1 Introduction General Information on Technetium
3
as compared with the age of the Earth (4.5 · 10
9 y (Zhang 2002)) and all primordial
99 Tc incorporated into the Earth crust at the time of its formation is expected
to have completely decayed since then. However, reliable measurements indicate
the presence of nonman-made
99 Tc in Earth materials today. Traces of natural technetium were found in African pitchblende (Kenna 1964). The concentration of its
long-lived isotope,
99 Tc, was determined at the level of (2.5–3.1) · 10
− 10 g per kg of
the uranium ore. Clearly, naturally occurring
99 Tc present today in the Earth’s crust
must be generated in other than stellar nucleosynthesis processes, e.g., by spontaneous fission (SF) of naturally occurring uranium. The half-life of this process is
equal to 1.5 · 10
16 y, 1.0 · 10
19 y and 8.2 · 10
15 y for
234 U,
235 U and
238 U, respectively
(Holden and Hoffman 2000). The SF taking place in 1 kg of natural metallic uranium
leads to a low but measurable emission of 13.5 neutrons per second (Reilly 1991).
It is reasonable to assume that the naturally occurring terrestrial
99 Tc is produced
mainly by spontaneous fission of
238 U. Knowing the SF cross-section of uranium
and the probability of Tc generation in this process, one may calculate the amount of
99 Tc nuclei produced as a result of spontaneous fission of a given amount of uranium,
e.g., present in a uranium-rich ore (Kuroda and Menos 1961; Kenna 1962; Parker and
Kuroda 1958). Thus, one may use Eq. (1.1). to calculate the amount of
99 Tc present
in the sample. N U−238 , N Tc−99 are the numbers of
238 U and
99 Tc nuclei, respectively.
λ U−238(fission) is the spontaneous fission decay constant of
238 U (equal to 2.8·10
−24
s
−1 ) and λ Tc−99 is the decay constant of
99 Tc (1.04 · 10
−13 s
−1 ). y Tc−99 denotes a
fission yield of
99 Tc and is equal to 6.12% (Katakura 2012, Katakura et al. 2016
Nuclear Data Center).
y Tc−99 N U−238 λ U−238(fission) = N Tc−99 λ Tc−99
(1.1)
Using Eq. (1.1) and the estimated uranium content in the oceanic waters of the
entire globe (Seko et al. 2003), which is equal to approximately 4.5 billion tons, one
may calculate that the Earth oceans contain a few kg of
99 Tc of natural origin.
More than 30 isotopes of Tc are known today (see e.g., Baum 2010; Nystrom
and Thoennessen 2012). All of them are radioactive with half-lives ranging from
milliseconds (e.g.,
112 Tc) to millions of years (
98 Tc). It must be stressed that the halflife of some of Tc isotopes has not yet been determined with acceptable reliability.
The history of the discovery of known technetium isotopes was reviewed by Nystrom
and Thoennessen (2012). The instability of the technetium nucleus was predicted by
isobar rule formulated in 1934 by Josef Mattauch. It states that at least one of isobars
of adjacent elements, e.g.,
98 Mo and
98 Tc, must be unstable. The stable isotopes
of the Tc neighbor with atomic number 44, i.e., ruthenium, are those with mass
numbers of 96, 98–102 and 104 (Baum et al. 2010), while for another Tc neighbor,
molybdenum (element 42), the stability is exhibited by the isotopes 92, 94–98 and
100. Therefore, the technetium isotopes with mass numbers from 94 to 102 are
not stable (Johnstone 2017). The instability of the technetium nuclei is related to
their nuclear shell structures (Mayer et al. 1951). The decay data for selected Tc
radionuclides are summarized in Table 1.1.
3
as compared with the age of the Earth (4.5 · 10
9 y (Zhang 2002)) and all primordial
99 Tc incorporated into the Earth crust at the time of its formation is expected
to have completely decayed since then. However, reliable measurements indicate
the presence of nonman-made
99 Tc in Earth materials today. Traces of natural technetium were found in African pitchblende (Kenna 1964). The concentration of its
long-lived isotope,
99 Tc, was determined at the level of (2.5–3.1) · 10
− 10 g per kg of
the uranium ore. Clearly, naturally occurring
99 Tc present today in the Earth’s crust
must be generated in other than stellar nucleosynthesis processes, e.g., by spontaneous fission (SF) of naturally occurring uranium. The half-life of this process is
equal to 1.5 · 10
16 y, 1.0 · 10
19 y and 8.2 · 10
15 y for
234 U,
235 U and
238 U, respectively
(Holden and Hoffman 2000). The SF taking place in 1 kg of natural metallic uranium
leads to a low but measurable emission of 13.5 neutrons per second (Reilly 1991).
It is reasonable to assume that the naturally occurring terrestrial
99 Tc is produced
mainly by spontaneous fission of
238 U. Knowing the SF cross-section of uranium
and the probability of Tc generation in this process, one may calculate the amount of
99 Tc nuclei produced as a result of spontaneous fission of a given amount of uranium,
e.g., present in a uranium-rich ore (Kuroda and Menos 1961; Kenna 1962; Parker and
Kuroda 1958). Thus, one may use Eq. (1.1). to calculate the amount of
99 Tc present
in the sample. N U−238 , N Tc−99 are the numbers of
238 U and
99 Tc nuclei, respectively.
λ U−238(fission) is the spontaneous fission decay constant of
238 U (equal to 2.8·10
−24
s
−1 ) and λ Tc−99 is the decay constant of
99 Tc (1.04 · 10
−13 s
−1 ). y Tc−99 denotes a
fission yield of
99 Tc and is equal to 6.12% (Katakura 2012, Katakura et al. 2016
Nuclear Data Center).
y Tc−99 N U−238 λ U−238(fission) = N Tc−99 λ Tc−99
(1.1)
Using Eq. (1.1) and the estimated uranium content in the oceanic waters of the
entire globe (Seko et al. 2003), which is equal to approximately 4.5 billion tons, one
may calculate that the Earth oceans contain a few kg of
99 Tc of natural origin.
More than 30 isotopes of Tc are known today (see e.g., Baum 2010; Nystrom
and Thoennessen 2012). All of them are radioactive with half-lives ranging from
milliseconds (e.g.,
112 Tc) to millions of years (
98 Tc). It must be stressed that the halflife of some of Tc isotopes has not yet been determined with acceptable reliability.
The history of the discovery of known technetium isotopes was reviewed by Nystrom
and Thoennessen (2012). The instability of the technetium nucleus was predicted by
isobar rule formulated in 1934 by Josef Mattauch. It states that at least one of isobars
of adjacent elements, e.g.,
98 Mo and
98 Tc, must be unstable. The stable isotopes
of the Tc neighbor with atomic number 44, i.e., ruthenium, are those with mass
numbers of 96, 98–102 and 104 (Baum et al. 2010), while for another Tc neighbor,
molybdenum (element 42), the stability is exhibited by the isotopes 92, 94–98 and
100. Therefore, the technetium isotopes with mass numbers from 94 to 102 are
not stable (Johnstone 2017). The instability of the technetium nuclei is related to
their nuclear shell structures (Mayer et al. 1951). The decay data for selected Tc
radionuclides are summarized in Table 1.1.