Материал: Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

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quantitative measurements, the instrument is calibrated to determine the quantity ki.

Evaporation is used to produce ions out of atoms of a solid volatile substance. For a heatproof substance, spark discharge, laser irradiation or ion (electron) bombardment is used.

The quantity of material in the sample is limited since:

∙normally, the experimental facility is served manually so its radioactive contamination should not exceed personnel safety standards;

∙the result of the experiment may be distorted due to the substances that have got into the sample, including those left from earlier analyses.

Less material in the sample gives less contamination of the instrument,

this making it possible to keep it highly sensitive .

A thin source has the sample atoms therein transformed into ions, these being directed to the analyzer via the electric field. Strip heat-emission sources are used extensively to analyze samples of heavy elements. In such a source, the substance to be analyzed is applied to the evaporator strip and ionization takes place near the ionizer strip.

Box-type sources are in common use nowadays. These have several samples placed therein to be analyzed in a series, thus saving time spent otherwise for sample replacement and evacuation.

The initial analysis phase when the sample is heated has the element composition in the vapor differing from that in the sample thanks to the predominant evaporation of light isotopes (the so-called discrimination by mass with a stronger effect for small-size samples). It is only after a while that true isotopic ratios are attained in the vapor. Due to the sample burning, the ion current decreases over time. These factors may bring systematic errors in the analysis data.

To control the discrimination by mass, the analyzed material has an

indicator added thereto, the latter containing two isotopes that differ in mass and are absent in the sample: 233U+236U to analyze a uranium sample

242Pu+244Pu to analyze a plutonium sample. By measuring the mass spectrum of the mixture, one may estimate the discrimination effect and correct the analysis result from the relative intensity of the lines of said isotopes.

Ions from the source get into the analyzer where the mass spectrum can be scanned by magnetic field. Depending on the field variations, ions have

Isotopic sensitivity is the relation of the background ion current in the mass spectrum interval of М±1 to the current of ions with a mass of М.

276

their mechanical trajectories changed, so that ions of a different mass get into the fixed detector. High-rate scanning, using variations of the electric field that accelerates the ions in the source, may be used instead of low-rate magnetic-field scanning. Electric-field scanning is employed more commonly when the range of the ion mass differences is small, while magnetic-field scanning is used when this is broad.

A variety of detectors and measurement modes are used to detect ions, including current-type ones to analyze large-size samples and pulse-type ones to analyze small-size samples. Often, electronic multipliers (VEU) with a signal amplification factor of 106 and Faraday cylinders are used for detectors.

Nowadays, multicollector detection systems are in common use. Each detector in these systems is tuned to a particular ion mass and the system measures simultaneously the content of several isotopes (up to 9) in the sample. For example, analyses of Pu give, at a time, the content of isotopes with the masses of 238 to 244. No scanning is involved in analysis. A multicollector system makes measurements several times as fast.

Mass spectrometry analysis of uranium isotopic composition

A pure uranium fraction is applied to the strip. Strips are placed inside the mass spectrometer’s ion source with air pumped out of the source. The sample evaporates as the strip is heated while the singly charged ions produced by thermal ionization are accelerated and focused with the aid of electrostatic ion lenses in the mass analyzer. By presetting respective variations of the magnetic field or fields and/or the accelerating potential, the beams of ions of a different mass are sequentially focused onto the detector.

Using automated scanning, one sample insertion lock, a high-rate evacuation system and digital data processing, two operators are capable of analyzing 12 to 16 samples per day, still it is more realistic to have 7 to 9 daily analyzed samples.

Measurement quality control requires at least one analysis of a reference standard with the enrichment close to that of the measured material to be done throughout the series of analyses.

Mass spectrometry is the preferred and broadly employed technique to control uranium isotopic composition. Depending on the instrument sensitivity, an analysis requires 10–8 to 10–5 g of U.

Surface ionization mass spectrometry is also commonly used for destructive analyses of Pu isotopic composition. A plutonium analysis

277

requires 10–9 to 10–6 g of Pu. Random errors of the Pu sample isotopic composition analysis are given in Table 5.15.

Table 5.15

Random errors in a Pu sample isotopic composition analysis by mass spectrometry method

Relative concentration, %

Relative error, %

 

 

0.01 238Pu

20

 

 

93.8 239Pu

0.10

 

 

5.8 240Pu

0.26

 

 

0.3 241Pu

0.81

 

 

0.03 242Pu

7.1

 

 

Isotope dilution method

Isotope dilution is a method used to find the quantity mx of the element Z in a sample of the analyzed material. One must know the isotopic composition of this element (хi) (measured in advance).

The technique is based on using a hanging indicator of the same element with however an isotopic composition differing from that of the element in the analyzed sample. The sample solution of the analyzed material has a

certain amount of the indicator m0 with the isotopic composition xi0 added thereto. After the mixing, a sample is taken. The isotopic composition of the element in the sample taken ( xi ) differs from that in the original sample

(xi) and in the indicator ( xi0 ):

 

 

 

 

∑xi

= ∑xi0 = 1 ;

(5.30)

 

 

 

 

i

i

 

 

 

 

 

i =

1

× [mx xi + m0 xi0 ] ∑

1

× [mx xi + m0 xi0 ],

(5.31)

x

 

 

 

 

 

 

Ai

i

Ai

 

where Ai is the atomic weight of the i-th isotope of the element Z.

278

The relation of the contents of the isotopes i and j in the mixture is equal

to:

 

 

i /

 

j =

1

× [mx xi + m0 xi0 ]

1

×[mx x j + m0 x0j ],

(5.32)

x

x

 

 

 

 

 

 

 

 

Ai

Aj

 

wherefrom the formula to determine mx is derived:

 

 

 

 

 

 

 

0

 

 

 

 

 

 

x A

 

 

 

 

 

 

 

 

 

 

 

 

x

 

 

 

x x

A

 

 

 

 

 

 

j

 

 

 

 

 

 

 

i

j

i

-1 .

 

mx = m0 xi0 1 -

 

 

 

i

i

 

x

 

 

 

 

 

 

(5.33)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

x

j

x0 A

 

 

i

x

j

x A

j

 

 

 

 

 

 

 

i

 

j

 

 

 

i

 

 

 

The sample and the indicator are mixed so that U (Pu) from the sample and from the indicator would acquire similar properties and behave in the same way in further transformations. This is done using multiple redox cycles.

Isotope dilution mass spectrometry (IDMS) is a commonly used technique to determine U and Pu (and isotopic composition thereof) in fuel processing.

Resin bead technique

This is a dedicated technique to determine U and Pu in high-level solutions of spent reactor fuel. It is used for cases where the sample is to be moved to a long distance between laboratories. This does not require shielding against sample radiation. The analytical procedure is as shown in Fig. 5.27.

279

A sample of the input fuel solution containing 1 mg of U and 10 µg of Pu, as well as added indicators of 233U and 242Pu

↓

Chemical operations to reach isotopic equilibrium

↓

Dissolution of 10 ml of the mixture containing 100 µg of U/ml and 1000 ng of Pu/ml in 8 M of HNO3

↓

Absorption of U and Pu from the solution in resin drops: 0.1 ml of solution + 10 drops

20–30 hours

↓1 drop per thread

Washing of resin drops in 3 M of HNO3

↓

Mass spectrometry

1) u-heating 1450oС, 2) U-heating 1800oС

↓

Determination of the Pu and U isotopic compositions and quantities

Fig. 5.27. Flowchart of a resin bead analysis. The potential accuracy of results is 0.6% for U and 0.9% for Pu

5.4. Combined use of NM measurement techniques

The most common NM measurement techniques have been discussed. Each technique has a limited application. Each task can be solved by a variety of different techniques.

Conventionally, all techniques can be interchangeable or complementary. So one may use Davis-Grey titration, isotope dilution mass spectrometry, densitometry and XFA to determine uranium content in a sample.

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