"Why are
you running external filters?
Dud"
?
It's all about the
Bremsstrahlung continuum.
?
In the X-Ray tube
electrons
are accelerated by a HV field, then are slowed down by a target. Notice I
didn't mention the filament. Some designs don't use a filament, they are called
Field Emission Tubes and are cold-cathode. Those aren't continuous output tho
and not used for XRF. Rather they build up a charge, 150 kV or more and
"fire" a pulse at the tube, similar to a photo flash. It's all over
in a few nanoseconds, but for X-Ray film it's OK, they just fire a dozen or
hundred pulses, really quickly.
?
For XRF we need a much
weaker ray, but it needs to be left on from minutes to hours- those tubes
require hot cathodes= filament to boil off some electrons free of the cathode
(the filament usually is also the cathode), a steady HV DC accelerates them,
they are focused into a beam by deflection plates, irises and magnetic focus
coils in a microfocus tube to get a small spot-.
Broad band Bremsstrahlung
radiation is created,?and if the peak HV is sufficiently above the Ka
binding energy of the target material, a huge peak at the targets
characteristic X-Ray energy(s) superimposes over the top of the broad Brems
continuum.
?
?
So? X-Ray tubes are
not by their very nature monochromatic.?
isotopes on the other hand
are monochromatic, but of many discreet energies.
?
It would be nice to have a
knob that said 0-50 kVp and mean it. Set it for 35 and you've actually set it
to "not over 35kVp", below it is a huge spray of energies up to the
PEAK HV available. kVp= kiloVolt Peak. Nor is most of the wattage in the peak
value, it is considerably below that, like 1/3 of the peak is the bulk of it.
?
My filters are just
absorbers that remove the lower energies, the one that happen to be exactly on
the energy peak you are trying to measure. Next the aluminum collimator absorbs
much the off-axis rays which do nothing but try to create interference from
your shielding.
?
Fortunately only a tiny
amount of power is needed to do XRF. By the way, Amptek specifically states
their warranty does not cover radiation damage to sensors, not to mention the
thin beryllium window is as fragile as a pancake probe.
?
Geo>K0FF
?
What
tube and power supply are you running?
Mine is
a W side exit window
Why are
you running external filters?
Dud
?
Cool,
that's a beast compared to mine. Max is 0.1 mA (100 microamp) here.
It
does have its own shielded housing, and I suspect the narrow measured beam cone
angle is controlled by an mechanical iris inside the sealed tube. I'm assuming
yours is also a side exit not and end-fire, those use transmission targets, the
kind I use have "reflection" tungsten targets.? The excitation
HV is less than the W Ka, so all I work with are Bremsstrahlung X-Rays, thus
the external filtering etc.
-----
Original Message -----
From: Dude <dfemer@...>
To: [email protected]
Sent: Sun, 09 Feb 2020 15:30:33 -0500 (EST)
Subject: Re: [XRF] Li battery shiny vs dull
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Geo,
My
Kevex¡¯s are PXS4-613W and ?run with a CU015 60 kV power supply.
Target
Voltage:? 5.0 to 60.0 kV
Current:?
0.01 to 1.00 mA
X-ray
source spot size: 0.01¡± x 0.020¡± (0.25mm x 0.45 mm)
Approximately
X-ray
Output: 101R/min at 60 watts
X-ray
Window: 0.005¡± (0.13mm) Be
Angle
of illumination: 35 degrees inclusive (Approx)
?
When
you first fire it up or when it¡¯s been sitting for a long
while ramp up the voltage in 20% steps at 10 % of max current steps ?to
allow
the tube to outgas. Do this over a 10- to 15 min period (1 minute minimum at
each step) before going to full power. Watch out for arcing. ?Don¡¯t run
prolonged operation at 5kV or less at high beam current as that will run the
filament
hotter than normal and will decrease the filament life.
They
warn that the X-rays are dangerous and ?may be fatal. The
X-ray tube window must be placed in a shielded chamber. It¡¯s also clearly
stated
not to let anyone named Geo operate this machinery.
Dud
?
?
Geo,
I have
the specs here someplace but I don¡¯t remember exactly. 16
seems about right and the microfocus spot was very small as I remember.
±õ¡¯±ô±ô
dig around and see f I can find the manual
?
?
That
sounds logical Dud, hey on your Kevex stand alone tube, do you know the spot
size and the cone angle? The one in my rig here in the counting room seems to
have a 16 degree cone- does that sound right?
