Johann versus Johansson crystals

[wrigke]

I have searched through this forum and unless I am mistaken, I have not seen any posts on Johann versus Johansson crystals.  It is my understanding that when talking about standard-sized crystals, modern instruments come with Johann geometry crystals.  Why is this?  My understanding of Johansson crystals is that the intensity would be substantially larger than that seen with a Johann crystal, without much degradation in resolution, if any.

Has anyone compared like crystals with different geometries (e.g. PET Johann versus PET Johansson)?  What did you find?

Karen

[Mike Matthews]

Hi Karen,

The main benefit is that Johansson crystals give better resolution over the entire spectrometer range. Because of the additional manufacturing difficulties (both bending and grinding compared to just bending for Johann) they're more expensive. The Johansson crystals are usually denoted with a J (e.g. PET for Johann and PETJ for Johansson). I certainly have a mix of both types on my instrument.

Hope this helps.

Mike

[Probeman]

I also believe the area of the diffracting crystal over which diffraction meets the Bragg focal circle criteria is larger for the Johansson crystals. Love, Scott and Reed (1995, 2nd Ed.) says:

A better solution was given by Johansson (1932, 1933) who bent the crystal to twice the radius of curvature of the spectrometer circle and then ground away the inner surface of the crystal to a radius equal to that of the spectrometer circle. This ensured that the angle of incidence equalled the angle of reflection and that the Bragg angle remained constant over the entire surface of the cryatal (see section 3.1). It resulted in practically perfect focusing and gave a high diffracted intensity because the cryatal size was not unduly restricted.

john

[wrigke]

Hi Mike,

Do you have a JEOL?  I have seen the "J" designation on JEOL crystals, but was unaware of what that meant.  I don't know if Cameca makes "J" type or not.  Certainly I do not have any.

Does anyone know if Cameca supplies "J" type regular-sized crystals?  Or perhaps who the supplier is for JEOL crystals?  Perhaps they could be convinced to make one for Cameca.

Cheers,

Karen

[Anette von der Handt]

Hi Karen,

my notes and vendor documentation and communications give:

for Cameca:
Johann: all (regular sized) TAP, PET, LIF, all synthetic PC crystals (both regular and L-size)
Johansson: all (large sized) LTAP, LPET, LLIF

The same for JEOL:
Johann: TAP, TAPH, PETJ, LIF and LDE synthetic crystals
Johansson: All H- and L-type crystals

Anette

[Ben Buse]

Hi,

I've been trying to do some measurements to determine if the crystals are Johann or Johansson. With Johann the resolution degrades to low angle.

I've measured FWHM on Ka lines for Si, Ti, V, Cr. And 2nd order ka lines for Ti, V, Cr, Mn, Co. Pure metal standards were used. The results are:



Now looking at the natural line widths - although increase with z, these are an order of magnitude less so should be safe to ignore.

What see is both PETJ and PETL behave the same, and appear Johann like.

The FWHM expressed as angle change in theta is ca. 0.1 max.

What is odd is that the 2nd order interferences are offset to worse resolution.

Ben

[Probeman]

Hi,

I've been trying to do some measurements to determine if the crystals are Johann or Johansson. With Johann the resolution degrades to low angle.

I've measured FWHM on Ka lines for Si, Ti, V, Cr. And 2nd order ka lines for Ti, V, Cr, Mn, Co. Pure metal standards were used. The results are:



Now looking at the natural line widths - although increase with z, these are an order of magnitude less so should be safe to ignore.

What see is both PETJ and PETL behave the same, and appear Johann like.

The FWHM expressed as angle change in theta is ca. 0.1 max.

What is odd is that the 2nd order interferences are offset to worse resolution.

Ben

Hi Ben,
I believe that PET crystals are always Johann geometry because they are not amenable to grinding the R/2 curvature.
john