请问专家：X射线荧光光谱仪的X射线的穿透能力

X射线荧光光谱仪的X射线的穿透能力取决于X射线发射的能量与材料的性质，一般情况下，对于金属（钢铁）也就穿透100微米左右，但对于塑料，X射线非常容易穿透，因此如果金属元件被包含在塑胶里，在测试塑胶的时候一定要把金属元件取出来。

X射线非常容易穿透塑料,那最多能穿透多少?

请问是否有这方面的文字资料,谢谢

1. there is one formula to calculate that. You need the major elements composition. But by heart if your sample is mostly organic there's a good chance you are going through it...
One way to check that, is to use a backing sample. Run your sample in the cup as is, and then run it again with another disk as a backing (pure Fe or Zn, etc...). If you are going through your sample the backing material will show on your results.

2.The penetration depth of the X-ray beam is inversely proportional to the density of the material; that is, the higher the density, the lower the penetration depth. Low-Z elements, e.g. C, are transparent to the X-rays in our machine ( Bruker, nee Siemens, SRS 300, Rh tube ), and high-Z elements, e.g. Pb, are essentially opaque. The masks of our sample holders are made of graphite for this reason.

Penetration depth is also related to the wavelength of the X-radiation,
with shorter wavelengths absorbed less than longer ones. This means that infinite thickness of samples varies with the element of interest. If a sample is infinitely thick, that means the intensity of the analyte line is not a function of the thickness of the sample *and* the concentration of the particular element, but of the concentration of the element of interest alone.

PVC is mostly C and H, with Cl replacing some of the H. The average Z is therefore very low, and, without doing the calculations, I'll guess that your 1-2mm sample is pretty well transparent.

There is an equation in Bertin, Eugene P., Principles and Practice of X-ray Spectrometric Analysis, Plenum Press, which allows calculation of infinite thickness. The ISBN is 0-306-30809-6. If you can find a copy in the library the equation is on page 624 of my second edition. If not, then let me know and I'll transcribe it for you.
method of 1. can be used to determine infinite thickness empirically, but remember that it will only demonstrate infinite thickness or the lack of it for the analyte line of the material used. For instance,
infinite thickness for Al K-alpha won't be infinite thickness for the much more energetic Pb L-alpha line. So, be sure to use a backing pellet in your experiment which is meaningful in terms of your analytical problem.

3. the penetration depth for an element analyzed by XRF is a factor of the energy of the emission line of the element, and how well the sample matrix aborbs the fluorescence emitted by the element. It's the depth to which the x-ray emission can escape from the sample. You can calculate it by knowing the average mass absorption coefficient of the matrix for the element, and correcting for instrument geometry. Both calculations are in Bertin. You'll need to know your instrument's incident and exit geometry.
The link below can calculate a penetration value by energy if you know the molecular formula of the sample (PVC). It can help to give you a rough idea. Good luck.

4. To calculate the penetration depth we use the Togel equation (1), that is:

t = (6,91/MAC.d) for a 99.9% radiation generated.

t: Penetration depth
MAC: Mass absorption coefficient
d: density of material
The equation is:
t = (6.91/MAC*d)

MAC is in Cm^2/g and d is in g/cm^3. As 6.91 is a Log, then has no unit, t must be in centimeter.

To calculate MAC, you just consider C,H and Cl as pointed earlier. The trace elements wont contribute a lot.

this must be a special case since there is no term defining the angles between impinging X-rays and surface; the general case is like:

t = 6.91 * (1/MAC*d) * (1/sin(x) + 1/sin(y))**-1 where x and y are the incident and exit angles; 6.91 = ln(1/(1-0.999))

5. The penetration depth depends greatly on the exitation wavelengths for, and the emission wavelength of, the analyzed element. In glass disks, 1 or 2 mm is barely enough to ensure infinite thickness for most elements except low-Z elements. In organic materials like 1-2 mm PVC, I would say that a large fraction of the radiations go through the sample. I did not check this with calculations.

6. From a practical point I would like to add my 5c worth by saying it is not really the most important question.

The real question should be:
From what depth are the analytical lines coming ?
And here we are immediately at the issue of infinite depth.
The tube lines will surely penetrate right through your PVC sample.
That is not a great problem in itself.
Oxygen K lines will come only from the surface of the sample, so
no problem. Even if you excite Oxygen K lines 1mm below the surface
they will not be able to get out, so, for Oxygen K at least, your sample is infinitely thick. Na, Mg, Al, Si K-lines originate from progressively deeper down in the sample.
A very different story for the heavy element K lines. Here you will
be measuring characteristic lines emanating from the far side of your
sample and beyond as the backing test will prove to you.
To do reasonable analysis you want your samples and standards to
be comparable. Either they should all be exactly the same thickness
so that losses are comparable or they should all be infinitely thick.
An easy way to make your sample infinitely thick, for a given element,
is to change the analytical line. For heavy elements, which will be the first to suffer from lack of infinite thickness problems, you can try using an L line rather than the traditional K line.
Zn L-alpha is comparable with Na K-alpha for example. (around 1 Kev)
You can confirm this again with the backing test.
Things to remember if you try this are that the tube excitation conditions should be optimised for light elements such as Na rather than for Zn.
The bad news is the sensitivity will be reduced and if you are analysing traces, this might present a problem.


以上是我从一个国外的XRF论坛上Copy来的帖子，不知道有没有帮助。