Prompt Fast-Neutron ActivationPromp

Prompt Fast-Neutron Activation
Prompt release of gamma-rays by fast-neutrons is the result of the excitation
of a nucleus and its immediate (within microseconds) return to
the ground state. The de-excitation of the nucleus can take place from
different nuclear levels, leading to several gamma-energies, the strongest
of which can be used to identify the emitting nucleus. It is important,
therefore, for the user to determine ahead the energies of the inelastic
gamma-rays for the elements of interest. Elements with high inelastic
cross-sections include C, Mg, Al, Si, and Fe. The excitation process can
be produced as a direct result of the neutron interaction with the nucleus,
or indirectly if the reaction product decays rapidly to a nucleus in
an excited state. An example of the latter is the reaction
which occurs at neutron energy of 9.63 MeV. The resulting decays
(with a 68% probability) by beta-emission with a half-life of 7.2 s producing
in an excited state that emits 6.13 MeV photons upon its
de-excitation. The transmutation of the nucleus by neutron absorption
can also result in the production of an excited nucleus; as in the case of
the reaction with releasing 3.09, 3.68 and 3.86 MeV
photons as the excited nucleus returns to its ground state. However, neutron
excitation of a nucleus is more often than not the result of inelastic
scattering,
Inelastic scattering requires a minimum energy at just about the excitation
energy of the target nucleus, typically over 5 MeV. Above 10 MeV
neutron-energy, the other non-elastic scattering reactions become competitive.
This makes it possible to use a variety of neutron sources in
this technique, including neutron generators and isotopic sources. Reference
[254] suggested that is an effective neutron-producing
reaction for a source suitable for bulk analysis, to a low level of detection,
of a wide variety of elements (Na, Mg, Si, P, Cl, K, Ca, Fe, Cu, An, Sn,
Ce, Ta, and Au). With an alpha-energy of 3.5 MeV, 5.6 MeV neutrons
are generated at zero angle with respect to the incident particle.
Table 8.6 lists the energies of the gamma-rays promptly emitted from
the first excited levels of some nuclei that can be activated by inelastic
scattering. The microscopic cross-sections for the reaction are also
given in Table 8.6 at an incident neutron energy of 14 MeV. It should be
Emission Methods
8.1.4.2
392
kept in mind, however, that the cross-section of inelastic scattering for
some elements can be higher at lower neutron energies, provided that
the energy is above the first excitation energy of the considered nucleus.
However, given the high neutron yield of 14-MeV neutron generators,
attention is paid here to neutrons at this energy. Table 8.6 also lists the
number of possible excitations levels up to an energy 14 MeV. The user
should examine various energy levels to determine which ones have the
highest cross-section, and which other elements present in the inspected
object can generate photons of similar energies that interfere with the
energies of interest. The evaluated nuclear data files (ENDF) cross-section
library, available on the Internet via reference [78], can be helpful in this
regard. The large number of energy levels shown in Table 8.6 for each
nuclide can make element identification with this reaction often quite difficult.
These many excitation levels also lead to background interference
problems, as other elements in the surroundings or the examined object
itself are excited by inelastic scattering. Prompt inelastic-activation is,
therefore, used for detecting the presence of a relatively large amount of
material, or high concentration of the element(s) of interest. Although
the cross-section for inelastic activation is generally lower than that of
the capture cross-section of thermal-neutrons, the ability to use unmoderated
fast-neutrons, emitted directly from the source, compensates to
some extent for the lower cross-section. Moreover, the lower degree of
attenuation of fast-neutrons enables the examination of larger objects.
Notice that the prompt emission of gamma-rays makes it unnecessary to
include the exponential decay terms in applying the measurement model
of Eq. (8.4) to prompt-activation analysis.

Kết quả (Việt) 1:

Kích hoạt nhanh-Neutron nhanh chóngNhanh chóng phát hành của tia gamma bởi neutron nhanh là kết quả của kích thíchcủa một hạt nhân và của nó trở lại ngay lập tức (trong miligiây)nhà nước đất. De-kích thích của hạt nhân có thể xảy ra từcấp độ hạt nhân khác nhau, dẫn đến một số gamma-năng lượng, mạnh nhấttrong đó có thể được sử dụng để xác định các hạt nhân phát ra. Nó là quan trọng,Vì vậy, cho người dùng để xác định trước các nguồn năng lượng của các không dản raGamma-quang cho các yếu tố quan tâm. Các yếu tố với cao không dản ramặt bao gồm C, Mg, Al, Si và Fe. Quá trình kích thích có thểđược sản xuất như là một kết quả trực tiếp của sự tương tác neutron với hạt nhân,hoặc gián tiếp nếu sản phẩm phản ứng phân rã nhanh chóng để một hạt nhân trongmột nhà nước vui mừng. Một ví dụ về sau này là phản ứngmà xảy ra vào năng lượng neutron của 9.63 MeV. Phân rã kết quả(với một xác suất 68%) bởi phiên bản beta-phát thải với một chu kỳ bán rã 7.2 s sản xuấttrong tình trạng vui mừng mà phát ra 6.13 MeV photon on của nóde-kích thích. Transmutation hạt nhân bởi sự hấp thụ nơtroncũng có thể dẫn đến việc sản xuất của một hạt nhân vui mừng; như trong trường hợp củaphản ứng với phát hành 3,09, 3,68 và 3.86 MeVphoton như là hạt nhân vui mừng trở lại để tình trạng đất. Tuy nhiên, neutronsự kích thích của hạt nhân một thường xuyên hơn không phải là kết quả của không dản ratán xạ,Tán xạ không đòi hỏi một năng lượng tối thiểu tại chỉ là về kích thíchenergy of the target nucleus, typically over 5 MeV. Above 10 MeVneutron-energy, the other non-elastic scattering reactions become competitive.This makes it possible to use a variety of neutron sources inthis technique, including neutron generators and isotopic sources. Reference[254] suggested that is an effective neutron-producingreaction for a source suitable for bulk analysis, to a low level of detection,of a wide variety of elements (Na, Mg, Si, P, Cl, K, Ca, Fe, Cu, An, Sn,Ce, Ta, and Au). With an alpha-energy of 3.5 MeV, 5.6 MeV neutronsare generated at zero angle with respect to the incident particle.Table 8.6 lists the energies of the gamma-rays promptly emitted fromthe first excited levels of some nuclei that can be activated by inelasticscattering. The microscopic cross-sections for the reaction are alsogiven in Table 8.6 at an incident neutron energy of 14 MeV. It should beEmission Methods8.1.4.2392kept in mind, however, that the cross-section of inelastic scattering forsome elements can be higher at lower neutron energies, provided thatthe energy is above the first excitation energy of the considered nucleus.However, given the high neutron yield of 14-MeV neutron generators,attention is paid here to neutrons at this energy. Table 8.6 also lists thenumber of possible excitations levels up to an energy 14 MeV. The usershould examine various energy levels to determine which ones have thehighest cross-section, and which other elements present in the inspectedobject can generate photons of similar energies that interfere with theenergies of interest. The evaluated nuclear data files (ENDF) cross-sectionlibrary, available on the Internet via reference [78], can be helpful in thisregard. The large number of energy levels shown in Table 8.6 for eachnuclide can make element identification with this reaction often quite difficult.These many excitation levels also lead to background interferenceproblems, as other elements in the surroundings or the examined objectitself are excited by inelastic scattering. Prompt inelastic-activation is,therefore, used for detecting the presence of a relatively large amount ofmaterial, or high concentration of the element(s) of interest. Althoughthe cross-section for inelastic activation is generally lower than that ofthe capture cross-section of thermal-neutrons, the ability to use unmoderatedfast-neutrons, emitted directly from the source, compensates tosome extent for the lower cross-section. Moreover, the lower degree ofattenuation of fast-neutrons enables the examination of larger objects.Notice that the prompt emission of gamma-rays makes it unnecessary toinclude the exponential decay terms in applying the measurement modelof Eq. (8.4) to prompt-activation analysis.