Author Topic: Detector windows  (Read 3678 times)

Anette von der Handt

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Detector windows
« on: January 01, 2016, 09:04:59 PM »
Dear all,

I just started thinking about detector windows, their material and thicknesses and have a hard time finding details (maybe I am looking in the wrong places).

So does anyone know the material and thickness for

JEOL
gas-flow, sealed

Cameca
low pressure, high pressure (is there a difference?)

and if there are any differences between different models?

Thank you!
Against the dark, a tall white fountain played.

UofO EPMA Lab

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Re: Detector windows
« Reply #1 on: January 02, 2016, 03:00:36 PM »
I just started thinking about detector windows, their material and thicknesses and have a hard time finding details (maybe I am looking in the wrong places).

So does anyone know the material and thickness for

JEOL
gas-flow, sealed

Cameca
low pressure, high pressure (is there a difference?)

and if there are any differences between different models?

Hi Anette,
John here (working at UofO on a Saturday!).

I really don't know much about the JEOL detectors (maybe some JEOL people will chime in), but I can tell you a bit about the Cameca detectors.  The basic difference between JEOL and Cameca detectors is that JEOL uses a combination of Ar (P-10) flow detectors at 1 atm with a polypropylene window and Xe sealed detectors with a Be window, while Cameca generally uses all Ar flow detectors (a 1 atm Ar flow detector for light elements and a 2 atm Ar flow detector for higher energies).

The advantage of the JEOL sealed Xe detectors is that they have better high energy sensitivity (e.g., Zn Ka) than the Cameca 2 atm Ar flow detectors, but the Cameca detectors don't get contaminated or pumped out over time and therefore don't have to be replaced every 3-5 years as the JEOL Xe detectors do.  That said, I believe a "tandem" detector system using an Ar flow detector for low energy photons and a pin diode detector behind it for high energies, would be the best option as described here:

http://probesoftware.com/smf/index.php?topic=193.msg1831#msg1831

As for details on the Cameca detectors, the high pressure (2 atm) flow detectors have a 1 um Be entrance window while the 1 atm Ar flow detectors have an approximately 1 um stretched polypropylene window as described here from the early days when I used to make my own detector windows at LBL:

http://epmalab.uoregon.edu/UCB_EPMA/technical.htm#stretched

The interesting thing about the Cameca Ar flow detectors is that they tend not to get contaminated over time (as the sealed Xe detectors so), therefore the noise levels stay low, so all your spectrometers can be used for trace element analysis routinely.

Also, the Cameca detectors have a 127 um Be exit window on the back of the detector body (for reducing secondary photo ionization inside the detector?), which allows x-rays of 6 keV or higher to pass right through.  As the first link above describes, if a pin diode detector were mounted behind this Be exit window, it would very nicely detect high energy x-rays (e.g., Zn Ka) with better efficiency and the 127 um Be window sort of acts like a high pass filter for these x-rays so the pin diode detector merely needs to act as a photon counter without any cooling.   

I will be insisting on this technology or something similar in my next instrument purchase...

Think about this: if you have these "tandem" detectors on every spectrometer, every spectrometer could in principle, detect the full energy range of every crystal.  You could perhaps have a 4 crystal spectrometer with LiF/PET/TAP and LDE crystals and have excellent sensitivity for all x-rays from 12 keV down to 0.1 keV. In practice, you'd probably only install these "tandem" detectors on LiF/PET spectrometers, to improve the high energy efficiency.
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Anette von der Handt

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Re: Detector windows
« Reply #2 on: January 04, 2016, 11:38:48 AM »
Thank so much. That was very helpful. When I found out from JEOL I will it also update here.
Against the dark, a tall white fountain played.