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 anywhere along 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 PNPI 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:
•
Thus as the energy goes up, volume capture
goes
down relative to volume reflection. 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) |
