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Uses of Isotopic Neutron Sources in Elemental Analysis Applications

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Uses of Isotopic Neutron Sources in Elemental Analysis Applications EG0600081 3rd Conference on Nuclear & Particle Physics (NUPPAC 01) 20 - 24 Oct., 2001 Cairo, Egypt USES OF ISOTOPIC NEUTRON SOURCES IN ELEMENTAL ANALYSIS APPLICATIONS A. M. Hassan Department of Reactor Physics Reactors Division, Nuclear Research Centre, Atomic Energy Authority. Cairo-Egypt. ABSTRACT The extensive development and applications on the uses of isotopic neutron in the field of elemental analysis of complex samples are largely occurred within the past 30 years. Such sources are used extensively to measure instantaneously, simultaneously and nondestruclively, the major, minor and trace elements in different materials. The low residual activity, bulk sample analysis and high accuracy for short lived elements are improved. Also, the portable isotopic neutron sources, offer a wide range of industrial and field applications. In this talk, a review on the theoretical basis and design considerations of different facilities using several isotopic neutron sources for elemental analysis of different materials is given. INTRODUCTION In principle there are two ways to use neutrons for elemental and isotopic abundance analysis in samples. One is the neutron activation analysis which we call it the "off-line" where the neutron - induced radioactivity is observed after the end of irradiation. The other one we call it the "on-line" where the capture gamma-rays is observed during the neutron bombardment. Actually, the sequence of events occurring during the most common type of nuclear reaction used in this analysis namely the neutron capture or (n, gamma) reaction, is well known for the people working in this field. The neutron interacts with the target nucleus via a non-elastic collision, a compound nucleus forms in an excited state. The excitation energy of the compound nucleus is due to the binding energy of the -25- neutro0<*©n with Wthe nucleus. This compound will almost instantaneously de-excite into a more stable configuration through emission of one or more characteristic prompt gamma-rays. In many cases, this new Configuration yield a radioactive nucleus which also de-excites (or decays) by emission of one or more characteristic delayed gamma-rays, but at a much lower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from a second to several years. Therefore, with respect to the time of measurement, neutron activation analysis falls in two categories: (1) the prompt gamma-ray neutron activation analysis (PGNAA), where the measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where, the measurements follow radioactive decay. In fact, the neutron activation analysis with both modes (PGNAA) and (DGNAA), is considered as a very sensitive analytical technique, which is very useful for performing both qualitative and quantitative multi-element analysis of major, minor and trace elements in samples from almost every conceivable field of scientific applications, this technique offers, sensitivities that are superior to those attainable by other methods, on the order of parts per billion or better. In addition, because of its high accuracy and reliability. The worldwide of this technique is so wide-spread it is estimated that approximately 100,000 samples undergo analysis each year as reported by Michael D. Glascock of Missouri University Research Reactor . Neutron activation analysis technique was discovered in (1936) when Hevesy and Levi found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. They quickly recognized the used of such nuclear reactions on samples followed by the induced radioactivity to facilitate both quantitative and qualitative identification of the elements present in the samples. In order to carry out the analysis of the samples by the neutron activation technique, a source of neutron is required. Also, instrumentation suitable for gamma-rays detection and a detailed knowledge on such nuclear reaction are needed. Nuclear research reactors, radioisotopic neutron emitters and accelerators are the main sources of neutrons. Also, there are three types of neutrons considered as the principal components (thermal, epithermal and fast neutrons) are used, in addition to cold neutron in some cases. Although, in absence of the nuclear research reactor facilities (which are no longer in use, due to different reasons) the neutron activation analysis using the low flux isotopic neutron sources is put to use in different areas of applications. So, a review on the uses some isbtopic neutron sources in multi-element analysis of several applications are given below. -26- ISOTOPIC NEUTRON SOURCES The possible choices for radioisotope neutron sources production are based on either spontaneous fission or on nuclear reactions for which the incident particle is the product of a conventional decay process. Accordingly, the neutron sources could be classified to tile following four categories: (a) The spontaneous fission sources: Many of the transuranic heavy nuclides have an appreciable spontaneous fission decay probability. Several fast neutrons are promptly emitted in each fission event, so a sample of such a radionuclide can be a simple and convinient isotopic neutron source. When used as a neutron source, the isotope is generally encapsulated in a sufficiently thick container so that only the fast neutron and gamma-rays emerge from the source. 252 ' The most common spontaneous fission source is Cf. Its half-life of 2.65 years is long enough to be reasonably convenient, arid the isotope is one of the most widely produced of all transuranics. Fig.(l) shows the double walled capsule of stainless steel surrounded the active Cf source. Table (1) gives the Cf capsule types and table (2) gives the content, activity, emission n/s and 252 the capsule number of Cf isotope neutron source taken from ref.(3). 252 Table(2) Cf content, activity, emission in n/s and the capsule number (ref.(3) emission ••;.; . capsule:' content' '•• activity n/sec-•'••••.','••.;•• mim 0-01 ug •.:;•:• 5nCi :•'. ••.:.:;.-X.1 X:'•;'•' ; 0-1 ua ; 54uCi ^•,,^2-3x10?x - •^•Xii-.-V-;;.. Tne 9 ig.(l) double walled 0-6ug .-.-:•. 268nCi 1-1BX10 , X.1 . • CVN.2 9 capsule of stainless steel ing •: ••••••• 536nC 2-3X10 feiX.1 .-;".:•:.:•.• CVN.3 B 252 2'ng /• •••"•'.•.• 1^07m<:r :,4'6x10 . X.1 CVN.4; fpr Cfsource.(ref.3) .:.-.'• ' • : : •. .••••.'••••.•; •••••;\- ;*.:--''i\\: :;•£?.•:•:'•'''}$'i v Bug . 2 7mC :•-::/:> •t'A&HAO^, X.1; CVN.B - 7 252 •.10'Mff-/ '•;.." 5'4mC . :2-3x10 . .'/'.'.•..•X;.1>:',-i•:•:• 20jig •;••: •••; •-;.• 10-7m(2i;-;vV:v-.4*03< lOT-v.- , X.1 Table (1) The capsules type for Cf (ref. 3) BO|ig;:••;.,• 27mCI 1-15x10". X.1 . threaded overall overall wall IAEA ISO capsule dla height thickness special classification Varfeus'i61BRS :offeifeiij' iot.s6\iicos Ih the fiptiowiSg type 'A' •B' C form ^tailS ore available bri rtquest mm mm mm X.3 22-4 31 1•2 SFC.9 C(E)64445 |- ••••;{: 2-3x1 Q*-;-- yj:C"^^ •:', '- X.4 22-4 48-5 1•2 SFC.10 E64445 'X.14 30 60 1•2 SFC.11 E64445 2O.QUg/-v-:•• 1 PTrtiCi :• -;• 4.r6 x 10°, ^ •••^V • .•/-;:.. : :500tig ;. .268nnCI•:.;.-.-.-1 -1.5x 10^.;•;-,.; ;•" Capsules X.3, X.4 and X.14 have 6M threads. Alternative threads to customers' specifications can be supplied on .request. -27- (b) Radio isotope (a ,n) Sources: It is possible to fabricate a small self- contained neutron source by mixing an a-emitting isotope with a suitable target material. This is due to the energetic a-particles which are available from the direct decay of a number of convenient radionuclides. The maximum neutron yield is obtained when Be is chosen as the target, and neutrons are produced through the following reaction: Which has a Q-value of+5.7 MeV. Some of the common choices for a-emitters and properties of the resulting neutron sources are listed in table (3) (taken from ref. 4). Table (3) Characteristics of Be(g,n) Neutron Sources . Pu/Be 24000y 5.14 9-33 210 Po/Be 138d 5.30 73 69 13 12 1JK Pu/Be 87.4y 5.48 79 "3TT Am/Be 433y 5.48 82 70 14 15-23 Cm/Be 18y 5.79 100 18 29 Cm/Be 162y 6.10 118 106 22 26 Ra/Be 162y Multiple 502 26 33-38 -^Daughters Ac/Be 21.6y Multiple 702 28 38 +Daughters 239 The Pti/Be source is probably the most widely used of the (a,n) isotopic neutron sources. However because about 16g of the material is required for ICi (3.7x10 Bq) of activity, sources of this type of a few centimeters in dimension are limited to about 10? n/s. Sources of 24lAm (Half-life of 433y) and "Vu (Half- life of 87.4y) are also widely used if high neutron yields are needed. In case of 241 Am/Be source, fig. (2) shows the stainless steel capsule types while table (4) gives its dimensions. Table (5) gives the activity, emission neutron n/s, capsule type and its code for this isotopic neutron source (taken from ref. 3). •28- Table(5): The activity, neutron emission in n/s, capsule types and their codes for 241 Am/Be source Fig.(2): The double Americium-241 /Beryllium walled stainless code activity emission capsule steel capsule of n/sec type 3 241 1mCi 2-2x10 X.2 AMN.11 3 3mCi 6-6x10 X.2 AMN.13 for Am/Be 4 lOmCi 2-2x10 X.2 AMN.16 source i 4 30mCi .. 6-6X10 X.2 AMN.16 X.2 30mCi 6-6x10* X.21 AMN.168 (X.1, X.211 5 100mCi 2-2x10 X.2 AMN.17 241 lOOmCi 2-2x10* X.20 AMN.170 Table(4): The dimensions of Am/Be capsules 6 300mCI 6-6X10 X.2 AMN.18 overall overall wall IAEA ISO X.3 capsule BOOmCl 1-1x10° AMN.19 type dia height thickness special classification 'A' •B' form 8 •c 1Ci 2-2X10 X.3 AMN.22 ; mm mm mm 3Ci 6-6x10° X.4 AMN.23 7-8 10 0-8 SFC.7 C(E)64344 5Ci 1-1x10' X.14 AMN.24 X.2.
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