The low particle-energy required fo

The low particle-energy required for XRF makes it also necessary to use
sources with thin windows, generally made of Be, to minimize absorption
of the emitted radiation within the source itself. The low energy of the
emitted x-rays also causes a significant amount of attenuation within the
inspected object. If the object is quite thick, x-rays at the characteristic
XRP energy may not be detectable, and the technique can becomeuseless.
Therefore, XRF is limited to the analysis of small samples, or low-density
gases or liquid solvents. Even if the sample is sufficiently thin, the
intensity of emitted x-rays will depend on the sample thickness, due to
the attenuation of both the incident and emitted radiation. In order to
demonstrate this effect let us construct a measurement model for XRF.
Measurement Model
Consider the case of excitation by photons. Assuming that the detector
is shielded from direct exposure to the source photons, the detector
count rate, at a certain energy corresponding to an excitation
level, say can be expressed as [238]:
where G is a system constant that takes into account the source strength,
detection efficiency and system geometry, and are the total crosssection
of the incident and emitted photons, respectively, is the
atomic density of the element that produces XRF photons at the
energy, with a fluorescence yield of and is the microscopic crosssection
for producing these XRF photons. The integration in Eq. (8.12)
is carried out over the path of radiation within the sample. Because
of the small size of the sample, the distance traveled by the source
photons within the sample is assumed to be equal to the distance XRF
x-rays travel in their way out of the sample. The exponential terms
in Eq. (8.12) account for the attenuation of the incident and emitted
photons, while the other terms represent the probability of interaction
once the photons reach a distance within the object. The microscopic
cross-section, for a photon source depends on the photoelectric
cross-section, The latter cross-section, as shown in section 3.4,
can be expressed by the relationship of Eq. (3.30), which shows strong,
but continuous dependence, on photon energy, and the atomic-number.
However, as schematically shown in Figure 8.2, exhibits strong jumps
at the energies corresponding to the energies of XRF emission, as conditions
become most favorable for the XRF process. Owing to these jumps

Kết quả (Việt) 1:

The low particle-energy required for XRF makes it also necessary to usesources with thin windows, generally made of Be, to minimize absorptionof the emitted radiation within the source itself. The low energy of theemitted x-rays also causes a significant amount of attenuation within theinspected object. If the object is quite thick, x-rays at the characteristicXRP energy may not be detectable, and the technique can becomeuseless.Therefore, XRF is limited to the analysis of small samples, or low-densitygases or liquid solvents. Even if the sample is sufficiently thin, theintensity of emitted x-rays will depend on the sample thickness, due tothe attenuation of both the incident and emitted radiation. In order todemonstrate this effect let us construct a measurement model for XRF.Measurement ModelConsider the case of excitation by photons. Assuming that the detectoris shielded from direct exposure to the source photons, the detectorcount rate, at a certain energy corresponding to an excitationlevel, say can be expressed as [238]:where G is a system constant that takes into account the source strength,detection efficiency and system geometry, and are the total crosssectionof the incident and emitted photons, respectively, is theatomic density of the element that produces XRF photons at theenergy, with a fluorescence yield of and is the microscopic crosssectionfor producing these XRF photons. The integration in Eq. (8.12)được thực hiện trên con đường của bức xạ trong mẫu. Bởi vìKích thước nhỏ của mẫu, khoảng cách đi du lịch bằng nguồnphoton trong mẫu được giả định là tương đương với khoảng cách XRFchụp x-quang đi theo cách của họ ra khỏi mẫu. Các điều khoản mũtrong Eq. (8,12) tài khoản cho sự suy giảm của các sự cố và phát raphoton, trong khi các điều khoản khác đại diện cho xác suất của sự tương tácmột khi các photon đạt được một khoảng cách trong các đối tượng. Các vimặt cắt ngang, đối với một photon nguồn phụ thuộc vào các quang điệnphần, phần thứ hai, như được hiển thị trong phần 3.4,có thể được thể hiện qua mối quan hệ của Eq. (3,30), trong đó cho thấy mạnh mẽ,nhưng liên tục phụ thuộc, vào năng lượng photon, và hạt nhân số.Tuy nhiên, như được schematically Hiển thị trong hình 8.2, thể hiện mạnh mẽ nhảytại các nguồn năng lượng tương ứng với các nguồn năng lượng phát thải XRF, như điều kiệntrở nên thuận lợi nhất cho quá trình XRF. Do các nhảy