Further Readings
9.3 Neutron Activation Analysis
9.3.1 Cross Section
Considering size alone, the probability that the incident particles will strike the nucleus should be proportional to its cross-sectional area,πR2.
Since R⫽R0A1/3, then the area ⫽πR2⫽πR02A2/3. The R0is a constant, 1.2 ⫻10⫺13 cm. For mass number A⫽100, the cross-sectional area will then be about 10⫺24cm2. The total cross section for collision with a fast particle is never greater than the geo- metric cross-sectional area of the nucleus. Therefore, fast particle cross sections are rarely much larger than 10⫺24cm2. Therefore, a cross section of 10⫺24cm2is con- sidered as big as a barn, and 10⫺24cm2has been named the barn, which is the unit of cross sections.
1 barn ⫽1 b ⫽10⫺24cm2(definition) 1 mb ⫽10⫺3b
If we measure the cross section for neutron energies between 0.4 and 1 MeV, we see so-called resonance peaksat discrete energies for a given isotope. Except for very few isotopes,e.g., Cd, Sm, Eu, Gd and Hg, the cross section changes as a function of 1/vup to 0.4 eV. This is called 1/vlaw where vis the velocity of neutrons. Above 1 eV, the cross sections show enormous fluctuations over a very small energy range due to resonance.
9.3.2 Neutron Sources
9.3.2.1 Laboratory Neutron Sources
These could be radioactive sources or neutron generators. In general, the main nuclear reaction for most laboratory sources is:
9Be(α, n)12C Figure 9.3 (n,γ) reaction followed by β−decay
The αs can be obtained from the radioactive α emitters, such as Ra and Po.
Spontaneously fissioning 252Cf sources, which are now available in mg quantities, are becoming increasingly more important.
As seen in Table 9.1, fast neutron can be produced by bombarding certain nuclides with photons or charged particles.
Of these, the last one (DT) is most often used for the fast NAA.
9.3.2.2 Research Reactors
Research reactors are the main sources of thermal neutrons.
235U ⫹n →236U*→X ⫹Y ⫹2.5n ⫹Q
where X and Y are the fission fragments. An average of 2.5 neutrons is emitted dur- ing the fission of a 235U atom.
The fission neutron spectrum shows a Maxwellian distribution ranging in 0–15 MeV, peaking at 1 MeV. Reactor neutrons are classified according to their energies into three groups: fast neutrons (fission spectrum), resonance neutrons and thermal neutrons. Fission neutrons are moderatedfor the propagation of the chain reaction to thermal energies.
9.3.3 Preparation of Samples for Irradiation
The experimental procedure for instrumental neutron activation analysis (INAA) vary greatly from laboratory to laboratory, depending on the type of irradiation facil- ities available, the counting equipment used, the elements to be determined, the type of sample and its matrix and the individual preferences of the experimenter. The flow diagram (Figure 9.4) shows an optimal sample preparation, irradiation and counting procedures to obtain as much information as possible.
Samples collected as explained in Chapter 4 should be unpacked and prepared in a particulate-free environment such as a “Class 100” clean room, or clean bench. A
“Class 100” work area means an area where the air contains less than 100 particles per cubic foot. All the metal surfaces are painted with epoxy paint, and a system con- taining filters circulates the air rapidly in the room. Elemental standards, flux mon- itors or both must be positioned at known locations in the irradiation container.
178 Chapter 9
Table 9.1 Production of fast neutrons by vari- ous reactions
Reaction Neutron Energy
9Be(γ, n)8Be Thermal, 0.025 eV
9Be(d, n)10B 4.4 MeV
2H(d, n)3He (“DD” reaction) 0.3 MeV
3H(d, n)4He (“DT” reaction) 14 MeV DD: deuteron–deuteron reaction; DT: deuteron–tritium reaction
Prior to any irradiation, samples can be counted for natural activity as well as activities released to the environment by man-made sources. This aspect has become very important after the Chernobyl nuclear accident especially for food samples.
Often, fission products can be measured, usually indicative of nuclear-weapon test- ing or reactor accidents. Debris from atmospheric tests may reach the stratosphere and return to the troposphere by storm action. Some isotopes of interest are listed in Table 9.2.
9.3.4 Short Irradiation
Extremely short irradiation (1–5 s) may sometimes be used to measure a number of isotopes, which have very short half-lives, such as 19O,20F and 77mSe (Table 9.3).
Figure 9.4 Flow diagram for sample preparation, irradiation and counting procedure in activation analysis
Counting times for γ-rays emitted from these isotopes usually are 10–30 s. If there are large amounts of sodium and/or aluminium in the sample, it may severely limit the use of this type of experiment because of high dead times and large Compton continua. The method developed by Chatt using Compton suppresser was very suc- cessful in measuring the selenium content of many Canadian vegetables and biolog- ical samples.2
For short irradiation, depending on the number of detectors available, 1–4 samples could be packed, separated and held in place by a strip of polyethylene foam. A nickel flux monitor (5–50 mg, depending on irradiation time) is placed in close prox- imity to each sample. Short-lived isotopes, used in first and second counts, their half- lives and γ-ray energies are given in Tables 9.4 and 9.5, respectively. Depending on the fast neutron flux and the amount of the Al and Si in the sample,28Al(n, p)27Mg and 28Si(n, p)28Al reactions might interfere on Mg and Al, respectively. Also there might be an interference from 843.8 keV γ-ray of 27Mg to 846.6 keV γ-ray of 56Mn.
Therefore one has to check all these and other possible interferences during the analysis of the γ-ray spectra. Fortunately available softwares make these corrections most of the time.
9.3.5 Intermediate and Long-Lived Isotopes, Long Irradiation
Samples that have been run for “shorts” are usually allowed to cool for several days prior to the re-irradiation. In many cases, it is preferable to completely re-bag the sample, if the contamination risk is small. In any case, suitable blanks must also be
180 Chapter 9
Table 9.2 Isotopes for natural or man-made radioactivity counting
Isotope Half-Life Eγ(keV)
7Be 53.3 day 477.6
141Ce 32.5 day 145.4
103Ru 39.4 day 497.1
137Cs 30.17 year 661.7
95Zr 64.0 day 756.7
131I 8.041 day 364.5
144Ce 284.4 day 133.5
90Sr 28.0 day β⫺
Table 9.3 Isotopes and their properties meas- ured in “short-shorts”
Isotope Half-Life (s) Eγ(keV)
19O 26.8 198.0
20F 11.0 1634.0
28Al 134.4 1779.0
46mSc 17.4 162.0
77mSe 18.7 142.5
prepared. For long irradiations, about 0.5% cobalt in aluminium wires is used as a flux monitor. Isotopes that are usually observed as a result of long irradiation in NAA of food and diet after 15–20 days cooling and after 1–2 months cooling are given in Tables 9.6 and 9.7, respectively.
9.3.6 Calculation of Activity Produced after Neutron Irradiation
Assume the simplest case: a thin foil or wire, containing natoms, is irradiated for a period of time t, with neutron flux,Φ. If the production rate R⫽Φσn, the rate of decay during the irradiation is
⫽ ⫺λN (9.8)
then the rate of change of the number of nuclides is
⫽
Φ
σn⫺N
dN ⫽R
⫺λ N
(9.9)ᎏdt ᎏdNdt
ᎏdNdt
Table 9.4 Isotopes and their properties meas- ured in short1
Isotope Half-Life (min) Eγ(keV)
27Mg 9.45 843.8
1014.4
28Al 2.24 1778.9
37S 5.06 3103.3
49Ca 8.72 3084.4
51Ti 5.76 320.1
52V 3.76 1434.2
66Cu 5.10 1039.0
Table 9.5 Isotopes and their properties meas- ured in short2
Isotope Half-Life (h) Eγ(keV)
24Na 15.02 1368.5
2753.9
38Cl 0.620 1642.4
2167.5
42K 12.36 1524.7
56Mn 2.576 846.6
1811.2
69mZn 13.90 438.7
87mSr 0.291 388.4
128I 0.417 442.9
139Ba 1.388 165.8