Ar- Analysis at X19A:

Adele and I checked the Ar presence at X19A while doing Cl exafs, using NaCl as sample. NaCL sample and PIPS detector were placed inside the large sample chamber for multiple samples. The chamber was evacuated to

10E-5 torrr vacuum and data collected. He flowing in I0 ion chamber.

Few Observations:

We did flow a small amount of Ar in I0 ion chamber to check the Ar XANES.

When the monochromator was detuned by 50% using the Burleigh power supply bias voltage, we observed a strong Ar edge in I0.(Fig 1).

The XANES, PIPS / I0 is (Fig 2)

Fig 1

Fig 2

If it was detuned 50% in a normal way, (using gain, reference and detune knob etc.) then the Ar edge in I0 disappears and a glitch appears at the Ar edge.(Fig 3)

XANES, PIPS / I0 in this case is (Fig 4)

Fig 3

It is possible that there is a monochromator glitch at Ar edge.

Fig 4

We flushed the Ar from I0 ion chamber by flowing He, and then using NaCl as sample. The I0 ion chamber exit window and the sample chamber in window were placed butting together and EXAFS spectra of Cl were taken passing Ar edge. We see a dip at the Ar edge (3203 eV)      ( Fig 5)

Fig 5.

Finally we put a quarter inch Teflon tube between the I0 exit window and sample chamber enter window and put a tape across to trap some Ar gas in the gap by flowing Ar through the tube. The Cl EXAFS in NaCl shows a huge drop at Ar edge as evident from  (Fig 6).

Fig 6

We started flowing N2 in the same tube to replace trapped Ar from the gap and started collecting few spectra. It is evident from (Fig 7) that as the nitrogen started replacing Ar the edge drop at Ar got smaller and smaller.

Fig 7.

OUR CONCLUSION:

Any trace amount of Ar produces the dip in the spectrum at Ar edge. Air has 0.9 % Ar and low grade He has 0.5% Ar. There is Ar all over.

At X24 Joe Woicik works at Cl edge, but his sample and detector are inside a vacuum chamber, which is connected to ring vacuum, so he does not see any Ar. We did not try to keep one Kepton window between the I0 ion chamber and the sample chamber and evacuating the sample chamber. Fear was that in case the Kepton middle window will burst the shock wave might break the 6 micron Be window, putting X19A out of commission and risking the X-ray ring vacuum. But full vacuum is the way to go in order to do Cl EXAFS and getting rid of Ar edge.