Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386

Neutron energy spectra inside a PET cyclotron vault room

H!ector Ren!e Vega-Carrillo*

Centro Regional de Estudios Nucleares, Universidad Autonoma de Zacatecas, Apdo Postal 336, 98000 Zacatecas, Zac. Mexico

Received 20 July 2000; received in revised form 3 November 2000; accepted 5 November 2000

Abstract

Neutron energy spectra were measured at three locations inside the vault room of a PET cyclotron. Measured

neutron fields were produced during 17.2MeV protons and 8.6MeV deuterons colliding with Faraday cup, and8.6MeV deuterons colliding with 14N. Measurements were performed using a Bonner sphere spectrometer whose active6LiI(Eu) scintillator was replaced by two pairs of thermoluminscent dosimeters. Modified neutron spectrometer was

calibrated using bare 252Cf and D2O moderated 252Cf neutron sources. Thermoluminiscent dosimeters were calibratedusing g-rays from 137Cs and 60Co sources. From this calibration, a single factor was derived that allow us to obtain thethermal neutron net signal used to unfold neutron spectra. MCNP 4A code was used to calculate neutron spectra at

those sites were measurements were taken. For MCNP calculations a source term for protons colliding with graphitewas calculated using a semiempirical model. Due the lack of source term for neutrons produced during 8.6MeVdeuterons nuclear reactions, with carbon and 14N, several models were tried. From experimental and calculated resultsa systematic behavior of neutron spectra was observed regardless the primary neutron source. This behavior was a peak

indicating the presence of evaporation neutrons due to the large amount of iron in cyclotron. # 2001 Elsevier ScienceB.V. All rights reserved.

PACS: 28.20.Gd; 29.20.Hm; 29.30.Hs

Keywords: PET; Cyclotron; Neutron; MCNP; Bonner spheres

1. Introduction

Positron Emission Tomography, PET, widelyused for diagnostic purposes, is a non-invasivemedical imaging technique used to determinelocation and concentration of physiologicallyactive compounds in human body [1].

In PET, 15O, 13N, 11C and 18F are used, thesehave short half-lives and are produced at hospitals,mostly using charged-particle accelerator facilities.PET isotope production is carried out usingnuclear reactions that produces undesirable neu-trons. Table 1 shows PET isotopes nuclearreactions features.

An accelerated beam of charged particlesproduce radiation as a consequence of interactionsbetween charged particle beam and surroundingmedia it collides, thus bremsstrahlung and char-acteristic X-rays, prompt g-rays, neutrons anddelayed radiation (b and g) are produced. The lack

*Fax: +1-52-492-2-70-43.

E-mail address: rvega@cantera.reduaz.mx (H.R. Vega-

Carrillo).

URL: http://cantera.reduaz.mx/�rvega.

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 3 4 - 0

of electric charge and the ways they interact withmatter, makes neutron field characterization adifficult problem. A single location to describeneutron production is difficult to establish becauseneutron-producing reactions can occur at manypoints inside the accelerator, such as target, beamstopper and beam deflector. According to NCRP51 [2], shielding design requires an estimate ofradiation emission rate from the accelerator. Tomake this estimation, parameteres like: species andenergy of accelerated particle, target material,particle-beam current, angular distribution andenergy spectrum of produced radiation, should beknown.

Nuclear reactions have been studied extensivelyfor a broad range of targets and particles. Ingeneral, it has been established that [3,4], even athigh proton energies, resulting neutron spectrumfrom thick targets may be described as consistingof three main components: Cascade neutrons,evaporation neutrons and epithermal and thermalneutrons arising from slowing down of evapora-tion and cascade neutrons.

In this investigation neutron energy spectra weremeasured at three locations inside the vault roomof a PET cyclotron. Neutron spectra weremeasured during 17.2MeV protons and 8.6MeVdeuterons colliding with Faraday cup, and8.6MeV deuterons colliding with 14N. Measure-ments were performed using a Bonner spherespectrometer, BSS, whose active 6LiI(Eu) scintil-lator was replaced by two pairs of thermolumins-cent dosimeters (two TLD600 and two TLD700).Modified BSS was calibrated using bare and D2Omoderated 252Cf neutron source. Thermoluminis-

cent dosimeters, TLDs, were calibrated usingg-rays from 137Cs and 60Co sources. From thiscalibration, a single factor was derived that allowus to obtain the thermal neutron net signal.

MCNP 4A code was used to calculate neutronspectra at those sites were measurements weretaken. For MCNP calculations a source term forprotons colliding with graphite was calculatedusing a semiempirical model derived by Alsmilleret al.

Due to the lack of a source term for neutronsproduced during 8.6MeV deuterons nuclear reac-tions, with carbon and 14N, several models wereused. From experimental and calculated results asystematic behavior of neutron spectra was ob-served regardless the primary neutron source. Thiswas a peak in the energy interval 0.3 to 4MeV,with a maximum at approximately 1MeV indicat-ing the presence of evaporation neutrons producedduring nuclear reactions that are modified by thelarge amount of iron in the cyclotron.

2. Materials and methods

2.1. Facility description

PET cyclotron is located at the ResearchImaging Center at the University of Texas HealthScience Center in San Antonio, TX. PET cyclo-tron is Scanditronix MC 17F capable of accelerat-ing protons or deuterons to 17.2MeV and8.6MeV, respectively. Maximum charged particlecurrent is 65 mA, with an extraction efficiency of

Table 1

Positron-emitting isotopes used in PET studies

Nuclear reaction Q-value (MeV) Positron emitter Half life (minutes)

11B(p,n)11C �2.764 11C 20.38514N(p,a)11C �2.922 11C 20.38516O(p,a)13N �5.217 13N 9.96512C(d,n)13N �0.280 13N 9.96515N(p,n)15O �3.536 15O 2.03714N(d,n)15O +5.073 15O 2.03718O(p,n)18F �2.437 18F 109.7720Ne(d,a)18F +2.792 18F 109.77

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386376

approximately 80% for protons and 60% fordeuterons [5].

Magnets, radiofrequency source, cooling sys-tem, target, beam probe, Faraday cup (beamstopper), and an ion source complete the cyclo-tron. Monitoring charged particle beam in thecyclotron, beam stability, particle selection, andtarget handling are all determined from the controlroom. Radioisotopes produced are automaticallytransported to hot cell at radiochemical labora-tory.

Cyclotron is housed in a 518� 518� 366-cm3

vault shielded by reinforced concrete, Fig. 1 showscyclotron facility. Vault walls are 152 cm thick,ceiling is 122 cm thick, access to vault is via amotor-driven concrete door 122 cm thick. Cyclo-tron is located at the first floor of the ResearchImaging Center building. Three of the cyclotron’svault walls are adjacent to equipment room,radiochemistry laboratory, and restrooms. Thefourth wall is adjacent to outside loading dockarea. A vault room plant view is shown in Fig. 2.

2.2. Bonner spectrometer modification andcalibration

There is not single spectrometer with idealresponse to neutrons from thermal to fast energies.BSS is often used to obtain neutron spectra over a

wide range of neutron energies (thermal to400MeV), but it has a poor energy resolution,and an unfolding code is required [6]. Pulse pileupand large dead times in intensive or pulsed neutronfields is another BSS drawback. To overcome thislast a passive thermal neutron detector can be usedinstead of active 6LiI(Eu) BSS detector. In thiswork was decided to use TLDs as thermal neutrondetector in the BSS.

The use of paired TLDs in mixed radiation fieldshas been widely used, however few works have

Fig. 1. PET cyclotron schematic diagram.

Fig. 2. Partial plant view of first floor at the University of Texas Health Science Center, where PET cyclotron vault room is located.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386 377

reported using pairs of TLDs in multispheres[7–10].

BSS was modified in the following manner: Ahigh-density polyethylene plug with the samegeometry and dimensions as the aluminum holderfor scintillator, light pipes, photomultiplier tubeand base was built. At plug upper section, foursmall holes were done to hold 4 TLDs, 2 TLD600and 2 TLD700. Bare, 200, 300, 500, 800, 1000 and12 inch-diameter polyethylene spheres were used.Two pairs of TLD-600 and TLD-700 in eachsphere were used to measure thermal neutrons.TLD-600 contains 95.6% 6Li, while the TLD-700contains 99.9% 7Li. Both have approximatelysame density and atomic numbers, and the sameresponse to g rays. For thermal neutrons, TLD-600 response is larger than TLD-700. TLDs are0.3175� 0.3175� 0.0889-cm3-ribbon type fromHarshaw (Solon, OH). When exposing TLD-600and TLD-700 in a mixed neutron g radiation field,measured response is the result of g-ray andneutron responses,

Rnþg600 ¼ Rn

600 þ Rg600 ð1Þ

Rnþg700 ¼ Rn

700 þ Rg700 ð2Þ

here, Rnþg600 is the total TLD 600 measured

response, Rn600 is neutron contribution, and Rg

600

is g-ray contribution to TLD-600 response. Rnþg700 is

the total response of TLD 700, Rn700 andRg

700 arethe respective contributions to TLD 700 responseof neutrons and g-rays.

When pairs of TLD 600 and TLD 700dosimeters are used to measure thermal neutrons,a common practice is to assume that both have thesame response to photons [7,11–15]. Because Rn

700

is practically null, the simple difference betweenresponses of both dosimeters is used as anindicator of net neutron response.

During this study, photon responses of bothdosimeters were measured with 60Co and 137Cs gsources. TLD 600 to TLD 700 g response ratio wasused as a correction factor, k, to equalize the g-rayresponse of both TLDs. Net neutron signal wascalculated using,

Rn ¼ Rnþg600 � k Rnþg

700 : ð3Þ

Bare 252Cf and D2O moderated 252Cf neutronsources were used to calibrate BSS with TLDs andBSS with 6LiI scintillator. During calibration,measurements with bare thermal detectors, 6LiIand TLDs, were carried out to check if statisticaldifferences in count rates show up due polyethy-lene plug that hold TLDs.

BSS with 6LiI and BSS with TLDs were used tomeasure neutron energy spectra of bare 252Cf andD2O moderated 252Cf neutron sources, this mea-surements were used to find the relationshipbetween BSS with TLDs and BSS with 6LiIexperimental responses.

2.3. MCNP calculations

2.3.1. Cyclotron and vault room modelingGeneral Monte Carlo Neutron Photon Trans-

port, MCNP, code version 4A [16] was utilized tocalculate neutron transport inside vault room.Faraday cup was modeled as 2.3-cm-diametercarbon sphere. Because information about thedeflector material was unavailable it was alsomodelled as a 2.3 cm diameter carbon sphere.Inside this spheres neutron source was located.Vault room concrete density was assumed be 2.3g/cm3, iron was used to model cyclotron’s mag-nets, meanwhile wires and cyclotron ‘‘dees’’were modelled as copper made. Air inside vaultroom was included in the model. For simplicityvault room door was modelled as thick as vaultwalls.

Neutron flux spectra were tallied at thoselocations where measurements with BSS withTLDs were carried out. For source terms used inMCNP calculations two different situations wereconsidered: neutrons produced by protons inter-acting with carbon and those produced whendeuterons interact with carbon and 14N.

2.3.2. Neutrons emitted during (p, C) reactionAt proton energies below 10MeV, dominant

interaction is by the compound nucleus formation,which is left in excited state with a number ofallowed decay channels. Compound nucleus tendsto reach ground state by particle emissiondescribed by an evaporation process. Energy

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386378

distribution of emitted neutrons can be estimatedby,

nðEÞ dE / E expð�E=TÞ ð4Þ

here, T is nuclear temperature, usually between 1and 10MeV. However, at higher energy develop-ment of intranuclear cascade become important.In this case, several neutrons can be emitted insuccession, while nuclear temperature falls from aninitial value to zero.

Intranuclear cascade develops through interac-tion of individual nucleons inside the nucleus.Using intranuclear-cascade-evaporation model,Bertini [17,18] generated a large amount of dataof non-elastic cross-sections and the energy andangular distributions of emitted neutrons andprotons. These data are for neutrons and protons,in the energy range 25–400MeV, colliding with avariety of targets. To make these data moreaccessible, Alsmiller et al. [19] developed analyticalmodels and tables of their coefficients, using least-square method to fit Bertini’s data. NeverthelessAlsmiller’s expressions are valid for 25–400MeVprotons it was assumed that coefficients andsemiempirical equations gave reasonable resultsfor 17MeV protons. Therefore coefficientsfor neutron emission and non-elastic cross-section models at 17MeV protons wereassumed to be the same as those at 25MeV,calculations were carried out as Nakamuraet al. [20]

To apply Alsmiller expressions, Faraday cupmodel was subdivided into n thin slabs, each of athickness Dx. As protons penetrates Faraday cup,was assumed protons loses energy according tocontinuous slowing-down model without changingdirection. Average kinetic energy ðEkÞ in the kthslab was calculated using,

Ek ¼ Ek�1 �Dx2

dE

dx

� �k�1

; k ¼ 1; 2; . . . ; n ð5Þ

where Ek�1 is protons average kinetic energy in thek� 1 carbon slab, and ðdE=dxÞk�1 is the stoppingpower of Ek�1 MeV protons in carbon. Protonstopping power was calculated using the computercode TRIM 92 [21].

Energy spectra of neutrons emitted within eachslab were calculated and summed over all thin

slabs as

nðE; yÞ ¼Xnk¼1

FkðE; yÞ þ1

4pGkðE; yÞ

� �

� AksðEkÞNDx10�27 ð6Þ

where sðEkÞ is the non-elastic cross-section ex-pressed in millibarns and was calculated using [22]

sðEkÞ ¼1

400exp

Xn

i¼0

aiEk400

� �i( )

ð7Þ

here, FkðE; yÞ stands for the cascade neutron-emission spectrum,

FkðE; yÞ ¼1

Ekexp

Xn

i¼0

biðyÞE

Ek

� �i" #

ð8Þ

and GkðEÞ represents the evaporation neutron-emission spectrum,

Gk ¼1

25exp

Xn

j¼0

cjE

25

� �j" #

: ð9Þ

Ak ¼ Ak�1 exp½�sðEk�1ÞNDx�, N is target’s atomicdensity, and ai, bi and ci are coefficients calculatedby Alsmiller et al.

2.3.3. Neutrons emitted during (d, C) reactionNo neutron source term was available for

deuterons as impinging particles, thus severaldifferent spectra were tried. A conservative ap-proach is to assume that source term is mono-energetic. For exoenergetic nuclear reactionssource term energy must be equal to maximumenergy achievable in the reaction. In case ofendoenergetic nuclear reactions source term en-ergy must be equal to the kinetic energy of theincident charged particle. Following this conser-vative criteria two monoenergetic neutron sourceswere used. Evaporation and Maxwellian sourcesterms were also used. Source terms used tocalculate neutron spectra produced during deuter-on interactions were,

1. Isotropic, monoenergetic neutron sources: 8.27and 13.92MeV.

2. Isotropic, Maxwellian source given by QðEÞ ¼CE1=2 expð�E=TÞ; where C is a normalizationfactor, and T¼ 5MeV.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386 379

3. Isotropic evaporation source given by QðEÞ ¼CE expð�E=TÞ; where C is a normalizationfactor, and T¼ 5MeV.

During calculations source terms were located atFaraday cup and deflector locations. From the six-neutron tally types available in MCNP we usedtally type 5, which gives the neutron flux at a pointdetector. To assure a statistical relative error lessthan 5% in each spectra’s energy bin, consideredreliable for point detectors, 106 histories were usedfor each run. No variance reduction techniqueswere used.

2.4. Bonner sphere measurements

Neutron field inside the cyclotron vault hasthree components: neutrons produced by chargedparticle interactions with cyclotron’s internalcomponents (primary source), those producedinside the cyclotron due to interaction of theprimary source neutrons with components of thecyclotron (secondary source), and neutrons thatafter leaking out the cyclotron, collide with bunkerwalls and are returned back to the room.Characterization and measurement of these com-ponents individually cannot be done becauseprecise localization of points where chargedparticles interact within the structure of thecyclotron is unknown. Also vault dimensions donot allow use of experimental techniques tomeasure the neutron room return spectrum.

Nuclear reactions at target area, with 14N orFaraday cup, and at deflector site are sources offast neutrons. These collide with cyclotron com-ponents, losing energy and leaking out fromcyclotron, then interact with walls and air in theroom. Some of these neutron are returned back tothe room, others continue interacting with theconcrete components loosing energy until they areabsorbed or leak out from the vault. Due to vaultwall thickness leaking neutron field is expected tobe highly thermal. A fundamental assumption inthis work was that neutrons are only produced incyclotron at two sites, the target and deflector.

BSS with TLDs was used during measurements,for Faraday cup, target and deflector sites, BSSwas located approximately 100 cm from the

assumed reaction point along the line of thecharged particle beam trajectory. The third mea-surement site was selected away target anddeflector locations. Fig. 3 shows sites where BSSwas located. Site 1 was used during protons anddeuterons reactions on Faraday cup, site 1’ wasused during deuteron reactions with the 14N target.For deuteron beam, BSS was located close todeflector, labeled as site 2 in Fig. 3. Site 3 was usedto measure neutrons produced during interactionsof deuterons with Faraday cup.

For 17.2MeV protons impinging on carbonFaraday cup (p, C), neutrons are produced ifnuclear reaction is with 13C. This isotope isapproximately 1% of the natural carbon; thenneutron production is weak. For this reaction,measurements were carried out only at site 1.

For 8.6MeV deuterons interacting with Faradaycup (d,C), measurements were taken on sites 1–3,meanwhile intections with 14N target, (d, 14N),were taken on sites 1 and 2.

TLDs annealing was carried out 24h beforemeasurements. Annealing cycle consisted of heatingTLDs for 60min at 4008C in a porcelain container,then letting them cool down to room temperatureby moving the TLDs from the porcelain containerto a stainless steel planchet. After cooling, TLDswere stored in a plastic container inside a cadmiumbox to reduce unwanted neutron exposure. In this

Fig. 3. Sites where BSS was located during measurements.

Position 1 is at 100 cm from Faraday cup. Position 10 is at

100 cm from target, site 2 is at 100 cm from deflector. Third site

is indicated by position 3.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386380

process each individual TLD was tracked. Afterannealing, TLDs were handled away from ultra-violet lights to avoid introducing TLDs responsebias [23]. In each measurement 5 TLD600 and 4TLD700 were used as controls to remove thebackground radiation contribution during TLDhandling. After reading TLDs, an average value ofthe TLD-600 and TLD-700 for each sphere, andcontrols, was calculated. Net neutron signal perirradiation time per microampere for each detectorwas calculated. This net neutron signal was usedduring unfolding process.

BSS have m spheres, the relationship betweendetector counts Cj, detector response functionRjðEÞ and neutron spectrum FðEÞ is described byFredholm first kind integral equation,

Cj ¼Z E max

E min

dERjðEÞFðEÞ for j ¼ 1; 2; . . . ; m

ð10Þ

Although several methods exist to get formalsolution of this equation [24–26], none of these areapplicable when response function RjðEÞ is notknown analytically, which is the case for BSS. Ingeneral, RjðEÞ is experimentally determined orcalculated, and usually approximated by a re-sponse matrix having discrete values. Thus, one isleft with the problem of solving m linear equationsin N unknowns, process named unfolding, wherem is the number of spheres used, and N is thenumber of energy points, or intervals, needed todefine the spectrum. Integral equation becomes,

Cj ¼XNi¼1

RijFi for j ¼ 1; 2; . . . ; m ð11Þ

where Cj are the counts from jth detector, Rij is thejth detector response to neutrons in the ith energyinterval, meanwhile Fi is neutron fluence in the ithenergy interval.

Since number of detectors is less that number ofpoints used to describe the spectrum, (m5N), nounique solution of Eq. (11) exists. Then approx-imate unfolding procedures must be applied tosolve it. Recently, several procedures have beenproposed to unfold neutron spectra from BSS[27–29]. BUNKIUT unfolding code was used toobtain neutron energy spectrum from the BSSmeasurements. The BUNKIUT input is the sphere

diameters with the thermal neutron detector read-outs and their experimental uncertainities, in theoutput the code shows an error that indicates theagreement of BUNKIUT’s solution with theexperimental data given in the input.

3. Results and discussion

The comparision between the differential neu-tron energy distributions of 252Cf and D2Omoderated 252Cf neutron sources measured withBonner spheres with 6LiI(Eu) and TLDs pairswere in good agreement [30].

From measurements with bare TLD pairs and6LiI thermal detectors using bare and D2Omoderated 252Cf neutron sources no differencewas observed in detectors output, therefore poly-ethylene plug, buit to hold TLDs, do not produceany neutron scattering effect different that 6LiIscintillator lucite light pipes.

From measurements to determine TLD600 andTLD700 response to 137Cs and 60Co g-rays afactor k ¼ 0:80� 0:06 was measured. This factorallows us to use Eq. (3) to obtain net neutronsignal from TLD600 and TLD700 readings.

Measured neutron spectra produced during(d, C) reaction at sites 1–3 are shown in Fig. 4,their uncertainities are 1.8, 8.6 and 2.1%, respec-tively. Here spectrum at site 1 shows a peak from0.2 to 7MeV with maximum at approximately2MeV, thermal and epithermal neutrons are verysmall. As measurement site moved to site 2, peak isshifted to energy range 0.1–3.7MeV with max-imum at approximately 0.45MeV, here a relativelylarger thermal and epithermal neutrons show up incomparision with site 1. At site 3 thermal neutronshave largest peak, meanwhile hard peak shiftsfrom 0.05 to 1.9MeV with maximum at approxi-mately 0.2MeV. This result is consistent withneutron moderation physics, harder spectra wereobserved near to sites where neutrons are pro-duced, site 1 and 2, meanwhile neutrons reachingsite 3 are strongly thermalized due to neutroncollisions with vault room environment.

Fig. 5 shows neutron spectra measured at site 10

and 2 during (d, 14N) reaction, their uncertainitiesare 4.3 and 3.1%, respectively. In both sites there

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386 381

is a neutron peak from 0.2 to 7MeV withmaximum at approximately 1MeV. These spectraare consistent with the fact that larger amount ofneutrons are produced at target site, and thosemeasured at site 2 are the contribution of neutronsproduced at site 10 that are transported to site 2plus those produced when deuterons collide withdeflector. Comparing neutron spectra from deu-terons colliding with Faraday cup at site 1, anddeuterons colliding with 14N at site 10, shows largeramount of thermal neutrons. The probable ex-planation is that 14N target is closer to vault roomwall than Faraday cup, then BSS detects thosewall returned thermal and epithermal neutrons.

Neutron spectra, measured at site 1, producedduring (p, C) nuclear reaction is show in Fig. 6, ithas an uncertainity of 6.4%. This spectra presentsa peak between 0.1 and 3.6MeV, with maximumat approximately 0.45MeV, these neutrons re-quires less collisions to thermalize, this could bethe explanation to the presence of a large amountof thermal neutrons, that is not observed at thesame site for reactions with deuterons. Tocompare MCNP calculated spectra, with spectra

Fig. 4. Neutron spectra produced during deuterons interacting

with Faraday cup at three locations inside PET vault room.

Uncertainities are 1.8% for spectrum at site 1, 8.6% for

spectrum at site 2 and 2.1% for spectrum at site 3.

Fig. 5. Neutron spectra produced during deuterons interacting

with 14N target. Measurements were carried out at site 10 and

site 2 inside PET vault room. Uncertainities are 4.3% for

spectrum at site 10 and 3.1% for spectrum at site 2.

Fig. 6. Neutron spectrum measured at site 1 inside PET vault

room, when produced when protons collide with Faraday cup.

Uncertainity is 6.4%.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386382

measured with BSS with TLDs, spectra werenormalized. For deuterons the best results wereobtained when Maxwellian ðT¼ 5MeVÞ sourceterm was used.

In Fig. 7 experimental and calculated normal-ized neutron spectra are shown. Calculated spec-trum was tallied at site 1 using MaxwellianðT¼ 5MeVÞ source term. Each energy bin has anerror less than 1.6%, only the bin with the largestenergy has an error of 2.5%. Experimental spectraare from neutrons produced during deuteronscolliding with 14N target, measured at site 10, anddeuterons impinging on Faraday cup and mea-sured at site 1. In this figure MCNP spectrum donot resembles any experimental spectra. MCNPspectra is softer than experimental ones, peak isbroader, however thermal contribution is close tothermal neutrons measured at site 10 during(d,14N) nuclear reaction.

Calculated normalized spectra tallied at site 2,using Maxwellian ðT¼ 5MeVÞ located at Faraday

cup and deflector is shown in Fig. 8. In this figureexperimental spectra measured at site 2, forneutrons produced for deuterons colliding with14N and Faraday cup. Here, MCNP calculation isclose to both experimental spectra, althoughMCNP spectrum is sligthy softer and peak isbroader. Each energy bin, but the last one, has anerror less than 2%, the last energy bin has an errorof 4.8%.

During calculations at site 3 all neutron sourceterms we used gives approximately the sameresults. In the energy range 1–20MeV, differenceswere observed only when monoenergetic sources(8.27 and 13.92MeV) were used as source term.Measured neutron spectra at site 3, produced bydeuterons impinging on Faraday cup is shown inFig. 9 with calculated neutron spectra, tallied atsite 3, using Maxwellian ðT¼ 5MeVÞ source termlocated at Faraday cup and deflector. Thisspectrum has an error less than 3% in all theenergy bins except the last one that has an error of

Fig. 7. Normalized, calculated and experimental, neutron

spectra at sites 1 and 10 for deuterons on Faraday cup and14N. Calculated is at site 1, using Maxwellian source term

located at Faraday cup and deflector. Calculated spectrum

error is 52.5%.

Fig. 8. Normalized, calculated and experimental, neutron

spectra at site 2 for deuterons on Faraday cup and 14N.

Calculated is at site 2, using Maxwellian source term located at

Faraday cup. Calculated spectrum error is 55%.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386 383

4.3%. MCNP and measured spectra are practi-cally the same, meaning that calculated spectrum isthe same regardless the source term used duringcalculations, except with monoenergetic source,where a peak at the upper energy section isexpected.

Normalized neutron spectra, measured andcalculated, at site 1 produced by protons interact-ing with Faraday cup is shown in Fig. 10.Calculated spectrum was tallied at site 1 and usingthe source term derived through Alsmiller’sformalism, this has an error less than 1.4% ineach energy bin. Meanwhile experimental spec-trum has an uncertainity of 6.4%. Both spectrahave the same peak features at the energy upperlevel section, but amplitude, the same difference isobserved in the thermal neutron section. Never-theless differences, one may conclude that sourceterm calculated through Alsmiller formalism givesacceptable results facing the fact of lack of bettersource term.

4. Conclusions

PET cyclotron is used to produce radioisotopesto perform a non-invasive imaging technique withmedical purpose. PET isotopes have short half-lives and they are produced in situ at healthcenters. During isotopes productions undesirableneutron field is produced making neutrons themain shielding problem to deal with.

Neutron shielding and health physics programdesign, require to know neutron energy spectrumas detailed as possible, neutron spectrum measure-ment is not trivial task. To measure neutronspectra inside a PET vault room BSS was modifiedchanging original active thermal neutron detectorby a passive thermal neutron detector using pairedTLD technique. A factor that equalizes TLD600and TLD700 g-ray responses was obtained forphotons produced by 137Cs and 60Co sources.Polyethylene plug used to hold TLDs at the centerof Bonner spheres do not produce any neutronscattering effect different from the original 6LiIplug array. BSS with TLDs was calibrated using a

Fig. 9. Normalized, calculated and experimental, neutron

spectra at site 3, produced when deuterons collide with Faraday

cup. Calculated was using Maxwellian source term located at

Faraday cup. Calculated spectrum error is 55%.

Fig. 10. Normalized, calculated and experimental, neutron

spectra at site 1 for protons on Faraday cup. Calculated

spectrum was using Alsmiller source term located at Faraday

cup. Calculated spectrum error is 1.4%.

H.R. Vega-Carrillo / Nuclear Instruments and Methods in Physics Research A 463 (2001) 375–386384

bare 252Cf and a D2O moderated 252Cf neutronsources, from this procedure a calibration factorwas obtained that correlates BSS with 6LiI withBSS with TLDs results.

Nuclear reactions with 17.2MeV protons and8.6MeV deuterons colliding with Faraday cup,electrostatic deflector and 14N were used. Pro-duced neutron fields were measured at three siteswith BSS with TLDs. Spectra were unfolded usingBUNKIUT code and UTA4 response matrix.Monte Carlo calculations were carried out, usingMCNP code, to determine neutron spectra pro-duced for both charged particles beam. The lack ofa source term for protons, a source term wasderived using Alsmiller formalism, and for deuter-ons several source terms were used.

Obtained results allow us to conclude: BSS withTLDs can be used to measure neutron energyspectra at locations were neutron field is strong,highly contaminated with g-rays or is pulsed. Withproper calibration factor unfolding can be doneusing response matrix for scintillators. Source termfor (d, C) reaction calculated with Alsmillerformalism gives acceptable results during calcula-tions. None of source terms used in the aim tocalculate neutron spectra produced by (d, 14N), atsite 10 or produced by (d, C) at site 1 reproduceexperimental spectra. However this source termcan be used to calculate neutron spectra measuredfor both pervious nuclear reactions at site 2. Fromcalculations at site 3, using Maxwellian ðT¼ 5MeVÞor Evaporation ða¼ 5MeVÞ source term, resulting(d, C) neutron spectra reproduce well the mea-sured spectrum regardless source term. This is alsovalid, from thermal to approximately 7MeV,when monoenergetic neutron source term wereused (8.6 and 13.97MeV).

Measurements and MCNP calculated spectraindicate a systematic behavior: intense peak in theenergy interval 0.3–4MeV, with a maximum atapproximately 1MeV indicating the presence ofevaporation neutrons produced during nuclearreactions, regardless nuclear reaction, this isconsistent with other facilities measurements [10,31–36]. Neutrons above 2MeV are scattered below1MeV by inelastic scattering with the massiveamount of iron in cyclotron magnets. Inelasticscattering threshold of major isotope in iron, 56Fe,

is around 850 keV. Neutrons below 2MeV areelasticaly scatterd with a smaller energy loss percollision, thus Cyclotron iron is the dominanteffect in determining the energy upper sectionfeatures of neutron field inside vault room.

Acknowledgements

This work was partially supported by CON-ACyT (Mexico). Help, support and assistancefrom faculty and staff of Nuclear EngineeringTeaching Laboratory of The University of Texasat Austin, and The University of Texas HealthScience Center at San Antonio is highly appre-ciated.

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