Californium-252 as a Neutron Source for BNCT
Californium-252 sources are used in several cancer institutions for clinical interstitial or intracavitary neutron brachytherapy. The dose deposited from the fast neutron and gamma irradiation components from a Cf-252 source is the main therapeutic agent
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Californium-252 as a Neutron Source for BNCT Albert Miller
Contents 5.1 Physical Properties of Californium-252 .....................................................................
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5.2 Californium-252 Medical Sources ..............................................................................
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5.3 Dosimetric Properties of Cf-252 Sources ...................................................................
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5.4 Clinical Applications of Cf-252 Sources.....................................................................
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References ..............................................................................................................................
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5.1
Physical Properties of Californium-252
In November 1952, the isotope Cf-252 was discovered in the debris from the MIKE thermonuclear test at Enewetak [1]. Early investigations of its properties indicated a half-life of between 2 and 3 years (actually 2.645 years) and significant branching fractions for decay by spontaneous fission (SF), making Cf-252 an especially good and compact source of neutrons. Due to its availability in macroscopic quantities, Cf-252 has been one of the most extensively studied transplutonium isotopes. Most of the effort has been directed at understanding the spontaneous fission properties, some of which are summarised in Table 5.1. These properties make Cf-252 one of the most useful neutron emitters out of all the ~3,000 known radionuclides. Though isotopes such as Cf-254 and Md-260 have higher rates of spontaneous fission, their half-lives are too short, i.e. weeks, to permit large-scale fabrication. The majority,
A. Miller Department of Radiation Medical Physics, Institute of Oncology, Vilnius University, Santariskiu-1, 06880 Vilnius, Lithuania e-mail: [email protected] W.A.G. Sauerwein et al. (eds.), Neutron Capture Therapy, DOI 10.1007/978-3-642-31334-9_5, © Springer-Verlag Berlin Heidelberg 2012
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Table 5.1 Decay and spontaneous fission properties of Californium 252 [3–5] Half-life 2.645 years Specific activity 536.3 Ci/g Decay mode a (96.908 %), SF (3.092 %) Neutron multiplicity 3.768 n/fission Mean fission neutron spectrum energy 2.13–2.15 MeV ~10/fission Prompt g-ray multiplicity (mean) 0.7–0.9 MeV Average prompt g-ray energy 80 70
× 1010neutrons/ µg•s
60 50 40 30 20 10 0 0
5
10
15
Neutron energy (MeV)
Fig. 5.1 Neutron energy spectrum from spontaneous fission of Cf-252 (total = 2.31 × 1012 neutrons/mg·s)
96.9 %, of Cf-252 decays are through alpha decay, but due to the nature of encapsulation, these He-4 nuclei do not escape the confines of the source. A small 3.092 % but significant proportion of Cf-252 decays via spontaneous fission which produces fission fragments, as well as a neutron yield of 3.768 n/fission (2.31434 × 1012 neutrons/s/g of Cf-252). The Cf-252 neutron energy spectrum is shown in Fig. 5.1. These neutrons have an energy spectrum which can be modelled as either a Maxwellian or a Watt fission spectrum. The National Bureau of Standards (NBS) evaluated this
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