isotopic signatures of environmental plutonium are generally used to assess the origin of the material . while decay counting is restricted to pu , pu , and pu
, all mass spectrometric methods can in principle detect the isotopes pu ( t1/2  =  24.1  ka ) , pu ( 6.65  ka ) , pu ( 14.4 a ) , pu ( 373  ka ) , and pu ( 80.8  ma ) .
however , there are several publications available concerning isotopic concentrations of the minor plutonium isotopes pu , pu , and pu in environmental samples .  
has assessed the global distribution of pu and pu , while   uses pu to distinguish chernobyl plutonium from global fallout . in   we
have shown how the isotopic vector of pu ( pu , pu , pu , pu ) can be measured by combining the respective best suited method for each isotope . due to the strong interference of u in mass spectrometers , pu can generally be measured with alpha - spectrometry only . also ,
while the combined activity of pu and pu can be determined efficiently by decay counting , their similar -energies are not readily separated , thus a mass spectrometer is usually required to determine the isotopic ratio pu / pu . the short half - life of pu renders -decay counting by lsc ( liquid scintillation counting ) the most efficient method .
for the long lived isotopes pu and pu only mass spectrometric methods are suitable . among the mass spectrometric methods , ams provides the most sensitive measurements , probably since in most cases the limit is imposed by background from u rather than by detection efficiency . the destruction of molecules by stripping and the combination of several spectrometers , permits ams to suppress background better than other methods .
if only ams is used , pu can not be measured , and a lower efficiency is achieved for pu . however , since no additional sample processing is required , the measurement of pu can be done at very little additional effort if the concentration of pu in the sample is high enough . during the last few years
, we have carried out several studies on environmental pu [ 57 ] . in most cases , the measurement of the pu / pu ratio was the main interest . however ,
if we encountered samples with sufficiently high count rates for these isotopes , we additionally performed measurements of the minor pu isotopes pu , pu , and in some cases even pu . dating with plutonium can be done very precisely by using the ratio am / pu ( e.g. ) .
the parent nuclide pu ( t1/2  =  14.4  yr ) and the daughter am are different chemical elements and thus this ratio yields the date of either the irradiation or that of the last chemical separation .
while this is a useful method for applications in the scope of nuclear safeguards , chemical fractionation occurring while the sample resides in the environment compromises its use for environmental studies . without the presence of the daughter isotope
am in the sample , the age since the irradiation can be assessed only if the initial isotopic abundance of pu is known .
this abundance , however , depends strongly on the production process , and thus is generally ambiguous .
we propose to use the isotopic ratios of the other plutonium isotopes to estimate the initial pu content . to check the accuracy of this estimate ,
we have applied the method to measured literature values for a thermonuclear weapons test , and for the chernobyl accident where we adopt the numbers obtained by   given by .
additionally , we have simulated thermal neutron irradiation of u. to demonstrate the practical applicability , we have measured environmental samples of different origin .
the short lived nuclide pu ( t1/2  =  5  h ) decays in a reactor before pu is produced , which results in cm being the isotope of mass 244 produced in a reactor . in a nuclear explosion , however , in which all nuclear reactions happen on a time scale of microseconds , pu can be produced directly via neutron captures on plutonium isotopes , and also by captures on uranium and subsequent -decays after the explosion .
results have been published for the thermonuclear weapons test  ivy mike  ( pu / pu  =  ( 11.8    0.7 )    10 )   and for the low - yield test 
( pu / pu  =  ( 2.3    0.4 )    10 ) .
local weapons test fallout in bikini atoll sediments and soils has been measured with ams recently by , yielding pu / pu ratios between 2.8    10 and 5.7    10 . in the outer layer of a deep - sea manganese nodule a pu / pu ratio of approximately 1 
  10 has been observed ( taken from fig . 3 of ) , which the authors attribute to a global fallout .
samples were obtained from three different projects . in   depth profiles of pu and pu ( among others ) were obtained from the region of nassfeld ( salzburg , austria ) . while cs and sr in this region stem from the reactor accident in chernobyl , pu / pu and am / pu activity ratios as well as the pu / pu isotope ratio determined by ams identified global fallout as the source of plutonium .
the samples used for this work ( t2f and t2 g in ) were collected on the mountain pasture  nafeld - alm  at an altitude of 2530  m asl , at 47.018  n , 13.012  e on august 25th , 1999 .
the chemical separation of the samples ( separating am from pu ) was carried out in summer 2006 , the ams measurement in august 2006 .
the discharge history of pu from sellafield is well documented . though releases extended from 19521992 , the peak discharge occurred between 1970 and 1980 ( fig .
1 ) . a marine sediment core was collected in 1993 at 54.416  n , 3.563  w by the research vessel gauss , federal maritime and hydrographic agency , germany .
this core was investigated for a number of isotopes in   and recently for pu isotopes and u with ams at vera . generally , the chronology of the 48  cm long core is unclear , since mixing of the sediment is suggested by the oceanographic situation of the sampling site and hence   expects an  average integral ratio  of the discharges in the core ; despite this , clearly discriminated peaks in the pu , pu , and pu are observed in . for the present work ,
the ams sputter targets prepared for   were re - measured for pu , pu , pu , pu and pu . the chemical sample preparation was performed in november 2009 , while the ams re - measurement was done in december 2011 . for some of the samples , pu / pu and pu / pu data have already been given in , however the data in the present manuscript stems from the new measurement .
the garigliano nuclear power plant was situated at sessa aurunca near the garigliano river in campania , italy .
it consisted of a boiling water reactor with a thermal power of 506  mw , and was in operation from 1964 to 1978 , and is presently in the decommissioning phase .
though mainly operated with 2.3% enriched uo2 , mixed oxide fuel , consisting of uo2 and puo2 , was also used . during operation and decommissioning ,
a core from the sediment inside the ( now dry ) drain channel ( 41.257  n ;
13.832  e ) and another one from the entry point of the channel into the river was collected in may 2006 ; pu was separated in autumn 2006 , and the ams measurement was performed in august 2007 . based on the pu depth profiles ,
in studies on the yield of heavy elements in thermonuclear explosions   a general exponential trend with increasing mass was observed .
this trend can be understood under the simplifying assumption of an identical cross section for all involved ( n, ) reactions ; the chance to produce a mass n masses above the initial nucleus is then the probability of absorbing exactly n neutrons , which is described by a poison distribution . as long as the neutron absorption probability is small ,
furthermore , the higher binding energy of even - even nuclei leads to a higher abundance of isotopes with even masses ; a factor of 1.58 was observed for the  ivy mike  device .
thus the initial ratio between mass 241 and 239 will be the same as that between mass 242 and 240:(1)241pu239pu0=242pu240puand thus we obtain the time t since irradiation(2)t = t1/2ln2ln(241pu239pu)(242pu240pu)with t1/2  =  14.4  yr the half - life of pu . despite the present - day availability of improved nuclear data for a more detailed simulation , calculation of the actual isotope yields
is complicated by the fact that a constant neutron flux throughout the uranium or plutonium used in the devices can not be assumed .
the resultant distribution will generally exhibit a higher yield of the heavier masses ( more frequently produced in the high flux spots ) than indicated by the lighter masses .
simulations are possible for simplified cases , especially for thermal neutrons , which corresponds roughly to the case of reactor production .
additionally , we assume that the duration of the irradiation is much shorter than the half life of pu .
both neutron capture and fission are taken into account , the corresponding cross sections were taken from the endf / b - vii.0 database .
the system of differential equations was solved numerically with mathematica ( wolfram research , champaign , il , usa ) for different assumed burn - up levels .
the methods to extract the plutonium from the environmental samples varied between the different projects ; the ( dry ) sample mass was 1  g for sellafield , 212  g for garigliano , and 20  g for salzburg samples , respectively . generally , the samples were leached , and plutonium was extracted with ion exchange .
after co - precipitation with 1 to 3  mg iron , the sample was calcined to produce plutonium oxide embedded in an iron oxide matrix . the ams facility vera is especially equipped for the measurement of heavy ions .
uranium pilot beams are used for tuning the spectrometer , which is then scaled to the various pu isotopes for the measurement .
the switching times between the different isotopes is typically 15  s , therefore fast variations of the ion source output are not completely averaged out and contribute to the uncertainty of the measured isotopic ratios .
the high - energy beam analysers are designed to allow the transport of 5  +  actinide ions at a terminal voltage of 3  mv .
oxygen is used as a stripper gas which results in a stripping yield of 5% for this charge state . for
most previously published data a time - of - flight ( tof ) detector with 25% efficiency was used to further suppress potential background from u ; but we have never observed such background , so measurements can also be carried out without tof , resulting in a theoretical detection efficiency of 4    10 . however , under typical sputter source conditions , for 10 pu atoms in the sputter target we obtain a count rate of 30  s ( for puo2 only 1/10 of this count rate is observed ) . since the sputter targets last for many hours , the available accelerator beam time usually limits the detection efficiency .
we hope that we can improve on this by reducing the amount of iron matrix used , which requires new handling procedures .
counting statistics were no limitation for the sellafield marine core samples , since their concentration of radionuclides is exceptionally high ; as already reported in , to avoid saturation of the detectors by the very high count rates we had to reduce the source output strongly , by running the cesium reservoir essentially at room temperature instead of the usual 150  c . therefore , these sputter targets appeared almost untouched after the measurement .
all data was normalized to a pulser running at a fixed frequency to correct for any detector dead time that arose as a result of high counting rates .
since all pu isotopes are measured in the same ionization chamber , we assume the same ion optical transmission .
the ams device can in principle introduce large fractionation , since each isotope is essentially measured with a different tuning of the ( relatively complex ) mass spectrometer , and the tunings may differ in quality . since we obtain our plutonium tunings by scaling from the u pilot beam the heavier plutonium masses are more likely to suffer from ion optical losses .
the resultant uncertainty is difficult to assess , since no plutonium standard with isotopic ratios suitable for ams is available so far , which would allow to monitor and correct machine fractionation for every measurement .
however , reference materials measured in the past suggest a typical precision better than 10% . to monitor whether the beam tuning degrades as a result of drift in the components , the u / u ratio of our in - house uranium standard vienna - kku is measured using three sputter targets per turn of the sample wheel .
it should be noted that systematic trends in the ion optical transmission of the different pu isotopes will partly cancel out in the double - ratio used in eq .
we expect little fractionation during chemical sample preparation , and blank material allows any laboratory contamination to be traced .
the beam analysers will not distinguish between the in - growing am and the remaining pu .
however , the time span between the separation of am and the ams measurement is still relevant .
am ( most likely ) has a different negative ion yield to pu , which introduces an uncertainty into the mass 241 determination .
we are not aware of any published ionization yield for am , but   has investigated other monoxide ions of actinides : the relative negative - ion formation probability of tho , uo , npo , and puo is approximately 1:3:5:7 . as an example , we assume that am does form negative monoxide ions even two times better than pu , and that the measurement is carried out one year after chemical separation . since 5% of pu will have decayed into am , the observed count rate for mass 241 will appear 5% higher , which corresponds to a 1  yr lower age . as another extreme example , if we assume that am does not form any negative monoxide ions at all ( which is definitively not the case , see ) , the sample will appear one year older - as it actually is .
since no information is available on the actual ion yield of am , which is moreover expected to vary with the matrix of the sputter target and with ion source conditions , we assume an additional uncertainty of 1  year for the age determination per year between separation and measurement .
fig . 2 and table 1 shows the results measured in this work , in comparison to the numbers given for the  ivy mike
43 counts of pu were successfully measured on salzburg soil within 1.5  h. the resultant isotopic ratio for pu / pu is ( 5.7    1.0 )    10 .
a contribution of reactor material in our samples , in particular from the chernobyl accident , could cause a lower ratio .
however , a significant contribution from chernobyl was excluded in . additionally , while a pu / pu ratio of 2.7    10 is expected for chernobyl debris in 2011 ,
our measured ratio of ( 1.43    0.06 )    10 is more than one order of magnitude lower , and lies perfectly on the eye guide for global fallout ( through masses 239 and 242 of salzburg soil ) .
we think that our value better represents the northern hemisphere global fallout ratio than that taken from . a possible explanation for the higher ratio observed in the deep sea crust ( va132 , 918 n , 14603 w , 4830  m depth ) would be influence from the various nuclear test sites in the south pacific .
we think this assumption is supported by the pu / pu of 0.25 observed in the nodule sample ( also taken from fig .
3 in ) , while the accepted northern hemisphere global fallout average is 0.185 ; this value is also observed in our salzburg soil samples .
( 2 ) for the measurements from literature and the simulations for thermal neutrons is shown in table 2 and fig .
since all ratios were calculated for the time of production , or corrected for pu decay , an age of 0 is expected in all cases .
while the mike test appears 6  years older than expected , the simulated thermal neutron results appear too young , and the deviation grows with the assumed burn - up .
the chernobyl data appears 5  years too young ; a more accurate estimate is obtained if we assume production by thermal neutrons . in this case
( 2 ) , but fit the simulation to the measured pu / pu ratio , which results in a total thermal neutron fluence of 0.0025 b , and in an estimated age deviating by less than 2  years from the known calendar date of the chernobyl explosion .
this agreement is surprising , considering that the processes in the actual reactor deviated significantly from a simple thermal neutron irradiation .
for the salzburg soil , an age of 52.5    1.5  yr is obtained , corresponding to the year 1954 .
this age is 8  yr before the maximum of the nuclear weapons tests in 1963 , and the deviation is similar to that for the ivy mike test . if this trend holds for all material produced in nuclear explosions , a correction could be applied reducing the deviation .
the garigliano results suffer from a larger statistical uncertainty , due to lower pu concentration in the samples .
the age is 42    8  yr , which corresponds to the year 1965 , in agreement with expectations . our new pu / pu data for the sellafield core agrees with previous measurements on the same sputter targets presented in , while the three pu / pu ratios given there have significantly lower precision .
however , one sample ( st60  k ) shows a deviation of about 25% from the previously published data , which is much more than allowed by counting statistics ( see table 1 ) .
we attribute this outlier to variations in the transmission of pu through the spectrometer , limiting the reproducibility of the present method .
since a deviation of 25% , corresponding to a dating error of about 5  years , is relevant for the present work , this emphasizes the need for an ams standard which would allow to trace and correct for such variations .
our sellafield dataset is well suited to illustrate the advantage of using the pu information for dating .
( 2 ) compared to an estimate of the initial pu / pu which is based only on pu / pu:(3)241pu239pu0=240pu239pu2 while the age shift due to the neglected even 
odd offset of this simpler method could probably be handled , the pu information establishes a monotonous age - depth curve for the sediment core , however the data covers only a short time span of seven years .
the time range 1977 to 1984 falls well into the later part of the sellafield discharges ; however , if the age is calibrated against a simulated thermal neutron irradiation , a fluence of 0.0015 b fits best , and leads on an estimated age correction of 5  years .
the conventional approach towards source assessment of an unknown material is to measure elemental concentrations and isotopic ratios , and to compare them with a database of all viable sources .
this works also for environmental plutonium isotopes , and probably would lead to the same source assignments as the method proposed in this work .
however , we think that the calculation of  predictive  values like burn - up , irradiation date , or production mode ( i.e. reactor vs. explosion ) makes the assessment more straightforward . the measurement of pu , pu , pu , pu and pu allows for more precise assessment of the history and origin of environmental plutonium .
additionally we have shown that the use of pu and pu allows dating of the time of irradiation accurate to within 6  years , while our data on the irish sea sediment core suggest that relative dating of material from the same source is possible with a precision of less than 2  years .
the information contained in the other plutonium isotopes may allow further improvement in the precision .
when a reactor origin can be assumed , possibly based on the absence of pu , and by calibrating the data against simulated thermal neutron data with the same pu / pu , an accuracy of 2  years is suggested by our data .
 vera , the vienna environmental research accelerator , is especially equipped for the measurement of actinides , and performs a growing number of measurements on environmental samples . 
 while ams is not the optimum method for each particular plutonium isotope , the possibility to measure 239pu , 240pu , 241pu , 242pu and 244pu on the same ams sputter target is a great simplification . 
 we have obtained a first result on the global fallout value of 244pu/239pu  =  ( 5.7    
 1.0 )    105 based on soil samples from salzburg prefecture , austria . 
 furthermore , we suggest using the 242pu/240pu ratio as an estimate of the initial 241pu/239pu ratio , which allows dating of the time of irradiation based solely on pu isotopes . we have checked the validity of this estimate using literature data , simulations , and environmental samples from soil from the salzburg prefecture ( austria ) , from the shut down garigliano nuclear power plant ( sessa aurunca , italy ) and from the irish sea near the sellafield nuclear facility . 
 the maximum deviation of the estimated dates from the expected ages is 6  years , while relative dating of material from the same source seems to be possible with a precision of less than 2  years . 
 additional information carried by the minor plutonium isotopes may allow further improvements of the precision of the method . 
Introduction
Samples
Methods
Results and discussion
Conclusions