Measuring Particle Speed at CERN: A Physicist's Guide

[chrisakel]

does anybody know how scientists and physicists at places such as CERN determine the speeds of the particles that they accelerate to nearly the speed of light? basically, what is their technique for determing the speed of the particles?

 

[ZapperZ]

does anybody know how scientists and physicists at places such as CERN determine the speeds of the particles that they accelerate to nearly the speed of light? basically, what is their technique for determing the speed of the particles?

They measure the kinetic energy. In most cases, depending on the resolution, particles such as electrons are essentially at c after they reach the order of MeV of energy.

Zz.

 

[chrisakel]

thanks, but do you have any idea how they measure the kinetic energy, there has to be a very specific method that they use due to the speeds of the particles?...I actually e-mailed a CERN scientist and asked him this very question but unfortunately I received no reply.

 

[ZapperZ]

thanks, but do you have any idea how they measure the kinetic energy, there has to be a very specific method that they use due to the speeds of the particles?...I actually e-mailed a CERN scientist and asked him this very question but unfortunately I received no reply.

It is similar to a mass spectrometer. You channel the beam into a bending magnet, and then look at where the beam hits the screen by adjusting the strength of the magnet. The strength of the magnet, if calibrated correctly using known energy/momentum of the particular charged particles, can directly tell you the energy of the particle.

Zz.

 

[chrisakel]

Thank you...this is probably quite a vague question, but it sounds to me like this technique is somewhat, if not fully dependent on light, does that sound right?...In other words, in the complete absence of light, would they still be able to determine the particles kinetic energy and thus its speed?
Thanks again.

 

[ZapperZ]

Light? What light?

Zz.

 

[chrisakel]

I must be getting confused when you say that you channel the "beam"...what is it a beam of?

 

[ZapperZ]

I must be getting confused when you say that you channel the "beam"...what is it a beam of?

A beam of charge particles that you are trying to measure! Channeling a beam of light through a bending magnet makes no sense.

Zz.

 

[chrisakel]

ok, I see...it makes sense now, I had a mental block going on...Thank you

 

[ice109]

couldn't you measure a moving particle's energy using relativistic kinematics, collide it with something big, see how fast it speeds away? or can you not guarantee conservation of energy?

 

[rcgldr]

IIRC, for a linear accelrator, simply time the transition of the magnets, and check to see if any electrons made it through the narrow timing window which requires high speed in order to sync up with the transition speed. Either you get a "hit" and an electron is going at a predetermined speed, or you get a "miss". With a D type accelerator, the radius of the path and the strength of the magnetic field can also be used to determine the speed of an electron.

Since the acclerators can't really "react" to a particle, the speed is basically predetermined by the strength and timing of the magnetic fields.

 

[ZapperZ]

couldn't you measure a moving particle's energy using relativistic kinematics, collide it with something big, see how fast it speeds away? or can you not guarantee conservation of energy?

1. You can't guarantee collision with any particles. This is the issue of luminosity.

2. High energy collisions, which by nature is required for these particles, will produce a zoo of effects. That's the reason in many cases to accelerate these particles in the first place. It is not that simple to get a calibration of their energies this way.

If you inject these particles into your beamline, you always get a range of momentum/KE for them AND the beam particles you want to measure. This is not very efficient to measure the beam's energy since you're introducing another unknown factor.

IIRC, for a linear accelrator, simply time the transition of the magnets, and check to see if any electrons made it through the narrow timing window which requires high speed in order to sync up with the transition speed. Either you get a "hit" and an electron is going at a predetermined speed, or you get a "miss". With a D type accelerator, the radius of the path and the strength of the magnetic field can also be used to determine the speed of an electron.

Since the acclerators can't really "react" to a particle, the speed is basically predetermined by the strength and timing of the magnetic fields.

Can you cite for me where this technique is actually done? It isn't done at SLAC as far as I know. Furthermore, magnets have hysteresis and self-induction. You NEVER get them to react that fast!

Zz.

 

[rcgldr]

From a wiki artiticle:

"As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at microwave frequencies, and so RF cavity resonators are used in higher energy machines instead of simple plates."

http://en.wikipedia.org/wiki/Particle_accelerator

My point is that the speed of accelerated particles is pre-determined by the rate of change and strength of magnetic fields. Accelerators don't "react" to the presence of the particles they accelerate, they are just "pre-programmed" to generate a pre-determined amount of acceleration.

In the case of linear accelerators, distance between fields can increase along the path to limit the rate of transition as a particle increases speed.

 

[ZapperZ]

From a wiki artiticle:

"As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at microwave frequencies, and so RF cavity resonators are used in higher energy machines instead of simple plates."

http://en.wikipedia.org/wiki/Particle_accelerator

My point is that the speed of accelerated particles is pre-determined by the rate of change and strength of magnetic fields. Accelerators don't "react" to the presence of the particles they accelerate, they are just "pre-programmed" to generate a pre-determined amount of acceleration.

In the case of linear accelerators, distance between fields can increase along the path to limit the rate of transition as a particle increases speed.

That wikipedia article was one of my poster child for what is wrong with wikipedia. That page is full of erroneous information.

Besides, you have completely misunderstood what was written. They are using the RF field as the accelerating mechanism in a LINAC. This has nothing to do with a "magnetic field" that is generated in coils. The speed of the particles are NEVER determined by the magnetic field! These are TM or TEM mode where the magentic field are not only transverse to the cavity, but plays no role in the particle dynamics close to the axis of a pill-box cavity, which is the most common element in a LINAC structure. Besides, since when can a charge particle gain any energy out of a magnetic field?

You can't simply "infer" the energy just because you put such-and-such field into the cavity. Nothing is "predetermined" just because you put in so-and-so amount of power. You still have to measure it because beam-loading effects, space-charge effects, and other external factors all conspire to change whatever it is that you have. Besides, this is not a static field. You'll be laughed at if you simply think that you can get away with claiming a certain amount of energy gained simply because it has been "predetermined". If it is, then all those plasma wakefield measurement group of the gradient achieve were wasting their time making an actual measurement of the energy gain by their electron bunch when all they could do is simply argue that it has been "predetermined".

Wikipedia citation is a no-no. This page in particular is exceedingly bad. I asked for a citation on where this is USED. Being familiar with particle accelerator, I have never heard of any accelerator facility that determines the particle's energy the way you mentioned.

Zz.

 

[ZapperZ]

I have mentioned this already a few times, but I'll highlight this again. There is a website for the US Particle Accelerator school, in which many of us who decided to go into this area, take courses offered over either the summer, or a period of several months. These course are fully accredited and carry equivalent college credits. Most universities in the US, and throughout Europe and Asia, recognize these courses (there are similar Asian and European particle accelerator schools). The webpage for the USPAS is

http://uspas.fnal.gov

Look at the course material. There are free lecture notes given in the course material section. In particular, look at lecture notes on the Fundamentals of Accelerator Physics and Technology, and the chapter on REAL Particle Accelerators.

http://controls.als.lbl.gov/als_physics/Fernando/USPASJun07Lectures/

.. and figure out how momentum and momentum spread of the particles are measured. These directly give you, with appropriate scaling law, the energy.

You'll find none of these appearing anywhere in Wikipedia's page on Particle accelerators.

Zz.

 

[rcgldr]

The speed of the particles are NEVER determined by the magnetic field!

I didn't mean to imply that. What I meant was for a given speed of a particle, the strength of the field determines it's path (radius). Getting back to the OP:

How do ... scientists and physicists determine the speeds of the particles that they accelerate?

It's mostly pre-determined by the accelerator. When acceleration is done, it involves cycling of fields at a high rate, and it's up to the particles to end up in a window of time that syncs up their position, speed, and acceleration with the cycling of the fields.

 

[Danger]

ZZ, in very simple terms, can you give me an idea of what 'beam-loading' and 'space-charge' effects are? I know next to nothing about particle physics (might as well make that nothing compared to you), but it sounds interesting.

 

[chrisakel]

Thanks for all the information