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Mass Spectrometer Modernization










Mass Spectrometer Modernization
Or
Teaching an Old Dog New Tricks

NBS 12-90 Upgrade
At Los Alamos National Laboratory


Foy 2001
ABSTRACT

























THERMAL IONIZATION MASS SPECTROMETRY
The solution is electroplated onto a rhenium filament, which is inserted into the ion
source of the mass spectrometer. A current is passed through the filament, which causes
the plutonium isotopes in the sample to ionize. The ions are accelerated through a
magnetic field, resulting in separation of the ions by mass, with heavier ions having more
momentum. An electron multiplier allows the number of ions of each isotope to be
counted. The amount of Pu-239 in the original sample is calculated by comparing the
number of those ions to those resulting from a known amount of Pu-242 spike. The Pu-
242 tracer was added to the sample prior to electroplating. These operations are
conducted in a Clean Room environment, and there are two mass spectrometers available
for sample analysis. The nominal detection limit for this analytical method is 0.5 fCi/l.
Approximately 300 samples, plus QCs, are analyzed per year for the LANL Bioassay
Project.

The Machine



The Mass Spectrometer consists of three major parts, a vacuum flight tube with a Source
at the front, a 90-degree bend with dipole bending magnet in the middle and a Detector at
the end. The modern electronics are as follows.

The Source

The Source has the sample filament and source lenses for drawing the ionized particles
into flight. The source lenses operate ~10Kv and are powered by eight 15Kv Bertan high
Voltage Power supplies. The filament supply is a Agilent precision 5Vdc, 20 A supply
that floats at the 10Kv or so operating voltage. The High Voltage supplies can be
controlled with GPIB or by means of a DAC. The DC filament supply may only be
controlled by GPIB through a fiber optic extender set in order to isolate the high voltage.

Bending Magnet

Once the Ion stream is focused and traveling the flight tube, the gaussian field on the
magnet is set to a value known to be able to deflect the desired particles toward the
detector. The magnet should not bend particles that are unwanted. The bending magnet
is a high impedance iron core dipole being of a 1960s design, requires a high voltage
low current power supply. The power supply will only be controlled by its remote DAC
input via the PXI chassis. A LakeShore 450 gauss meter/ controller measures the
Gaussian field and is the controlling feedback to the PXI chassis using GPIB.
The Detector

The detector is an Electron Multiplier Tube, which is powered by a 5Kv power supply, is
AC coupled to a low noise amplifier, fed to a peak counting discriminator, and counted
by the frequency counters. We also have an air actuated Faraday Cup which can be
inserted and counted by a precision pico-ammeter.

Support

All is to be controlled by a National Instruments PXI 8156B. The vacuum pumps, the ion
gage, the ion pump, the door interlocks, the pyrometer that measures the filament/sample
ionization temperature. High Voltage Run Permit will depend on the system seeing all
functions in a Go condition. Control software will be a combination of LabVIEW,
Component Works, and Visual Basic. All Data is to be downloaded into Excel files
which are then analyzed by the Nuclear Chemists. The Source will be automated with
stepper motors so that five samples can be rotated into place and increase our
productivity.