End Section


This is the last section of the experiment where the actual detection of the resultant Hydrogen molecules occur. After the quadrupole lens cleans out the charged particles from the stream only neutral H atoms and H2 molecules remain (Actually the quadrupole can pass protons created when atoms strip close to the electrodes, so a Cleaner electrode 12.8 cm downstream of the quad removes them from the stream). At this point the ~100 molecules/sec of H2 must be removed from the 1011 atoms/sec of H. To do this we use a gas cell to strip a portion of this stream so that afterwards the charged particles can be seperated by an electrostatic energy analyzer.

Gas Cell

The neutral particles then enter our gas cell which has a length of 78.7 cm, beginning at the Quad. The entrance aperture is a circular 1.6 cm diameter tube. He gas is let in via a leak valve. Typically 2x10-4 Torr of Helium pressure is used, while base pressure is ~3x10-7 Torr. The pressure is meausred by a hot ion guage. In order to count the amount of H2 molecules we need to know what portion of these are stripped when passing through the gas. This is done by using measured stripping cross sections and determining our He column density through the gas cell. The cross section used is 1.04x10-16 cm2 (Browning et al, J Phys B, 1970). From the cross section and our determined column density we find that 5% of our H2 moleucles are stripped into H2+.

Energy Analyzer

The H2+ is seperated from the rest of the stream by an electrostatic energy analyzer. The intial H- ions typically have energies of 10 keV, so an H2+ molecule has 20 keV and is easily seperated.

Channeltron Electron Multiplier

After the H2+ is seperated it is counted by a Channeltron Electron Multiplier (CEM). The CEM has a conical opening leading to a serpentine shaped tunnel. The insides of the cone and tunnel are coated with lead glass material. When a potenial difference is applied across this tunnel, incident particles will crash into the walls, splashing off electrons. The electrons pinball their way through the tunnel gaining more electrons everytime they hit the walls, the electron cloud is saturated at 108 electrons. This electron avalanche makes its way through until it hits the end anode where the pulse of electrons is measured. Output pulses are easily measured using a discriminator, preamplifier and a counter.

https://docs.astro.columbia.edu/attachment/wiki/SavinGroup/MergedBeams/EndSection/cem.jpg?format=raw https://docs.astro.columbia.edu/attachment/wiki/SavinGroup/MergedBeams/EndSection/PositiveHV.gif?format=raw
Sjuts website (http://www.sjuts.com/Introduction_Principles.html)

Neutral Cup

Here are the drawings for a neutral cup that will be installed soon that will be installed to monitor the neutral H atom beam.

The Neutral cup measures our neutral H atom current. This is done by measuring the secondary electrons that are splashed off by H atoms crashing into a collision plate. The plate is within a tube which will have a positive potential on it (+300-400 V). The secondary electrons are sucked onto this tube and the resulting positive current is read off of the collsion plate. The postitive electrode tube has to be protected by the incident beam, so a grounded shielding tube protects the postive tube from the beam. The cup must be calibrated because with different vacuum conditions the amount of electrons that come off of the collision plate can change. All parts were made of stainless steel.

The length of the positive electrode tube should be at least 2 times the diameter. This cup needs to be calibrated for the number of secndary electrons to incident neutral particles. This ratio is referred to as Gamma. Typical values of Gamma in other designs range from 1 to 1.5.