Volume channeling effects  updated October 25, 2007  D. Carrigan carrigan@fnal.gov (subject line must be sensible)

Channeling Formulary

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What to call Fliller-Still effect shown to the right?

  • Whole arc channeling (gets at characteristic of process)
  • Volume reflection (probably correct)
  • Volume capture (probably wrong)
  • Vorobiev-Taratin effect (they sort of predicted but does not include accelerator aspect)
  • Fliller shoulder (yes, RHIC was initial observation, but Tevatron confirmation helped)
  • Fliller-Still shoulder (includes Tevatron confirmation)
  • L5 effect (silly but 42% of Fliller-Still is "l"s)

Possible explanations for whole arc or Fliller-Still effect

  • Miscut angle for crystal
  • Something else

Effects are summarized in table to the right

Were there volume channeling effects in the Tevatron extraction experiment?

Fliller-Still effect

Fliller-Still effect
coherent bend effects

Volume reflection (Taratin-Vorobiev)

Particle bounces off plane at some place in passage through a curved crystal. Deflection order of ψc

Volume capture (Sumbaev, Samsonov/PNPI)

Particle scatters into channel with lower transverse energy and remains there. Continues to end of bend.

Volume reflection (top)


Volume reflection was discovered in simulations by Taratin and Vorobiev in the eighties [Phys.
Lett A, 119, 425 (1987 for English language version]. In essence particles reflect off of planes when they are nearly parallel and are deflected on the order of a critical angle away from the bend. The process will occur over the whole arc of the bend. It can be cumulative for many passes.

Since the expected deflection is O(θc) the deflection will go as 1/(pβ)½. R does not appear but θc for a bent crystal will be a function of R. As a result the effect will diminish more slowly than volume capture as the energy increases.

This is why many expect the whole arc effect seen at RHIC and the Tevatron is due to volume reflection.

Useful to understand p, R scaling since we are extrapolating to LHC



Volume capture (top)

“Volume capture” is the putative process whereby particles outside a channel in a bent crystal diffuse into the channel. It was first investigated at Gatchina by Samsonov, Sumbaev and their colleagues at 1 GeV. Volume capture should deflect in the direction of the bend.  This diffusion process is an analog of dechanneling where the particles diffuse in to the channel. The process occurs over the whole arc of the bend. Deflections can range up to the whole arc of the bend.

In their book Biryukov, Chesnokov, and Kotov (Crystal channeling …) give a formula (BCK 5.27) for the transition probability to diffuse into the channel as:
formula
Thus as the energy goes up, volume channeling goes down. As R gets smaller (tighter bend) it also decreases. Biryukov, et al., have shown that this relation holds true for 70 GeV protons and is characteristically small compared to ordinary bent crystal channeling (Fig. 3.30 in BCK)