M A S S S P E C T R O M E T R I C M E T H O D O F I S O T O P I C

A N A L Y S I S O F X E N O N F O R M E D IN N U C L E A R F I S S I O N

Yu. A . S h u k o l y u k o v , Y a . S. K a p u s t a , a n d A. B. V e r k h o v s k i i

UDC 539.173.8

~ne nuclides 129-136Xe are formed in the f ission of any nuclei by neutrons or charged part icles . Isotopic analysis of xenon may be of in teres t for both nuclear physics r e s e a r c h and the solution of cer ta in applied p ro - blems of nuclear engineering. In the presen t ar t ic le we descr ibe a procedure developed for the mass spec t ro - met r ic investigation of xenon with a maximum sensit ivi ty of detection of ~ 10 -14 cm 3 (~ 105 atoms) of individual nuclides. We used a recons t ruc ted MI-1201 mass spec t romete r , an analog-to-digi tal conver ter , and a univer - sal Nairi-2 computer .

Extract ion of Xenon f rom Solids. In order to separa te xenon f rom mater ia ls of var ious composit ions it is neces sa ry to produce a sufficiently high t empera tu re in a closed volume with vacuum puri ty and a low xenon background. A vacuum furnace (Fig. 1) is recommended for the complete separat ion of xenon f rom pract ical ly any solid containing f iss i le nuclei. The heating element and shell of screens were located in a vacuum of 10 -5 ram Hg; the samples being investigated were f i r s t degassed at 10 -7 mm Hg and 200~ for 5-10 h and then placed in the working volume of the furnace - a molybdenum tube. The tube was f i rs t pumped out at 1900-2000~ for 2-5 h to e n s u r e the r emova l of chemical ly act ive gases (hydrogen, nitrogen, carbon dioxide, organic compounds) and xenon f rom the walls. The completeness of the outgassing of the tube was checked with the mass spec- t r o m e t e r by running dummy tests . After carefu l outgassing of the tube the background of a tmospher ic la6Xe did not exceed ~ 10 -14 cm 3 (~ 105 atoms).

In extract ing f ission product xenon f rom solids it is sufficient to maintain the neces sa ry t empera tu re for 1-1.5 h in order to separa te more than 90% of the xenon capable of migrat ing at the given tempera ture (in the 500-2000~ range).

Separation of Xenon f rom Chemically Active Gases and Helium. The gases separated f rom the sample under study a re passed through a solid carbon dioxide (-78~ cold t rap 4 (Fig. 2) in which liquid nitrogen or oxygen cannot be used since xenon is retained on the walls of the trap at - 1 8 3 to -196~ In 15 rain al l the res idual gases a r e sorbed at -196~ on activated charcoa l in ampul 5. Helium, often contained in samples , cannot be sorbed on activated charcoal under these conditions, and is pumped out through valve 1 for 3 rain. Then with sylphon valves 1 and 2 closed the charcoal is heated to 250~ Simultaneously the tempera ture of the steel tube 6 with sponge titanium is ra ised to 900~ In 15 rain the chemical ly active gases a re absorbed by the titanium. Following this the tube is taken f rom the furance and cooled to room tempera ture . When valve 2 is opened and valve 3 is closed, the res idual gases a re sorbed on the activated charcoal in ampul 7 in 15 rain. Then, closing valve 2 the xenon purification process is repeated using the titanium getter 8. The xenon sepa- rated in this way is discharged into the mass spec t romete r through valve 3.

All the equipment except the tube with the samples was made of steel . Before beginning the operat ing cycle the equipment was pumped out at 300~ for 24 h by two m e r c u r y diffusion pumps. During this process the tubes with t i tanium getter were heated to 950-1000~ and the ampul with activated charcoal to 300~

Measurement of Xenon on Mass Spectrometer . It is impossible to measure an u l t rasmal l amount of f is- sion products such as xenon isotopes on a commerc i a l type mass spec t romete r . It was necessa ry to r econ- s t ruc t the vacuum par t to ensure steady vacuum conditions for the measurements . Two steel pipes filled with SPN-3 getter were attached to the ends of the MI-1201 m a s s - s p e c t r o m e t e r chamber . After heating to 600-700~ (with evacuation by the diffusion pumps) the getter at room tempera tu re ensures maintenance of the operating vacuum conditions in the m a s s - s p e c t r o m e t e r chamber for 3-4 h with the chamber valves closed. Before s ta r t - ing a cycle of measurements the m a s s - s p e c t r o m e t e r chamber had to be outgassed for at least 24 h, 3-5 h of which were at 300~

Trans la ted f rom Atomnaya ]~nergiya, Vol. 47, No. 6, pp. 389-391, December, 1979. Original ar t ic le submitted March 27, 1978.

0038-531X/79/4706-1001507.50 �9 1980 Plenum Publishing Corporat ion 1001

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Fig. 1. High- temperature vacuum furnace for extract ing xenon f rom solids : 1) h igh-vacuum pump; 2) molybdenum screens ; 3) the rmo- couple vacuum gauge; 4, 5) outlets to purif i - cation and evacuation sys tems respec t ive ly ; 6) window for optical pyromet ry ; 7) multiply charged s t ruc ture ; 8) heating elements ; 9) water cooling; 10) h igh- tempera ture hea ter ; 11) tung- s t e n - r h e n i u m thermocouple; 12) cooled cur ren t lead-ins.

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~b I 2 J'

4~ 5 8 7 8

Fig. 2. Schematic d iagram of high-vacuum equipment for separat ing xenon: a) to high- tempera ture fur nace; b) to evacuation sys tem; c) to mass spec t romete r .

The cooling sys tems for the vacuum traps and the electr ic and water supply sys tems of the mass spec- t rome te r were recons t ruc ted to ensure automatic a round- the-c lock vacuum pumping. ~[he flow of liquid ni t ro- gen into the mass spec t rometer t raps was regulated by a clockwork mechanism, a control sys tem, and a com- pressor .which produced an ove rp res su re in the Dewar flasks as commanded by the clockwork mechanism. The sensit ivity of the meas twement of ion cur ren ts in the receiving end of the mass spec t rometer was increased by install ing an open louver type e lec t ron multiplier which made it possible to measure ion currents f rom 1.5 - 10 -t3 to 1 �9 10 -17 A. The background current of the multiplier was no more than 2 �9 10 -I~ A. The xenon back- ground of a sample of a commerc ia l mass spec t rometer was lowered by a factor of 10~: f rom approximately 10 -9 to 10 -14 cm s (~ 10 ~ atoms) in the closed chamber .

A meter ing device - a gas pipet - was attached to the vacuum chamber for continuous monitoring of the sensi t ivi ty of the mass spec t rometer and to determine the amount of xenon in the samples being studied. By compar ing the ion cu r ren t in the mass spec t romete r obtained f rom xenon f rom samples containing a known

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amount of it with that f rom the pipet it was possible to cal ibra te the pipet, i.e., to calculate the amount of xenon of a tmospher ic isotopic composi t ion in it. In addition, by compar ing the xenon ion cu r ren t of the samples under investigation with that of the pipet it was possible to calculate the amount of xenon in the sample under study. In addition, the xenon in the pipet enables an es t imate to be made of the mass discr iminat ion in each exper i - ment : the sys temat ic deviation of the measured isotopic rat ios in a tmospher ic xenon of the pipet f rom tabu- lated data - a measu re of the isotopic mass discr iminat ion of the instrument. In an MI-1201 mass spec t rom- e te r it genera l ly does not exceed 0.3% per amu,

The basic operating reg ime of the mass spec t romete r scan is d iscre te . At most 11 isotopes a re mea- sured. The control automat ical ly switches f rom one mass to another, stopping at each mass for 2, 4, 8, or 16 sec depending on the p r o g r a m specified. This makes it possible to integrate the ion currents over a specified t ime of 2 to 16 sec. The integration is per formed on a PRM at tachment which combines two functions : an analog- to-digi ta l conver te r and a d i sc re te function integrator . The mass number of the isotope, the intensity, and the running t ime are recorded on punched tape.

P rocess ing of Experimental Results . In measur ing an isotope of mass m in the output cu r ren t f rom the dc ampli f ier is

I (m, t) = (N i m~ ~}- Nr m~ + h~m~ q- N~ mr) Km t q- I%,

where Nim t is the number of ionized atoms of the isotope being measured ; Nrm t is the number of ionized atoms of isotope of mass m remain ing in the m a s s - s p e c t r o m e t e r chamber af ter e~acuation s tops; Nmm t is the num- ber of ionized atoms of the isotope of mass m resul t ing f rom the "memory" effect; N/mt is the number of ionized atoms of mass m resul t ing f rom inleakage (quasistatic regime) ; Kmt is the m a s s - s p e c t r o m e t e r con- ve r s ion factor which depends on masses and var ies with t ime as a resu l t of instability of the fields of the in- s t rument ; I0t is the initial output cu r r en t of the dc amplif ier .

The value of Nrm depends on the degree of evacuation of the m a s s - s p e c t r o m e t e r chamber ; Nmm and N/m are ze ro a t the instant of admission. Consequently, in order to determine a true isotopic ra t io it is neces sa ry to measure all isotopes at t=0 (the t ime of admission). A ser ies of measurements of mass spect ra is p e r - formed, a co r rec t ion to I% is introduced, and the measured ion currents a re extrapolated to the t ime of admis - sion.

The time dependence of the ion cur ren t s can be approximated by an n-th degree polynomial equation. Choosing the degree of the polynomial is a problem. A low-degree polynomial will give a crude descr ipt ion of r physical p rocess , and a h igh-degree polynomial will not smooth out the ~noise" of the experiment. A ru le for choosing the optimum degree of the polynomial is given in [1].

The mass spec t romet r i c information was processed by using a universal Nairi computer . Input infor- mation was by the punched tape obtained f rom the output of the mass spec t romete r . The p r o g r a m developed includes : taking account of I0t, choosing the degree of the extrapolating polynomial, extrapolating the ion c u r - rents to t ime t = O, calculating the isotopic ra t ios ; introducing a cor rec t ion for the mass discr iminat ion of the instrument, and calculating the amount of xenon in the sample. The information is p rocessed while the exper i - ment is in p rogress . The mean square e r r o r of the determinat ion of the isotopic ra t ios of atnaospheric xenon (10 -l~ cm ~ for an integration t ime of 8 sec and the record ing of 10 mass spectra) is no worse than 0.3%.

This new var iant of the mass spec t romet r i c procedure is a l ready in use in pract ice and can be employed to solve various physical and engineering problems. F rom a knowledge of the isotopic composit ion the shape of the f ission f ragment mass distr ibution curve in the range 129 -<A-< 136 can be found for the spontaneous f i s - sion of nuclides with v e r y long half- l ives. The mass spec t romet r i c procedure descr ibed for the isotopic analysis of xenon can be used to search for hypothetical t ransuran ium elements in nature [2]. The method developed is used to sea rch for t races and to investigate the manifestation of a chain process of the f ission of 23SU in nature [3], and for neutron dos imet ry in the study of samples i rradiated in a nuclear r eac to r [4].

By using the p rocedure developed it is possible f rom the content of xenon in the monitor and in the s a m - pies being investigated to p e r f o r m an analytic determinat ion of f issi le nuclides. For a f ission c ross section of

1 0 2 b, a fluence of ~ 1019 neut rons /cm 2, an amount of f iss i le nuclides ~ 1 0 -1 2 g can be determined with a r e l a - tive e r r o r of N 15%. The e r r o r can be decreased to 3-5% if the amount of iodine or bar ium in the samples and monitor is determined before i rradiat ion. Then, s imultaneously with xenon f rom fission 128Xe or i31xe is formed, and consequently the concentra t ion of the f issi le nuclide in the sample will be determined by the ra t io Xef/~28Xe or Xef/131Xe in the sample and the monitor.

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lo 2. 3. 4.

L I T E R A T U R E C I T E D

D. Hudson, Statistics for Physicists [Russian translation], Mir, Moscow (1967), p. 182. G. Sh. Ashkinadze et al., Geokhimiya, No. 7, 851 (1972). Yu. A. Shukolyukov and Vu Min'Dang, Geokhimiya, No. 12, 1763 (1977). Yu. A. Shukolyukov, Ya. S. Kapusta, and A. B. Verkhovskii, Geokhimiya, No. 4, 572 (1979).

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