How can we measure the wavelength of gamma-rays?

  • B
  • Thread starter avicenna
  • Start date
  • #1
46
3
How can we experimentally measure the wavelength of gamma-rays, say for about 0.7MeV?
Can it be done without gamma-ray spectrometry?
 

Answers and Replies

  • #2
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2019 Award
25,664
8,854
We don't measure wavelengths. We measure energies. If we want wavelength (and why?) we calculate it.
 
  • #3
Astronuc
Staff Emeritus
Science Advisor
18,929
2,245
How can we experimentally measure the wavelength of gamma-rays, say for about 0.7MeV?
Can it be done without gamma-ray spectrometry?
What wavelength does one calculate from the stated energy?

Bristol Instruments provides instruments to measure wavelength using interferometry, but the instruments are limited to light with wavelengths of 375 nm.
https://www.bristol-inst.com/bristol-instruments-products/wavelength-meters-scientific/

Otherwise, as V50 indicated we measure energy and calculate wavelength or frequency.
 
  • Like
Likes vanhees71
  • #4
hutchphd
Science Advisor
1,985
1,324
I am confused by these answers. What then does Bragg diffraction of X-rays from a crystal measure?
 
  • #5
34,955
11,143
It gives direct wavelength measurements in the x-ray range, but I doubt this will give useful results at 700 keV.

Measuring the energy and calculating the wavelength will be better.
 
  • Like
Likes vanhees71
  • #6
hutchphd
Science Advisor
1,985
1,324
Yes it certainly is easier. I am undecided as to whether that is actually a measurement of wavelength It is a measurement of energy from which wavelength may be inferred. This is largely a semantic point I guess (but not entirely so?) I guess all measurement requires a chain of inference.
As a practical matter it looks like the limit is indeed about 300keV photons for doing useful crystalline diffraction (this is my cursory literature search).
 
  • #7
Astronuc
Staff Emeritus
Science Advisor
18,929
2,245
I am confused by these answers. What then does Bragg diffraction of X-rays from a crystal measure?
I thought of Bragg diffraction, but 'measuring' the wavelength requires 'knowing' the distance between atoms in crystal. How does one 'know' or 'measure' the distance between atoms? X-ray diffraction.

Here is a paper that indicates "Measurement of X-ray spectral line wavelengths by using two Bragg reflections"
https://www.sciencedirect.com/science/article/abs/pii/0895399690900079

The term 'measuring' is important, in contrast to 'determining'. For example, we can measure distance or displacement, but we do not measure stress. Stress is calculated.
 
  • #8
34,955
11,143
How does one 'know' or 'measure' the distance between atoms? X-ray diffraction.
Or atomic force microscopy, these days. By far less precise than x-ray diffraction of course.
You can also measure it using interference with visible light (at large incidence angles), relating the x-ray wavelength to a measurement of visible light wavelengths that can be done with other methods.
 
  • #9
Astronuc
Staff Emeritus
Science Advisor
18,929
2,245
Or atomic force microscopy, . . .
I thought about AFM, but wasn't sure. I'm curious about the resolution. I could not find a definitive statement, other than noise on the order of 10s of pm. A 1 MeV gamma ray has a wavelength of ~1.24 pm (~0.00124 nm), so diffraction methods are too coarse. Atomic radii of most atoms are on the order of 100 pm, and lattice constants slightly larger.
 
  • #10
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2019 Award
25,664
8,854
We're going far afield from the original question.

You can calibrate the x-ray wavelength from diffracation without being circular. You use x-ray diffraction to get the crystal structure - you don't need the spacing here, just the pattern. From the density and the atomic weight plus the crystal structure you get the atomic spacing. From the atomic spacing and the diffraction pattern you can get the wavelength.
 
  • Like
Likes Astronuc, hutchphd and mfb
  • #11
34,955
11,143
I thought about AFM, but wasn't sure. I'm curious about the resolution. I could not find a definitive statement, other than noise on the order of 10s of pm. A 1 MeV gamma ray has a wavelength of ~1.24 pm (~0.00124 nm), so diffraction methods are too coarse. Atomic radii of most atoms are on the order of 100 pm, and lattice constants slightly larger.
You gain a lot from the angle. If you can determine the height of a layer to be e.g. 100 pm +- 3 pm your relative uncertainty on the wavelength from the lattice constant will just be 3%, even if you measure wavelengths below 10 pm because the diffraction angle is small. But, as V50 noted, you can also measure the mass of an atom and the mass of a macroscopic object.
 
  • Like
Likes Astronuc

Related Threads on How can we measure the wavelength of gamma-rays?

  • Last Post
Replies
7
Views
1K
Replies
22
Views
2K
  • Last Post
Replies
2
Views
7K
  • Last Post
Replies
4
Views
4K
  • Last Post
Replies
1
Views
3K
Replies
1
Views
612
  • Last Post
Replies
2
Views
3K
  • Last Post
Replies
1
Views
2K
Replies
6
Views
839
Replies
6
Views
2K
Top