Study of the neutron field around a PET cyclotron - IRPA

IRPA11, Madrid, 23-28 May, 2004 2004 Méndez et al., Neutron field around PET cyclotron
Study of the neutron field around a PET cyclotron
R. Méndez 1 , M.P. Iñiguez 1 , H.R. Vega-Carrillo 2 , J.M. Martí-Climent 3 , I. Peñuelas 3 and R.
Barquero 4
1 Depto. de Fisica Teórica y Atómica, Molecular y Nuclear, Facultad de Ciencias,
Universidad de Valladolid, 47011 Valladolid, Spain. E-mail: piluca@fcualab.fam.cie.uva.es
2 Unidades Académicas: Estudios Nucleares, Ingeniería Eléctrica y Matemáticas de la Universidad
Autónoma de Zacatecas, Apdo. Postal 336, 98000 Zacatecas, Zac. Mexico.
3 Clínica Universitaria de Navarra, Spain
4 Hospital Río Hortega, Valladolid, Spain
Abstract. In this work the neutron field around a cyclotron during 18 F production is studied by Monte
Carlo calculations and thermoluminescent measurements. Considered (p, n) nuclear reaction occurs with 18
MeV protons in a Cyclone 18/9 from IBA (Ion Beam Applications) cyclotron placed at Clínica
Universitaria de Navarra. The neutron source intensity was experimentally determined by TL
measurements while its energy distribution, with a mean energy of 2.5 MeV, was taken from theoretical
estimations. The MCNP 4C Monte Carlo code was used to obtain the neutron spectrum at four points
around the cyclotron. During cyclotron modeling, coils, magnets and electromagnetic shielding were
included, while bunker walls were included for modeling the vault room. For measurements, 6 LiF:Mg,Ti
thermoluminescent chips were located at the same points where calculations were carried out. During
experiment 30 microAmperes of 18 MeV protons were utilized to bombard a H
18
2
O target. Thermal
neutron fluence rate was obtained through the difference between the high temperature peak intensities in
the glow curves of bare and Cd covered TLD600 chips. The obtained value of 3.57E11 n/s for the neutron
source intensity agrees with the IAEA recommended physical yield of 1171 GBq/Coulomb for the 18 F
production reaction. From the neutron spectra features, ambient doses equivalent from some tenths to some
hundreths of mSv/microA-hour were derived resulting in agreement with Berthold LB6411 monitor
measurements. A discussion is made about the neutrons coming from other 18 O(p,x) neutron producing
reactions different to the 18 F production.
1

IRPA11, Madrid, 23-28 May, 2004 2004 Méndez et al., Neutron field around PET cyclotron
1.Introduction
In neutron capture therapy neutrons are usefull therapeutical particles whose cytotoxicity is caused by a
nuclear reaction between 10 B and thermal neutrons. This technique is used in treating brain tumors [1] and
it has been proposed to treat rheumatoid arthritis [2] . In both cases the cells to be eliminated are artificially
loaded with a 10 B-enriched compound and subsequently are irradiated with low energy neutrons.
By the contrary, in other medical applications neutrons arise as undesired products of nuclear reactions
occuring inside the facility. This is the case of linear electron accelerators where neutrons reach the
patients [3] and of cyclotrons for positron emission tomography imaging (PET) where neutrons constitute a
radiation protection hazard [4 - 6] .
During the last years PET technique has expanded in several countries where radionuclide imaging has
become a standard method of non-invasive exploration. [1] The fluorodeoxyglucose, a PET widely use
compound, is labelled with the 18 F radioisotope that is formed during the 18 O(p, n) 18 F nuclear reaction
where 18 O enriched water is bombarded with 18 MeV protons, producing a neutron field. [6]
A gamma-ray field is also produced during the decay and de-excitation of nuclides generated along the 18 F
producing reactions and from the neutron induced nuclear reactions with the vault room walls and
cyclotron materials. This mixed radiation field becomes an important issue of radiation shielding design. [4,
6]
Neutron field features, such as energy and angular distributions, have been experimentaly determined for
proton energies up to 10 MeV. [7] For larger proton energies the information required to validate the
cyclotron shielding, to estimate the concrete activation and to review the radiation protection protocols is
based in theoretical cross section data and computer codes.
[5, 8]
A physical yield, expressed in terms of activity per electric charge, is defined as the ratio between the
number of produced 18 F atoms and the amount of protons reaching the target. The yield increases with the
beam energy, and for 18 MeV protons its value is 1171 GBq/C. [9] With a half life of 110 min the 18 F
production rate is 1.115 x 10 10 s -1 -µA -1 producing a strong neutron radiation field that, inside the vault
room and around the cyclotron, means large neutron doses. Mostly of the neutron fields encountered in
radiation protection practice have a broad energy distribution, knowledge of the neutron spectrum is
necessary for assessing the radiation protection conditions around these facilities. [10]
In this investigation the neutron spectra were simulated at four locations inside the vault room of a PET
cyclotron by means of Monte Carlo methods. Concerned neutrons are mainly produced by evaporation
during the 18 O(p, n) 18 F nuclear reaction having a peak close to 2 MeV. These emitted neutrons interact
with the cyclotron materials and the vault room walls producing neutron spectra whose features depend
upon the site, being harder close to the target and softer as the site is away the target . The thermal portion
of the spectrum was measured with LiF:Mg,Ti dosemeters, and from these measurements the source
intensity was obtained, after comparisson with the simulated spectra.
2.Materials and Methods
Facility description
The cyclotron is a negative ion accelerator made by Ion Beam Application Radioisotopes model Cyclone
18/9 located at the Navarra University Hospital. It is housed in a 480 x 400 x 363-cm 3 vault room shielded
by concrete walls of up 200 cm thick. [5, 6] The cyclotron is operated during some minutes at a proton
beam current of 30 µA for the TLD's irradiations.
Thermoluminescence
Ribbon type thermoluminescent LiF:Mg,Ti dosemeters 0.3 x 0.3 x 0.1 cm 3 from Harshaw were used. They
were covered with a Cd foil to determine the thermal neutron fluence rates at four sites inside the vault
room. Those are labelled A, B, C and D, and are at 100, 100, 176 and 50 cm from the cyclotron surface,
and with an orientation relative to the target: at 90º, 180º, 0º and 0º. TLD600 contains 95.6% of 6 Li that
allows to detect thermal neutrons while TLD700 has 99.9% of 7 Li that is insensitive to thermal neutrons.
The two TLD's have the same response to gamma rays of not as much energy as to produce photonuclear
2

IRPA11, Madrid, 23-28 May, 2004 2004 Méndez et al., Neutron field around PET cyclotron
reactions. [11] These, were calibrated in the neutron field produced by a water-moderated 1.11 x 10 11 Bq
AmBe source. During the TLDs reading they were preheated to 100 o C and heated up to 350 o C at a rate of
10 o C/s. The area under the high temperature peak (~ 270 o C) of the glow curves was used to determine the
neutron fluence rate. During thermal neutron irradiations TLDs were located in small methacrylate boxes,
one covered with the Cd foil and other without it.
Simulations
Neutron spectra and neutron doses were calculated at the four sites above mentioned. Vault room and
cyclotron were modeled and neutrons transported using the Monte Carlo code MCNP 4C. [12] The "Des",
Cu coils, iron magnets and electromagnetic shielding were explicitly included. Because its complexity
details about the target were omitted, instead a point-like source was located at 1 cm-depth in the cyclotron
upper section. The source term was taken form Carroll [8] that is similar to an evaporation model given by,
Q(E) = 0.59 E 0.45 Exp[ - E/2.7] (1)
here, E is the neutron energy in MeV. In Fig. 1 the evaporation and Carroll´s models are shown together.
Figure 1. Initial spectrum in the simulations
The number of histories utilized during calculations was 10 6 , this was large enough to have uncertainties
less than 1% in the integral parameters and less than 3% in the neutron spectra energy bins.
Remmeter
An active probe Berthold LB6411 with a 3He thermal detector inside a 25 cm diameter polyethylene sphere
was utilized.
3.Results and discussion
The area under the high temperature peak in the glow curve of TLD600 contains the information of any
energy neutrons, while the respective area under Cd-covered TLD600 only contains information about
neutrons whose energy is larger than approximately 0.4 eV. From the difference of bare and Cd-cover high
3

IRPA11, Madrid, 23-28 May, 2004 2004 Méndez et al., Neutron field around PET cyclotron
temperature area peak the thermal neutron fluence rate, E < 0.4 eV, can be obtained, after calibration of the
TLD’s in the moderated AmBe source. Resulting values are shown in Table 1.
Site Bare TLD600
coulombs
Cd covered TLD600
coulombs
A 1404 277
B 997 188
C 3270 763
D 4070 1433
Table 1. TLD readings and corresponding thermal neutron fluences.
Thermal neutron Fluence
n cm -2 s -1
7.13 . 10 5
5.12 . 10 5
1.59 . 10 6
1.67 . 10 6
On the other hand, the simulated thermal neutron fluence rates for a unitary source, C th , are given in Table
2. Also is given the percentual contribution of thermal neutrons % respect to the total energy range.
Site C th n cm -2 s -1
A 2.55 . 10 -6
B 1.84 . 10 -6
C 3.98 . 10 -6
D 3.52 . 10 -6
%
36.1
45.9
16.0
4.12
Table 2. Simulated unitary thermal fluences.
From it one sees that the proportion of low energy thermal neutrons varies strongly with the distance to the
target. The neutron production strength was derived by averaging the results at the four points.
From the comparisson between Tables 1 and 2 a source intensity can be derived, which after being
averaged for the four considered points results in a value of 3.57. 10 11 n/s. Taking into account the proton
courrent used of 30 µA, the corresponding IAEA recomended value would be 3.345. 10 11 n/s which is very
similar. Resulting neutron dose rates are in good agreement with the Berthold LB6411 readings which are
21, 7 and 210 mSv/µA h at sites A, B and C, respectively, and out of range at site D.
4.Final conclusions
The neutron fluence rate around a PET cyclotron, DURING 18 F PRODUCTION, has been determined through
Monte Carlo calculations and thermoluminescence measurements.
An agreement between the obtained neutron intensity at the target and the IAEA recommended value was
found from the thermal neutron fluence measurements along with the spectrum simulations. If initial
energy spectrum and other ingredients assumed in the simulations are accurate enough, that agreement
reveals that neutrons coming from other nuclear reactions different from just that producing 18 F could be
neglected. An experimental measurement of the neutron spectrum is in progress.
Acknowledgements
Authors thanks the colaboration with Universidad de Cantabria, Universidad Politécnica and CIEMAT,
Madrid, and the Hospital Arturo Eyries of Valladolid.
5.References
1. Cerullo, N., Daquino, G.G., Muzi, L. and Esposito, J. Development of a treatment planning system
for BNCT based on positron emission tomographydata: preliminar results. Nuclear Instruments and
Methods in Physics Research B 213(1) (2004): 637-640.
2. Vega-Carrillo, H.R. and Manzanares-Acuña, E. Neutron source design for boron neutron caputre
synovectomy. Alasbimn Journal 6(22), (2003): 1-9.
3. Barquero, R., Méndez, R., Iñiguez, M.P:, Vega-Carrillo, H.R. and Voytchev, M.
Thermoluminescence measurements of neutron dose around a medical linac. Radiation Protection
4

IRPA11, Madrid, 23-28 May, 2004 2004 Méndez et al., Neutron field around PET cyclotron
Dosimetry 101(1-4), (2002): 493-496.
4. Vega-Carrillo, H.R.. Neutron energy spectra inside a PET cyclotron vault room. Nucl. Instrum.
Meth. Phys. Res. A 463 (2001): 375-386.
5. Martí-Climent, J.M., Peñuelas, I. and Richter, J.A. Radiation protection in a clinical PET center.
Eur. J. Nucl. Med. 23 (1996) 1245.
6. Martí-Climent J.M., Méndez, R., Peñuelas, I., Iñiguez, M.P., Serra, P., Barquero, R., Vega-
Carrillo, H.R. Neutron dosimetry around a cyclotron during positron emission radionuclide production. J
Nucl Med 44(5), (2003): 327p-328p
7. Carroll, L.R. Estimating the radiation source term for PET isotope targets. Ninth International
Workshop on Targetry and Target Chemistry. Turku Finland (2002)
8. Carroll, L.R.. Predicting long-lived induced activation of concrete in a cyclotron vault. AIP
Conference Proccedings 576(1), (2001): 301-304.
9. IAEA. Charged particle cross-section database for medical radioisotope production: Diagnostic
radioisotopes and monitor reactions. International Atomic Energy Agency Technical document 1211.
(2001)
10. ICRU. Determination of operational dose equivalent quantities for neutrons, International
Commision on Radiation Units and Measurements Report 66, Journal of the ICRU 1(3), (2001): 27-33.
11. Méndez, R., Iñiguez, M.P., Barquero, R., Mañanes, A., Gallego, E., Lorente, A. and Voytchev, M.
Response components of LiF:Mg,Ti around a moderated Am-Be neutron source, Radiation Protection
Dosimetry 98(2), (2002): 173-178.
12. Briesmeister, J.F. (editor) MCNP TM -A general Monte Carlo N-particle transport code. Los
Alamos National Laboratory report LA13709-M, (2000).
5