Take the controls
A simplified simulation of the EMMAS 3 MV desk — the same sequence our operators follow
Getting an ion beam onto a target is a sequence, not a switch. You pump the beam line down, bring the terminal up to voltage, tune two magnets so that only the ion you want survives the trip, and then let the beam through to the chamber. Everything below runs on the real physics the machine obeys — magnetic rigidity, the tandem energy equation, and Rutherford kinematics — so the numbers you read out are the numbers an operator would read out. Follow the checklist and you will end up with a genuine RBS spectrum of your sample.
This is a teaching model, not the control software. Timescales are compressed and many interlocks are omitted.
Vacuum & interlocks
Ion injector
Terminal & stripper
Switching magnet
Beam profile monitor
Sample & acquisition
Run checklist
- Beam line under vacuum
- Source on, injector tuned
- Terminal at voltage
- Switching magnet on port
- Beam on target
- Spectrum acquired
Operator log
Beam-line synoptic
Multi-channel analyser — backscattered particle energy
What the console is doing
Three equations run the whole machine
Magnetic rigidity
A magnet bends a charged particle onto a fixed radius only if its momentum matches the field. For a beam of mass m, charge state q and energy E, the field the magnet must supply is B = 0.1439 · √(mE) / (qρ). Both magnets in the simulation are mass–energy filters: turn the knob away from the right value and the beam simply is not there. That is how an operator picks one isotope out of everything the source produces.
The tandem trick
Negative ions from the source fall through the terminal voltage once, get stripped of electrons in the argon canal at the terminal, and — now positive with charge state q — are pushed back out through the same voltage again. The beam therefore leaves with E = (1 + q) · VT + Vinj. It is why a 3 MV machine can deliver far more than 3 MeV, and why the carbon foil, which strips harder than the gas, buys energy at the cost of beam lifetime.
Rutherford kinematics
An ion that bounces backwards off a target nucleus keeps a fraction of its energy that depends only on the two masses and the scattering angle — the kinematic factor K. Heavy nuclei give the projectile back almost everything; light ones absorb the recoil. Read the energy of an edge in the spectrum, invert K, and you have the mass of the element that produced it. Depth adds up as a tail below each edge, because ions lose energy on the way in and on the way out.