Nuclear Instruments and Methods in Physics Research A 652 (2011) 124–128

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

tgozani

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

Linac based photofission inspection system employing noveldetection concepts

John Stevenson, Tsahi Gozani n, Mashal Elsalim, Cathie Condron, Craig Brown

Rapiscan Laboratories, Inc., 520 Almanor Avenue, Sunnyvale, CA 94085, USA

a r t i c l e i n f o

Available online 30 August 2010

Keywords:

Photofission

Special nuclear material (SNM)

Radiography

Cargo inspection

Fission detection

Neutron detection

High-Z material detection

Nuclear material detection

Photofission based inspection

Threshold activation detectors

Delayed gamma detection

High energy radiography

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

016/j.nima.2010.08.047

esponding author. Tel.: 408 961 9711; fax: 40

ail addresses: jstevenson@rapiscansystems.co

@rapiscansystems.com (T. Gozani).

a b s t r a c t

Rapiscan Systems is developing a LINAC based cargo inspection system for detection of special nuclear

material (SNM) in cargo containers. The system, called Photofission Based Alarm Resolution (PBAR) is

being developed under a DHD/DNDO Advanced Technology Demonstration (ATD) program. The PBAR

system is based on the Rapiscan Eagle P9000 X-ray system, which is a portal system with a commercial

9 MeV LINAC X-ray source. For the purposes of the DNDO ATD program, a conveyor system was

installed in the portal to allow scanning and precise positioning of 20 ft ISO cargo containers.

The system uses a two step inspection process. In the first step, the basic scan, the container is

quickly and completely inspected using two independent radiography arrays: the conventional primary

array with high spatial resolution and a lower resolution spectroscopic array employing the novel

Z-Spec method. The primary array uses cadmium tungstate (CdWO4) detectors with conventional

current mode readouts using photodiodes. The Z-Spec array uses small plastic scintillators capable of

performing very fast (up to 108 cps) gamma-ray spectroscopy. The two radiography arrays are used to

locate high-Z objects in the image such as lead, tungsten, uranium, which could be potential shielding

materials as well as SNM itself.

In the current system, the Z-Spec works by measuring the energy spectrum of transmitted X-rays.

For high-Z materials the higher end of the energy spectrum is more attenuated than for low-Z materials

and thus has a lower mean energy and a narrower width than low- and medium-Z materials.

The second step in the inspection process is the direct scan or alarm clearing scan. In this step, areas

of the container image, which were identified as high Z, are re-inspected. This is done by precisely

repositioning the container to the location of the high-Z object and performing a stationary irradiation

of the area with X-ray beam. Since there are a large number of photons in the 9 MV Bremsstrahlung

spectrum above the photofission ‘‘threshold’’ of about 6 MeV, the X-ray beam induces numerous

fissions if nuclear material is present. The PBAR system looks for the two most prolific fission signatures

to confirm the presence of special nuclear materials (SNM). These are prompt neutrons and delayed

gamma rays. The PBAR system uses arrays of two types of fast and highly efficient gamma ray detectors:

plastic and fluorocarbon scintillators. The latter serves as a detector of fission prompt neutrons using

the novel threshold activation detector (TAD) concept as well as a very efficient delayed gamma ray

detector. The major advantage of TAD for detecting the prompt neutrons is its insensitivity to the

intense source related backgrounds.

The current status of the system and experimental results will be shown and discussed.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Active interrogation systems are currently being developed andtested as the most promising solutions for the detection of shieldedspecial nuclear material (SNM). In active cargo interrogation, anexternal source is directed into the cargo to induce fission, if nuclearmaterial is present. The results of the stimulated fission, in the form

ll rights reserved.

8 727 8748.

m (J. Stevenson),

of prompt and delayed neutrons and delayed gamma rays aredetected by an array of various detectors surrounding the inspectedcontainer. The active systems are not dependent on the relativelyweak and readily concealable natural emission of radioactivity frommost nuclear materials. Since the early 1970 both photofission andneutron induced fissions were extensively employed as part ofnuclear material safeguards effort to detect presence of SNM invarious points in the nuclear fuel cycle. From nuclear fuel scrap tonuclear waste drums and larger objects [1–3].The scanned objectswere generally smaller than the conveyances inspected today andthe main fission signature used was the delayed neutrons.

Fissiondetectors

9MVLinac

Conveyed containerRegular & Z-Specradiography arrays

Conveyor

Fig. 3. Current PBAR system design.

J. Stevenson et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 124–128 125

Rapiscan Laboratories (RapLabs) is developing and building,under the DHS/DNDO sponsorship two inspection systems; one isbased solely on automated high-energy X-ray radiography as analarming system and photofission based system as a clearing step.The other inspection system is based on neutron fission andphotofission [4]. This paper describes the first system based onautomated high-energy radiographic alarming followed with aphotofission clearing stage. The system is called PhotofissionBased Alarm Resolution (PBAR) and being developed under aDHD/DNDO Advanced Technology Demonstration (ATD) program.The PBAR system is based on the Rapiscan Eagle P9000 X-raysystem, a portal system with a 9 MeV LINAC X-ray source (seeFig. 1). The PBAR concept is illustrated in Fig. 2. For the purposesof the ATD program a conveyor system is installed in the portal toallow scanning and precise positioning of 20 ft ISO cargocontainers. The current PBAR system design is shown in Fig. 3.

2. Concept of operation

The system uses a two-tier inspection process. For the firststep, the ‘‘basic scan,’’ the container is completely and rapidlyinspected using two independent co-linear radiography arrays,

Fig.1. Rapiscan Eagle P9000 portal X-ray system.

Fig. 2. PBAR concept: Z-Spec transmission spectroscopy detectors are added in-

line to the standard X-ray detectors. Novel fission detectors, which detect

simultaneously prompt and delayed gamma rays, are attached to the C-shape

X-ray detector housing for high efficiency detection of photo induced fission

events.

the primary array and the Z-Spec transmission spectroscopy array[5]. The primary array uses 544 cadmium tungstate (CdWO4)detectors with conventional current mode readouts using photo-diodes, providing images with very high spatial resolution. The Z-Spec array uses about one fourth of that number of fast plasticscintillators with spectroscopic readouts using fast photomulti-plier tubes. The transmission spectroscopy detectors measure thetransmitted X-ray spectrum and can work at peak counting rateswell in excess of 100 MHz. The transmission spectroscopydetector electronics digitize the detector anode signal at asampling rate of 500 MHz and perform digital signal processingincluding pile-up correction and rejection to provide the pulseheight of individual transmitted X-rays. Fig. 4 shows the digitizedanode signal for three X-ray beam bursts, each approximately 2microseconds long.

Each of the independent radiography arrays are used to locatehigh-Z objects in the image such as lead, tungsten, and uranium,which would be potential shielding materials as well as thenuclear material itself.

3. Z-spec transmission spectroscopy method

The transmission radiography detectors work by measuringthe energy spectrum of transmitted X-rays. For high-Z materialsthe high energy portion of the X-ray spectrum, from about5 MeV to the endpoint 9 MeV are more attenuated than for low(e.g., polyethylene, graphite) and medium-Z (e.g., iron, copper)materials. The transmitted energy spectrum through high-Zmaterials has thus a lower mean energy and a narrower widththan through low- and medium-Z materials. This is seen veryclearly in the transmission spectra calculated for a wide range of Zabsorbers having all the same energy attenuation value of 5000(see Fig. 5). The energy attenuation, i.e. the ratio of the energydeposited by the unattenuated X-ray beam in the X-ray detectorto the same quantity with the absorption present, is theattenuation measured in regular radiography systems. The cleardependence of the transmission spectra on the atomic number ofthe absorber is very obvious even though the X-ray transmissionis exactly the same. Another way to express the strong Zdependence of the Z-Spec method is to plot the ratio of the lowenergy to high energy spectral regions of interest of the spectrashown in Fig. 5 vs. the value of Z at the same X-ray attenuation(see Fig. 6). As one can see there is a very strong dependenceof the spectral ratio on the Z value of the absorber. The strongspectral Z dependence is the result of the different energydependence of the photon interaction cross-sections of thevarious elements with matter. The magnitude of the effectchanges in a known and measured way as a function of the

Fig. 4. Three linac X-ray beam bursts each roughly 2 microseconds long with individual X-ray pulses are shown. Negative spikes in the pulse are individual X-rays and are

roughly 15 ns wide. The sampling interval of the ADC is 1 ns. The signal is DC offset by 120 mV to utilize the full range of the bipolar ADC.

Transmission spectra of various elemental absorbers witha constant energy attenuation of 5000

0

0.001

0.002

0.003

0.004

0.005

0.006

0Photon energy

Flu

ence

(arb

it. u

nits

)

174cm C106.6cm Al31.9cm Fe30.6cm Sn10.2cm W9.5cm U

6

74

92

50

26

13

1 2 3 4 5 6 7 8 9 10

Fig. 5. Z-Spec transmission spectra of absorbers with atomic number Z from

carbon (z¼6) to uranium (z¼92). The thickness of the absorbers are chosen to

keep the energy attenuation (which is the quantity measured in a normal

radiography system) constant and equals to 5000.

Spectral index ratio [(2-3MeV)/(5-9MeV)]vs Z for 9MV x ray source with a

constant attenuation of 5000 (calculatedfor plastic scintillator)

Z = 5.7x + 5.2

0

10

20

30

40

50

60

70

80

90

100

0Index ratio value

Ato

mic

num

ber Z

Sn

Al

UPb

W

Fe

C&CH2

5 10 15

Fig. 6. Transmission spectroscopy spectral index shows strong Z dependence.

J. Stevenson et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 124–128126

attenuation, which is precisely provided by the regular X-raydetectors. Thus the Z-Spec method applies to the entire range ofX-ray attenuation encountered in actual cargo inspection.

Fig. 7 shows the measured transmitted X-ray spectra (from9 MV linac) through samples of graphite, steel and tungsten. Thethree samples have roughly the same overall attenuation butresult in very different transmitted spectra.

These high-Z detection techniques cannot however, distin-guish potential shielding materials such as lead or tungsten fromnuclear materials. This is done in the second tier.

4. Novel fission signature detectors

The second step in the inspection process is to inspect thelocation identified by the automated X-ray system as a possiblealarm. This is done by a longer stationary direct scan of thatlocation. In the direct scan, areas of the container image that were

identified as high-Z are re-inspected. This is done by preciselyrepositioning the container to the location of the high-Z objectand doing a stationary irradiation of the area with the X-ray beam.Since the X-ray beam has a continuous spectrum of X-rays withan endpoint of 9 MeV, some of the X-rays are above the energyrequired to cause photofission (�6 MeV). SNM threats, as well asall fissionable materials will fission and produce fission signatureswhile in the X-ray beam. The PBAR system looks for two types offission signatures to identify that fission is taking place. These areprompt neutrons from the direct fission process and delayedgamma rays from the decay of the fission products. The PBARsystem uses an array of forty 16 in.�16 in. fission detectors todetect these signatures of fission. There are two types of detectorsin the array: plastic scintillators and fluorine based thresholdactivation detectors (TAD, see Ref. [6]) in the form of fluorocarbon

Measured normalized transmission spectra(9MV linac)

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0Photon Energy (MeV)

Frac

tion

of s

pect

rum

per

cha

nnel 3"W

9"Fe42"C

W

C

Fe

1 2 3 4 5 6 7 8 9 10

Fig. 7. Measured transmission spectra for a 9 MeV bremsstrahlung beam through

samples of graphite, steel, and tungsten.

Fluorocarbon response to sources ofgamma ray and fission neutrons

0.1

1

10

100

1000

10000

100000

1000000

0Pulse height (MeV)

Cou

nts/

unit

ener

gy

Cf252 DataTh228(2.61 MeV)Co60(1.17+1.33 MeV)Cs137(0.66 MeV)

Cs137Th228C060

Cf252 (n,α)activation

1 2 3 4 5 6 7 8 9 10 11

Fig. 8. Response of fluorocarbon detector to various gamma rays and fission

neutron source (Cf252) a threshold fission neutron detector.

Photofission signatures measured betweenlinac pulses with FC and PS scintillators

0.01

0.1

1

10

100

1000

0Pulse hieght distribution (MeV)

Cou

nts

per s

econ

d/un

it en

ergy

FC

PS

5 10 15

Fig. 9. Pulse height distribution of the two main detectors of PBAR, plastic and

fluorocarbon scintillators measured between 2 microsecond wide 9 MeV linac

pulses. Linac ran at 100 Hz.

J. Stevenson et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 124–128 127

liquid scintillators. The plastic scintillators can detect delayedgamma rays only. The fluorocarbon detectors can detect bothdelayed gamma rays and prompt neutrons via the reaction19F(n,a)16N, which has an effective threshold of 3 MeV and isinsensitive to most photoneutrons due to the typical highthreshold for photonuclear reactions. Exceptions are berylliumand deuterium which have thresholds for producing 3 MeVneutrons of about 5 and 8.2 MeV, respectively. Beryllium is arare cargo that will cause neutron alarms but will have noaccompanying delayed gamma ray signature. The backgroundfrom deuterium in normal hydrogenous materials is small. Theisotope 16N beta decays with a 7.1 second half-life. There are twomajor beta decay modes: 10.4 MeV endpoint (26%) and 4.3 MeV(68%). The detection of the prompt fission neutrons by thefluorocarbon detector is achieved by the close to 100% efficiencydetection of the high-energy beta decay rather than the alphaparticle in the (n,a) reaction which occurs during the X-ray pulse.The major advantage of detecting delayed gamma rays andprompt neutrons using the fluorocarbon detector is that signalsare delayed, relative to the fission event and the X-ray pulse. TheX-ray pulse temporarily blinds the detectors but they recover

between the bursts. Fig. 8 shows the response of one of the fluoro-carbon fission signature detectors to gamma rays from 137Cs, 60Co,and 228Th as well as the beta decays from 16N decay followingirradiation with 252Cf fission neutrons.

The ability of the PBAR to detect simultaneously the mostprolific fission signatures, delayed gamma rays and prompt fissionneutron is shown in Fig. 9. In both detectors the energy rangebelow 5 MeV shows typical fission delayed gamma rays. Abovethis energy the fluorocarbon shows the prompt neutrons via thedetection of high-energy (10.4 MeV) beta particles.

5. Conclusions

Extensive simulation corroborated by thorough laboratoryexperiments show that the PBAR system concept is very viableand will deliver high sensitivity to fissile and fissionable materialswith throughput consistent with high-energy X-ray radiographysystems. A full-size PBAR has been designed and it is now beingmanufactured.

The use of high-energy X-ray to stimulate fission via thephotofission process and the detection of the strongest fissionsignatures, i.e., prompt neutrons and delayed gamma rays, assurea fast clearing process of the reduced number of alarms generatedby the radiography systems and filtered by the automatic radio-graphy system, and aided by the Z-Spec transmission spectro-scopy technique.

Acknowledgments

The research was funded under a contract from the U.S.Department of Homeland Security, Transformational and appliedResearch Directorate (TARD) Domestic Nuclear Detection Office(DNDO).

References

[1] T. Gozani, D.E. Rundquist, R.O Ginaven, R.L Bramblett, Safeguards Techniques,IAEA-SM-133 (45) (1970) 143.

J. Stevenson et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 124–128128

[2] R.L. Bramblett, T Gozani, R.O. Ginavn, D.E. Rundquist, Nuclear Technology 14(1972) 33.

[3] T. Gozani, Active nondestructive assay of nuclear materials—principles andapplications, US Nuclear Regulatory Commission book, NUREG/CR-0602,January 1981, pp. 307–313.

[4] Tsahi Gozani, Timothy Shaw, Michael J. King, John Stevenson, Mashal Elsalim, CraigBrown and Cathie Condron, Combined dual species interrogation of cargo using

Photoneutrons and X-rays, CAARI 2010, AIP Conference Proceedings submitted forpublication.

[5] T. Gozani, John Stevenson, Craig Brown, Mashal Elsalim, Willy Langeveld andTimothy Shaw, Z-SPEC—a new method for Z discrimination based on X-rayspectroscopy, submitted for publication.

[6] T. Gozani, J. Stevenson, M.J. King, Neutron threshold activation detectors (TAD)for the detection of fissions, submitted for publication, this issue.