Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 253-259, 200599

Tc in the Environment: Sources, Distribution and Methods
Manuel García-León*
Facultad de Física Universidad de Sevilla Apdo. 1065, 41080 Sevilla Spain
Received: July 31, 2005; In Final Form: September 30, 2005
Tc is a β-emitter, Emax = 294 keV, with a very long half-life (T1/2 = 2.11 × 105 y). It is mainly produced in the fission of 235U and 239Pu at a rate of about 6%. This rate together with its long half-life makes it a significant nuclide
in the whole nuclear fuel cycle, from which it can be introduced into the environment at different rates dependingon the cycle step. A gross estimation shows that adding all the possible sources, at least 2000 TBq had beenreleased into the environment up to 2000 and that up to the middle of the nineties of the last century some 64000
TBq had been produced worldwide. Nuclear explosions have liberated some 160 TBq into the environment. In
this work, environmental distribution of 99Tc as well as the methods for its determination will be discussed.
Emphasis is put on the environmental relevance of 99Tc, mainly with regard to the future committed radiationdose received by the population and to the problem of nuclear waste management. Its determination at environmental levels is a challenging task. For that, special mention is made about the mass spectrometric methods for
its measurement.
99

on the topic of environmental 99Tc, as for instance that presented by Prof. K. Yoshihara.4

1. Introduction
The discovery of element 43 was announced at the beginning of the 20th century after the observation by Noddack andTracke1 of an unknown radioactive isotope in uranium ores.
Results were not reproduced in further experiments and thescientific community did not accept this finding.
Nowadays it is fully accepted that the element Technetium(Z = 43) was discovered by Perrier and Segré in 1937.2 It was
produced by irradiating for some months a molybdenum target
with deuterons at the Berkeley cyclotron.
Ten years later, Paneth3 suggested the name of technetium,
after the Greek word τεχνικοζ (artificial), for the new element
due to the fact that it was the first chemical element artificially
produced.
So far 24 isotopes of the element are known with 9 nuclear
isomers. All of them are radioactive with half-lives ranging
from a few milliseconds to a few million years.
From an environmental viewpoint, only 99Tc seems to be relevant because of its high production rate in the fission of 235U
and 239Pu, around 6%, similar to that of 137Cs, and its nuclear
characteristics. Indeed, it is a β emitter with Emax = 294 keV
and T1/2 = 2.11 × 105 y. Such a long half-life assures its presence
in nature once released from a given source. Therefore, it isconsidered to be an important contributor to the future collective dose to the population due to the use of nuclear energy,
playing an important role in the problem of nuclear waste management.
On the other hand, due to its chemical characteristics 99Tc
can be used to trace the evolution of natural compartments. For
instance, 99Tc conservatively behaves in water. This is the reason why it is a potential tracer of marine water dynamics orother water masses movement.
In what follows a comprehensive account on the sources,
distribution and behavior of 99Tc in the environment will begiven. The methodology for its measurement in environmental
samples will also be reviewed.
Of course I will feel satisfied if this paper could be considered as a good complementary reading of other excellent views*Corresponding author. E-mail: manugar@us.es. FAX: +3495-4460145.

2. Sources
At the beginning of Tc research, scientists did not believe that
Tc was present in nature. In fact, this was an important reason
why they called technetium. Nowadays, it is well known that it
can be found in nature as produced by natural means.5 Indeed,
it was 30 years after its discovery that Kenna and Kuroda6 wereable to detect it in pitchblende samples. And on the basis of the99
Tc/ 238U isotopic ratio, they concluded that 99 Tc was mainly
produced by the spontaneous fission of 238U, although the neutron-induced fission of 235U could be a minor way of production. As an example, it can be said that 99 Tc/238U ratio of around10−12 is found in U-mineral samples5 which gives an idea on theexperimental difficulty for the characterization of this production way. These findings are scientifically relevant. However,the production of 99 Tc by the spontaneous fission of 238U is anegligible source of 99 Tc in the modern environment.
The most important way of 99 Tc production and, consequently, the main source in the modern environment is thenuclear fission in nuclear weapons and reactors.
Estimations by Aarkrog et al.7 showed that about 140–160
TBq of 99Tc have been produced by nuclear weapon detonation.
This amount, although important, is at least one order of magnitude smaller than that produced by the nuclear industry. The
weapon pulse is nowadays masked by it and cannot be found inenvironmental samples with the probable exception of ice and,to a less extent, sediment cores.
99
Tc produced in reactors enters the nuclear fuel cycle andfrom the different steps of the whole cycle it is introduced into
the environment. It is calculated that the inventory in PWR fuels
is about 9 kg of 99 Tc per 1 GW(e) year. Thus, it is estimated that,up to 1983, around 15000 TBq of 99 Tc was produced in nuclear
power stations worldwide.8 From that date to 1993, the production of 99 Tc is estimated to be 49000 TBq.4 Calculations madeby F. Luykx in 19868 showed that up to such date about 1000
TBq of 99 Tc had been released into the environment mainly fromthe nuclear fuel reprocessing, since the release from nuclear
power stations is very low, roughly 4 GBq.9
However, according to Kershaw et al.10 only from the Sellafield© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

254 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

García-León

99

plant, about 950 TBq of Tc was released into the environment
from 1978 to 1998. The radioactivity release trends have beenvery different at Sellafield and La Hague nuclear reprocessing
plants. From 1982 to 1991, 102 TBq was discharged from La
Hague, while 40.2 TBq was from Sellafield.11 This tendency
drastically changed after that the Enhanced Actinide Removal
Plant (EARP) began its operation in 1994 (see following sections for details).
It is not easy to estimate the total 99 Tc radioactivity actuallystored in nuclear power plants or classified as low to medium
radioactive wastes, especially bearing in mind that the whole99
Tc inventory related to the former Soviet Union countries is
not known. With the previous data a very rough calculationshows that a lower limit could be 60000 TBq.
Inputs of 99 Tc from the use of nuclear medicine 99Mo-99m Tc
generators should also be taken into account although its contribution on a global scale is clearly less important than theother sources. According to Reference 9, a medium-sized
Nuclear Medicine Service produces some 200 kBq/y in the
average. Thus, a gross calculation would give that this source
has contributed to the total inventory with a few GBq of 99 Tc
during the last 10 years.
The major accident of Chernobyl introduced 0.75 TBq into
the environment.12
Generally speaking, it can be concluded that the most intensesource to the environment is the reprocessing of nuclear fueland it is also apparent that the amount of stored 99 Tc in nuclear
wastes is at least one order of magnitude greater than theamount so far introduced in to the environment.
3. Distribution and Behavior
Sea Water. Due to its chemical characteristics, 99 Tc is known
to behave as a conservative radionuclide in marine and fresh
water under oxidizing conditions.13 Indeed, the most probable Tc
compound in water is TcO4− which is very stable and soluble,14
showing very low affinity to particulate matter and sediments.15
Since the main source of 99 Tc in the environment is associated to the release of radioactive wastes into the sea, many
works have been devoted to the determination of 99 Tc in marine
water. A pioneer work by Golchert and Sedlet,16 reported on
99
Tc concentration in water, of unknown origin, of about sometens of mBq per liter. I will refer later to this work, since theauthors present one of the first procedures for Tc measurement
in water appeared in the current literature.
More recent determination has been carried out in the vicinity of the European fuel reprocessing plant of Sellafield and La
Hague. For instance, Riley and Siddiqui17 found concentrationlevels ranging from 15 to 75 mBq/L in water samples from theNorth Sea. Parallel measurement in the English Channel coasts
by Patti et al.18 showed that concentrations lay between 5 and 21
mBq/L. Some 500 mBq/L has been measured by Eroglu et al.19
in water samples collected close to the location of Sellafieldrelease area in the Irish Sea. All these results show the influence of the reprocessing plant on the close environment.
Far from the plants, it is also possible to find their fingerprints
in the form of enhanced 99 Tc concentrations with regard to the
expected fallout level. Several factors make complex the relationship between the release from a reprocessing plant and theconcentration increase at a location far away from the release
point. Some of them are: 1) the amount of discharge, which is
not constant with time (see Figure 1), 2) the ocean conditions,which can vary with time, and 3) the existence of a certain timeand space dispersion of pollutants in the sea, which arrive to thecollection places as successive tides.
Taking into account these factors, a transit time from La
Hague to Kattegat was calculated13 to be in the range from 15
to 18 months. In the case of Sellafield, the transit to Northern
Norway takes about 40 months. The data have recently been

Figure 1. Taken from Reference 23. Technetium-99 activity concentration in Fucus serratus (primary axis) from Särdal (56.76 ˚N, 12.63 ˚E) in
Sweden, 1967–2000, and annual 99 Tc activity discharge from Sellafield(U.K.) and La Hague (France) reprocessing plants (secondary axis).

updated, as shown in Reference 20. These authors reported
levels of some 1 mBq/L from 1990 to 1992 in Norwegiancoastal waters. The concentration reached the value of up to 9
mBq /L for the period from 1996 to 1997, just after the
Enhanced Actinide Removal Plant (EARP) at Sellafield began
its operation in 1994.
The influence of EARP was also noted by Smith et al.21 who
measured the concentration of up to 60 mBq/L in seawaterfrom the open Irish Sea. This peak coincided with the maximum release of 99 Tc from Sellafield which happened around1997. Then in 1998, the authors observed a decrease in thecontamination till about 20 mBq/L.
The influence of Sellafield is also observed in the Greenland
coasts and, in the North Pole environment. Many literatures
are certainly relevant to these regions. As an example, I cantake the Reference 22 which provides the data on the concentration of 99 Tc, in Eastern Greenland water with special mention on the origin of the water masses. Polar water is found tocontain about 70 mBq/m3 of which the author estimates that 10
mBq/m3 are the weapon fallout contribution.
A recent long-term study on 99 Tc in the marine environmentof the Swedish coasts23 shows that the more important source
has been the liquid discharges from the reprocessing plant of
Sellafield. Levels in seawater of about 1 Bq/m 3 were found.
The authors also claim that 99 Tc is one of the best transit tracers
for the water circulation in the Baltic Sea.
Better analytical procedures allowed the determination of99
Tc in seawater that was not affected by intense radioactivity
sources. Momoshima et al.24 reported levels as low as 1 mBq/
m3 in the sea water taken in 1993 at Fukuoka, Kyushu, Japan.
Nevertheless, Hirano and Matsuba 25 were able to detect theactivity concentration as high as 200 mBq/m3 in water samples
collected from the Isozaki Coast, during the period from 1987
to 1990. The authors carefully analyze the data and conclude
that the observed peak can be attributed to the operation of the
nearby nuclear installations although the exact source could
not be identified.
Seaweed. The determination of 99 Tc in sea water is a challenging task. For that, in many cases, the use of bioindicators
has been the choice to determine 99 Tc in the marine environment. Brown seaweeds, mostly Fucus vesiculosus, serratus or
spiralis, have been the most popular bioindicators for 99Tc inmarine systems. The mechanisms of 99 Tc incorporation into
the seaweed are not precisely known. At the very beginning of

99

Tc in the Environment: Sources, Distribution and Methods

research, surface absorption was the suggested mechanism.
A good, although not the only, example on how good 99 Tc
concentrations in brown seaweed represent those in sea water
can be seen in Reference 18. The authors studied the presenceof 99 Tc in the marine environment close to La Hague plant.
The dependence on the distance from the plant release point isalmost identical for water and seaweed. Concentration valuesof the order of 104 L/kg were found for Fucus spiralis. Someauthors found a concentration of 105 L/kg for Fucus vesiculosus.9 Actually this figure significantly changes among different environment and cannot be taken as a fixed number. But
the potential of such algae species as a 99 Tc bioindicator is
obvious.
This finding can be traced back to the work by Dutton andIbbett.26 They studied the 99 Tc content in Fucus vesiculosus
collected in the Irish Sea. After these first studies, the use of
Fucus species as spatio-temporal indicators of the 99 Tc evolution in the marine environment became a common practice. In
particular, it has permitted to evaluate the impact of Sellafieldand La Hague on the North European marine environment,
including Greenland and Polar surroundings. Examples can beseen in the above cited works for sea water (see Figure 1). Of
course, the concentration in sea water can help to estimate thelevels in algae. It could be also said that these studies had provided with a plenty of information on the water circulation in
the North Atlantic area.
Almost all the studies on 99 Tc concentration in seaweed havebeen carried out in the North Atlantic transect due to the contaminant character of the nuclear fuel reprocessing plants at La
Hague and Sellafield. However, the measurement of 99Tc concentration in algae samples has been carried out in the otherenvironments. For instance, levels in Fucus vesiculosus samples taken at Galicia, Northwestern Spain, in 1984 reached values of some 0.4 mBq/g dry weight, which were attributed to
fallout background in such area.27 Manjón et al.28 measured
99
Tc in several seaweed species collected at the Southern
Spanish coasts during 1988 and 1989. 99 Tc was detected basically in Fucus spiralis and vesiculosus and, in general terms,
the concentration values of some 0.1 mBq/g were found andinterpreted as general background in the absence of local
sources of radioactivity. For the sake of comparison, levels up
to 10 4 mBq/g were found for Fucus vesiculosus taken at the
Irish Sea after the beginning of EARP operation.21
Hirano and Matsuba 25 presented very interesting results of99
Tc in Sargassum algae collected in Japan. The authors foundconcentration values of 104 L/kg for this algae species.
Atmosphere. There are not so much works published on
the presence of 99 Tc in the atmosphere. Attrepp et al.29 and later
Ehrhardrt and Attrepp30 communicated the data on rainwater(14 samples in total) taken at Texas from 1960 to 1970. Concentrations of the order of 10−2 mBq/L were reported in these works.
In 1993 results were published31 on rainwater taken at the Southof Spain from 1984 to 1987. Levels ranged from 10 −3 to 10−1
mBq/L and are compatible with those found by the previous
authors. The American and the Spanish groups found anomalously high 99 Tc/ 90Sr and 99 Tc/137Cs activity ratios in the samples from Texas and Spain compared to what it is expected if
fission were the only source of 99 Tc in the environment, i.e.
~10−4. Furthermore, the ratio steadily increased since 1960 till
1987. Calculations showed that 98Mo neutron activation was
not a significant source of 99 Tc which could explain the deviation from the expected ratio. Some hypothesis was done on thepossibility that a slightly higher residence time of 99 Tc in thestratosphere could account for the discrepancy.31 The answer
to this problem still remains unresolved, since the influence on
a global scale of gaseous emissions from nuclear reprocessing
plants cannot be discarded as an additional explanation for thisfact. Nevertheless, as far as I know, there is no work in the
open literature supporting this possibility. The only observa-

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 255
tion related to this effect is found in Reference 32, which gave
results on the environmental impact of the 99 Tc gaseous
releases from the Karslruhe nuclear reprocessing plant. The
detected effects were local in any case.
The concentration of 99 Tc was also measured in samples
taken after the Chernobyl accident.31 It was found no relevant
difference between the concentration found before and after
the Chernobyl accident. This is consistent with the results ofReference 12. For the same samples, it was found that the levels of 137Cs increased some two orders of magnitude after the
Chernobyl accident.
It is interesting to note that the same group33 measured 99 Tc
in atmospheric filters taken in the south of Spain during thesixties. Its maximum concentration was found to be 3 µBq/m3
in 1967. E. Holm34 calculated the 99 Tc/137Cs activity ratio withthese data and found higher ratios than expected. In the samereference, the data on air filters taken at Sweden and Monaco
showed also such anomaly.
Finally, the data of Holm and Rioseco35 on 99Tc in carpets oflichen and soils deserve attention since they could inform on
the behavior of 99 Tc in the atmosphere.
Sediments and soils. As it was stated before, 99 Tc is very
soluble and mobile in seawater. For that reason, it is not easily
measured in open sea sediments. Reducing conditions in waterand in the sediment, or in the sediment, facilitate its absorption
on the sediment. Interesting works on the process that could
lead the incorporation of 99 Tc onto the sediment are those ofReferences 36 and 37. In these works, chemical speciation of99
Tc was studied in real samples. The authors found thatorganic material appeared to be the most important binding
phase for Tc(IV). Apparently, highly reducing conditions
enhance the organic uptake of Tc, which, in fact, was alreadystated by Tagami.38 Low organic matter content, on the other
hand, enhances the Tc association to carbonates.
The majority of the works on the concentration of 99 Tc inmarine sediments have been carried out in the surroundings ofintense release sources. From this viewpoint, the North
Atlantic area is fairly well studied. A very complete work can
be found in Reference 39 on the 99 Tc inventory in sediments of
the Irish Sea close to Sellafield. Up to 2 Bq/cm2 can be foundin the most contaminated samples.
A very unique work is that reported by Aarkrog et al.40 on
the presence of 99Tc in river sediments. The authors found the
maximum activity peak (780 mBq/g) at a sediment-core depthof 14 – 18 cm that corresponded to the year 1950. This datecoincides with the maximum releases from the MAYAK plutonium production plant at the Urals. According to them some
100 PBq of low to medium liquid wastes was discharged into
the Techa river from 1949 –1952. The authors supposed theexistence of reducing conditions to explain the presence of 99Tc
in the sediment.
For the same reason as in the case of sediments, the measurement of 99 Tc in soils is difficult due to the low concentration in them, in the absence of a direct source. In spite of this,global fallout 99 Tc distribution in Japanese soils has been determined by Tagami and Uchida.41 The values found range from4 to 88 mBq/kg. The observed high differences were attributed by the authors to the variability of soil types. The
99
Tc/137Cs activity ratio in the soils was reported to be one orderof magnitude higher than that theoretically expected. The
authors explained such fact not only on the basis of the ratio
during the deposition but also to the irrigation practices andbehavior in soils of the two radionuclides. The vertical distribution of 99 Tc in soil cores shows that it migrates faster than137
Cs. Nevertheless the bulk of 99 Tc still remains within thefirst 20 cm of soils.
The described set of data can be compared to those obtainedin the soils from the Marshall Islands by the same group.42 The
area was presumably contaminated by nuclear tests and the256 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
obtained results confirmed it, since levels of some 0.1 to 1.1
mBq/g were found.
Morita et al.43 also presented concentration profiles of 99 Tc
in a soil and compared its behavior to the other long-lived
radionuclides. The authors found that 99 Tc and the othernuclides were retained also in a part of the profile which contained more organic matter.
As stated in Reference 4, it is not easy to understand the
behavior of 99 Tc in soils. It depends on many variables and their
combinations. What it seems clear is that under aerobic conditions 99 Tc is very mobile since it is expected to be as TcO4 −. Inthat case, its availability to plants is high.44 At anaerobic conditions, 99 Tc is reduced at lower valence states and its mobility
decreases. 99 Tc needs, therefore, the existence of reducing conditions to be absorbed in soils. If this happens, its transference
to vegetal species decreases very much, which is certainly relevant to agricultural soils.38 The last two cited works are thekey to understand the behavior of 99 Tc in soils as well as its
availability to plants.
4. Methods
In general terms, the concentrations of 99 Tc in the environment are very low. Therefore, very sensitive methods areneeded for its determination. In all the cases, the chemical
separation of Tc from a sample is required and the addition of ayield tracer is necessary. Once Tc is separated, radiometric or
non-radiometric methods can be used for 99 Tc determination.
Radiochemistry. Broadly speaking, the chemical separation method for environmental Tc consists of three steps: sample pretreatment, Tc extraction, and final conditioning forcounting. A brief discussion of the methods found in the literature is given below on the basis of these three steps.
As for the pretreatment of water samples, pioneer works
were based on its evaporation to smaller volumes. Many
authors claim that it has to be done at basic pH to avoid lossesof Tc during the process.16 Ulterior tests showed that this condition is not totally necessary when the temperature was kept
under 90 ˚C.38 The method rapidly appeared as unpractical forlarge water volume. An interesting advance was represented
by the use of anion exchange resins.17 The authors used suchmethod for analyzing as much as 10 L of seawater. A similar
way for the preconcentration of Tc is that presented by Holm etal.45 They use activated carbon filters to preconcentrate Tc insitu.
Larger volume of water can be pretreated with a reductioncoprecipitation process. In this case, Tc is reduced by adding a
reducing agent, FeSO 4 (Ref. 27) or K 2S2O5,24 in to the water
sample. Under these conditions, Tc precipitates with Fe(III) at
basic pH. The advance is clear since the analysis of huge volume of water is substituted by some grams of precipitate.
Either the residue or the precipitate are then dissolved with
HCl, HNO3, H2SO4 or even aqua regia and introduced into theextraction process.
For solid samples, seaweed, sediments or soils, the calcination has been the most popular way of sample pretreatment.
The temperature of 500 or 600 ˚C is used in the literature.46, 47
Distillation has been used also, but as far as I know, it is not themost widely used method.26, 48
After the calcination, the ashes are dissolved and Tc is
extracted according to the different methods described below.
Liquid-liquid extraction with TIOA,16 TBP 47, 49 or C6H10 O(Ref. 50) as extracting agent has been widely used. An anion
exchange resin, Dowex AG1-X8, for instance, is an interesting
option as well. Riley and Siddiqui17 incorporated Tc in aslightly acidic solution into the resin. Basic media were used
in References 40, 50, and 51.
More recently, TEVA resins (see for instance References 52
and 53 have proven to be highly selective with regard to Mo

García-León
99

and Ru, the main interfering isotope in mass spectrometricmethods, including accelerator mass spectrometry (AMS). In
this case, Tc is introduced into the resin under slightly acidic
conditions.
The final step of sample conditioning for counting essentially depends on the physical method of 99 Tc measurement.
Radiometric methods, based on the counting of the β radiation from 99 Tc, usually need an electroplating process in orderto prepare a thin counting sample so as to avoid the self-absorption of relatively soft radiation. The electrodeposition is carried out under basic conditions, usually a 2 M NaOH solution.47
When liquid scintillation counting is carried out, electrodeposition is not needed and the extracting agent in a liquid-liquid
extraction, TBP for instance, is dissolved directly in the scintillator.49
In the case of mass spectrometric methods, the final sourceof Tc has to be in a form which depends on the method. For
instance, a slightly acidic solution, 2% HNO3, is used for ICPMS methods. 54 Resonance ionization mass spectrometry(RIMS) uses electroplated 99Tc sources onto Re filaments. 55
The form of the final source for AMS is somewhat different
since the sample is injected into the accelerator as an oxide. It
is thoroughly mixed with highly pure aluminum powder 56
which acts as the sample holder.
The yield of the chemical separation must be measured toquantify the concentration of 99 Tc. Early works used Re as a
tracer. 29 This method is based on the chemical similarity
between Tc and Re. But this practice is not common nowadays.
A recent intercomparison exercise52 has shown that the use of
Re as a yield tracer systematically overestimates the 99 Tc activity in the sample. Mas et al.57 show, however, that the use of Re
or 95mTc for the measurement of 99 Tc in seaweed is equivalent.
Certainly the more comfortable possibility is the use of 99mTc(T1/2 = 6.01 h), because it is easily available58, 59 and measurable, since it practically emits only a γ ray. But 99m Tc decays
into 99 Tc, which provokes some prevention with its use.
Nevertheless, even in very sensitive methods it is an acceptable
alternative, although with some limitations.59
97m
Tc (T1/2 = 90.5 d) is also an option which was proposed by
Kaye et al. 51 97mTc has a little more complex decay scheme,
emitting mostly conversion electrons, and longer half-life than
99m
Tc. Thus, it is less accessible than 99mTc.
95m
Tc (T1/2 = 61 d) is probably the most suitable option. Its
half-life is not too long and emits γ-rays with enough intensity.
As far as we know, it was first used by Thein et al.60 Nevertheless, some authors highlight the fact that the commercial solutions of 95mTc often contain detectable 97Tc, 98Tc, and 99Tc which
does not advise its utilization.38 The direct production of 95mTc
in a cyclotron through the reaction 93Nb(α,2n) 95mTc is proposed
by Sekine et al.61 as a cleaner alternative. This is, however,accessible to a few laboratories.
A complete method for 99Tc extraction can be seen in Figure
2, taken from Reference 62.
Physics. Radiometric methods are based on the counting ofthe β–radiation emitted by 99 Tc. Since its concentration innature is very low, techniques of low-level counting (LLC) are
required. A good text on the principle and practice of LLC isthat was written by P. Theodorsson.63
Gas ionization detectors of Geiger-Müller or proportional
types have been widely used for 99 Tc determination.17, 35, 51 For
24 h counting time, typical instrumental detection limits
between 1 to 5 mBq can be obtained.
Holm et al.58 use a surface barrier detector for 99 Tc determination. The spectrometric response of the detector could allow
the identification of interfering β-emitters. Nevertheless, for
16 h counting time a detection limit of 10 mBq can be reached,i.e. slightly higher than those obtained with gas counter.
Scintillation counters have been an option used from the
beginning.46, 49 Detection limits of some 40 mBq in 10 h count-

99

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 257

Tc in the Environment: Sources, Distribution and Methods

MDA (mBq)

100

Semiconductor detector

101Geiger-Müller
Liquid Scintillation
0

5

10

15

20

Countingtime (hours)
Figure 3. Taken from Reference 64. Minimum Detectable Activities
for different detectors as a function of the counting time.

Figure 2. An example of a whole radiochemical method for 99Tc determination in water. Taken from Reference 62.

ing time are reported by the authors.
In Figure 3, taken from Reference 64, a comparison is doneon the minimum detectable activity (MDA) obtained for 99Tc
by the published radiometric methods. MDA is conventionallydefined as the smallest activity in a sample that can be detectedwith a 5% probability of erroneously detecting radioactivity,
when in fact none was present (Type I error) and also, a 5%
probability of not detecting radioactivity, when in fact it is present (Type II error). Liquid scintillation method seems to giveslightly smaller MDA than gas counters.
The conventional radiometric techniques have, however,limitations due to the long 99 Tc half-life. A simple inspectionof the popular activity equation
A = λNshows that for a long half-life is more effective to measure
directly N than A. That is the case for 99 Tc. Non-radiometricmethods can help to overcome the difficulties.
Historically, they have already played their roles. In fact,
very early works used neutron activation for 99 Tc determination.65 The authors separated 99 Tc from the sample of interestand then irradiated it in a reactor to obtain 100 Tc (T1/2 = 15.8 s).
After the separation and purification of the 100 Tc, they measured the radioactivity by a low-background gas counter.
According to the information that the authors gave in the paper,for a thermal neutron flux of 5 × 1013 ncm−2s −1, the detection
limits for filter papers and vegetation samples are 5 and 9 pg of99
Tc, which in radioactivity terms are about 3 and 6 mBq,
respectively. The method is complicated taking into accountthe very short half-life of 100 Tc. An alternative is that presented
by Ikeda et al.66 The authors used reactions of the type (n,n´)
to obtain 99m Tc which has longer half-life than 100 Tc and, thus,
is easy to separate and measure. The 99 Tc radioactivities ofsome 10 Bq are reported by the authors.
A refined approach used other reactions as (γ, γ´), instead of
the traditional (n, γ) activation reaction.67 The authors also
obtained 99m Tc by using bremsstrahlung radiation from theelectron linear accelerator of Tohoku University.
Nevertheless, only mass spectrometric techniques can nowadays compete with radiation counting.
The technique that has demonstrated to be more accessible
and realistic so far is ICP-MS. Many works can be found in
the literature on the use of ICP-MS to detect 99 Tc in environmental samples. Examples are References 24, 50, 52, 54, 62
and 68. Detection limits of a few mBq, obtained with counting
times of a few minutes, are presented by the authors, whichmakes ICP-MS superior to radiometric methods. A deep study
on the tuning of conventional quadrupole ICP-MS for 99 Tc
atom counting and of the interferences in 99 Tc determination
can be found in Reference 69. When high resolution ICP-MS
is used, the detection limit falls to a few µBq,70 the best so far
obtained with ICP-MS.
The use of RIMS for 99 Tc determination was reported.55, 71
The authors claimed to reach sensitivities of some 107 atoms
which means 10 fg or 10−2 µBq. Nevertheless, the application of
RIMS to environmental determination has not been yet done.
AMS is increasingly competitive with regard to radiometricmethods.56, 72, 73 Tc is accelerated up to 125 or 225 MeV andsensitivities from 106 to 108 atoms, i.e. from 10−2 to a few µBq
were obtained by using about 250 mL of sea water.
5. Conclusions
99

Tc is a relevant nuclide from an environmental viewpoint.
Its long half-life assures its presence in the environment once
introduced in it. It behaves as a conservative element in water
which serves to trace, for instance, the water current pattern inthe ocean. It is also very mobile in soils and in, general terms,
available to plants under aerobic conditions. A reducing environment provokes a decrease in its mobility and availability toplants. However, lack of knowledge still exists on its behavior
in the environment. Intensive speciation studies have to be carried out to overcome this problem.
The use of non-radiometric methods, specially conventionalor high resolution ICP-MS as well as AMS can help to overcome the difficulty of its measurement at environmental levels.
Acknowledgements. Some time before starting the redaction of this work an old colleague and co-author of my first
papers on 99 Tc died. Many practical details that made possible
our first 99 Tc determinations were directly solved by Prof. C.
Piazza, a good chemist and best friend. I want to dedicate him
the paper. It is not too much but is what I can do to keep him
in the mind of the “Technetium community”.
Prof. G. Manjón and Dr. J. L. Más, Guillermo and Pepe, are258 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
responsible for many parts of this paper. They were first students of mine, then colleagues and now colleagues and friends.
Thank you. The help of María Dolores Roldán Villena has
been (and is) simply invaluable.
Finantial support by the Spanish Ministry of Education andScience, through the project FIS2004-00495, is deeply acknowledged.
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