highenergy g ray from Pbðn,gÞ, the layer of the lead blocks surroundingone of the experimental holes is replaced by bismuth blocks of10–15 cm in thickness.

Evaporation neutrons from the photonuclear reactions inthe neutron production target are initially slowed down byinelastic and elastic scatterings in the lead material. In the energyregion below 0.57 MeV which is the energy of the first excitedstate of 207Pb, only the elastic scattering becomes dominant inthe slowing down process. In this energy region, the neutrons areasymptotically slowed down, where the cross-section for theelastic scattering is almost constant. The energy of the neutrons isderived from the relation En¼K/t2, where t is the slowing-downtime. The slowing-down constant K was determined to be17872 keVms2 which was deduced by the measurement of theneutron resonance peaks of Au, Cu, Co, and In using a BF3 or Arproportional counter covered with these materials.

Since it is difficult to determine the number of neutronsreaching to the sample, the reference method is employed toobtain the absolute value of the cross-section for the neutron-induced fission. In this method, the fission fragments emittedfrom a sample of interest and reference one are detectedsimultaneously. Under the assumption that the fissions occur bythe same numbers of the incident neutrons on both samples, thefission cross-section is calculated by

sðEnÞ ¼

Y241Amðn,f ÞðEnÞ

Y235Uðn,f ÞðEnÞ

NUOU

NAmOAms235Uðn,f ÞðEnÞ ð1Þ

where Y(En) is the fission yield at the neutron energy En, N is thenumber of atoms in the sample, and O is the solid angle for thefission fragments. The fission cross-section s for the referencesample is taken from the evaluated library and broadened by theneutron energy resolution of the KULS, typically 40% [3].

The 241Am and 235U samples were used in the present work.Each sample was purified by a chemical separation and depositedby the filtration method [4,5], where the hydroxide precipitationof the sample is filtered on an alumina membrane filter (What-man Anodisc). The deposited area is 20 mm in diameter. Eachnumber of atoms deposited on the membrane filter was (1.147 70.005) �1016 for the 241Am sample and (1.132 70.040) �1016

for the 235U reference sample, determined by a�ray spectrometrywith a silicon surface barrier detector. Here, only the statisticalerrors for the counts of the a particles are considered. A smallamount of the 234U impurity was found to be less than 0.035% inthe 235U reference sample. Since the 243Am impurity in theoriginal solution of 241Am was determined to be less than 0.54%by mass spectrometry, the contribution of the 243Am impurity andits decay daughter, 239Pu, to the cross-section for 241Am(n,f) isexpected to be negligible. No other impurities were found in the aspectra.

The experiment was carried out under the condition of theelectron energy of 30 MeV, the pulse repetition of 150 Hz with thewidth of 47 ns, and the averaged beam current of about 20mA.

4. Data analysis and results

A pulse-height correlation between the first and secondanodes in 241Am(n,f) shown in Fig. 5(a). The spectra for allevents are shown by the dashed lines in Figs. 5(b) and (c). As canbe seen from the dashed lines, the fission fragments cannot beclearly selected because of the large backgrounds caused byelectric noises at the injection of the electron beam. The solid linesshown in Figs. 5(b) and (c) are the projections of the coincident

events onto the x- and y-axes, respectively. The single and doublehumped structures corresponding to the light and heavyfragments are observed. The spectrum for the sum of bothanode signals is shown in Fig. 5(d). A good separation wasachieved by the coincident signals between the first and secondanodes. It should be noted that the selection of fission eventsbecame easier than that using the BTB chamber which has beenused in the previous experiments with the KULS.

Because of the existence of the many resonance peaks in thefission cross-sections, the ratio of the energy-dependent yieldsbetween 241Am(n,f) and 235U(n,f) is distorted. In the present study,the neutron spectrum was deduced by 10B ðn,aÞwhich shows a 1/v-dependence in the neutron energy. Then, the relative cross-sectionfor 241Am(n, f) was obtained using this neutron spectrum.

The relative cross-section for 241Am(n,f) is normalized to theabsolute value which was obtained using the 235U referencesample in the energy region between 100 eV to 1 keV. Theinfluence of the resonance peaks is thought to be negligibly smallin this energy region. The calculation procedure of the cross-section is expressed as

s241AmðEnÞ ¼ ZðEnÞ �

Y241AmðEnÞ

Y 010BðEnÞ

� s10BðEnÞ ð2Þ

ZðEnÞ ¼

P1 keVEn ¼ 100 eV

Y241AmðEnÞ

Y235U

ðEnÞ �NUOU

NAmOAm� s235U

ðEnÞ

8<:

9=;

P1 keVEn ¼ 100 eV

Y241AmðEnÞ

Y 010BðEnÞ

� s10BðEnÞ

8<:

9=;

ð3Þ

where Y and Y0

are the yields of the (n,f) and ðn,aÞ events,respectively. N is the number of atoms. O is the solid angle for thefission fragments. s is the evaluated data taken from JENDL-3.3[6] which is broadened by the neutron energy resolution forthe KULS, typically 40% [3]. Z is the normalization factor to theabsolute cross-section.

The cross-section for 241Am(n,f) is shown in Fig. 6. The resultobtained in this work is shown by the closed circles. The opensquares [7] and open triangles [8] are also shown for comparison.The smooth curve is the JENDL-3.3 evaluated data broadened withthe energy resolution of the KULS.

The error bars shown in the figure contain the statisticaluncertainties evaluated from the yields of 241Am(n, f) and

10 -2 010101 -1 2 310 0 10 1

Neutron energy (eV)

Fiss

ion

cros

s se

ctio

n (b

)10

10

10

10

10

2

1

0

-1

-2

This workS.Yamamoto(1997)V.F.Gerasimov(1997)JENDL-3.3(broadened)

Fig. 6. The cross-section for 241Am(n, f). The open squares and open triangles are

from Refs. [7,8]. The smooth curve is the evaluated data JENDL-3.3 broadened with

the neutron energy resolution of the KULS.

K. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 621 (2010) 379–382382

10Bðn,aÞ. As the other source of errors, the systematic uncertain-ties of the number of atoms N and the solid angle O for the fissionfragments were considered for both samples. It was estimatedto be 5.4% altogether, where the uncertainties of the half-lives,the a�branching ratios [9], the solid angles and statistics ina spectrometry were taken into account. At the resonance around3 eV, the present result shows a good agreement with that ofRef. [8] shown by the open triangles which are hidden by our datapoints. On the other hand, our data agree with those of Ref. [7] inthe energy region above 40 eV.

5. Summary

The multi-layered parallel plate ionization chamber (MLPPIC)has been designed and built for the cross-section measurements

of the neutron-induced fission of the minor actinides. Theperformance of the MLPPIC was examined with the spontaneousfission of 248Cm. A good discrimination between fission fragmentsand a particles was achieved. The cross-section for 241Am(n,f) wasobtained in the neutron energy range from 0.03 eV to 2 keV usingthe KULS. The result showed an agreement with the previousdata and the JENDL-3.3 evaluation, which suggests the successfuluse of the MLPPIC for the measurements of the neutron-inducedfission. The cross-section for the neutron-induced fission ofcurium isotopes is being measured with the system developedin this work.

Acknowledgements

The authors would like to thank the staffs at the KURRI-LINACfor the accelerator operation.

Present study is the result of ‘‘Study on nuclear data by usinga high intensity pulsed neutron source for advanced nuclearsystem’’ entrusted to Hokkaido University by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan(MEXT).

References

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Center, Japan Atomic Energy Agency.[7] S. Yamamoto, et al., J. Nucl. Sci. Eng. 126 (1997) 201.[8] V.F. Gerasimov, et al., Conference Report: Joint Institute for Nuclear Research

Dubna Reports, No. 97, 213, 1997, p. 348.[9] Table of Radioactive Isotopes /http://ie.lbl.gov/toi/radSearch.aspS, Lawrence

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