Path: santra!tut!draken!kth!mcvax!uunet!shelby!labrea!brooks@sierra.Stanford.EDU
From: brooks@sierra.Stanford.EDU (Michael B. Brooks)
Newsgroups: alt.fusion
Subject: Re: ptl sum. of st. fe wksp. (off the wall mechanisms)
Summary: ions, energy losses in Pd, LONG
Message-ID: <140@sierra.stanford.edu>
Date: 29 May 89 22:02:18 GMT
References: <TED.89May24004251@kythera.nmsu.edu> <1989May24.115628.22755@cs.rochester.edu> <41@anasaz.UUCP> <1554@hudson.acc.virginia.edu> <137@sierra.stanford.edu> <138@sierra.stanford.edu>
Sender: brooks@sierra.STANFORD.EDU (Michael B. Brooks)
Reply-To: brooks@sierra.UUCP (Michael B. Brooks)
Lines: 103

Distribution, 

A mail note from Dale Bass made me realize that I have been less than
careful with my discussion of charged He and its energy losses.

According to the "range distribution" data I cited in my last post,  a
30MeV He (alpha) could escape to the Pd surface from an interior 
generation site (at which the CNF event occurred) if it was less than
0.627 mm deep.  It would not escape if deeper because it's energy would
be drained away by processes that do not produce gammas---necessarily.

The processes are:
     "The interactions are usually separated in binary elastic collisons
with the screened nuclei of the target atoms, and inelastic collisions with
the entire electron system of the target." [from U.Littmark and JF. Ziegler,
Range Distributions for energetic Ions in All Elements, see my last post]

Put another way by Chu, Mayer and Nicolet [see last time also], p.39:

     "For the light projectile atoms and the energy range of interest to back-
scattering spectrometry, the two dominant processes of energy loss are the 
interactions of the moving ion with the bound electrons or free electrons in 
the target [Bethe-Bloch ionization losses and electronic "frictional" losses
respectively], and the interactions of the moving ion with the screened
or unscreened nuclei of the target atoms [nuclear stopping].

The nuclear stopping is less important at higher energies but actually causes
the particle to come to rest after it has lost much KE; it is not equal to 
a nuclear collision of high energy.  Instead, it may be thought of as a series
of "small angle scattering collisions of the projectile with the atomic
nuclei of the target."  

All of these processes are "electrostatic in nature" and compete with higher
energy processes to reduce a particles` energy.  They do not produce gammas,
though other excitations such as xrays can result (as well as Auger processes).

Some of the other processes that compete are

1. Primary Gamma Emission (instead of a
high energy particle from the CNF event) leading to a whole variety of 
things including the pair-production<-->gamma reemission mentioned last time.
This is the "cascade" producing phenomena that should be readily detectable.

2. Radiation Loss of Electrons: if high energy electrons are produced 
at some point in the process following CNF, then bremsstrahlung
radiative process become important.  According to Perkins (Introduction to
High Energy Physics, 3rd Ed., 1987, Addison-Wesley), pp 42-45;

     E(subc) = 600/Z [in MeV] describes a critical Energy Ec where the

rate of ionization losses (Bethe-Bloch again) are equaled by high energy 
photon losses due to the bremsstrahlung.  For Z = 46 (Pd), this is 13 MeV.

3.  Gamma Ray Attenuation: through one of three processes (mentioned two
posts ago), photoelectric absorption, Compton scattering and pair 
production.  The latter is dominant for E>10MeV (see before).

4.  Direct nuclear collisions or reactions: my catch all group for all sorts
of high energy products. These might be created from the  collision of
a high speed
particle, like an alpha, with Pd nuclei encountered closely enough so that the 
strong forces play a role in energy loss.  This is distinctly not an 
elastic process, unlike the "nuclear stopping" referred to above.

This is far from a  complete list, and I wish someone with an atomic physics
background would elaborate on it, as I am getting out of my field of 
expertise.  However, with this in mind some conclusions can be drawn:

*  For high energy alphas, much if not most of the energy loss comes from
the "electrostatic-like" mechanisms described initially. The cross-sections
for the processes are large in the energy regimes of interest.  As far as
"channeling" of particles is concerned---it won`t happen in a poly-
crystalline material.  The ranges are fairly large anyway.  Paul Deitz 
mentioned the problem of directionality; meaning that if some alphas go
out of the Pd, others must go in.  This is a valid point, yet IF the only
processes are these, it makes no difference whether they go in or out,
a gamma free process may (Note, may) result.  One could look for xrays
and embedded He (via SIMS) to help check on whether alphas are important
or not.

*  The cross-sections for the competing processes 1-4 above are not 
negligible---indeed 1 may be the central CNF energy loss means---so
radiation of some sort of gammas is very probable.  A brief calculation
for attenuation of 13MeV electrons shows that these can reduce their energy
to 1/e of the initial value in ~0.8cm through bremsstrahlung  (Perkins,
pg. 43 Eqn 2.14, radiation length guessed at ~8-10 g/cm^2, see Table
2.2).  Lots of radiative possibilities there.  Likewise, 10MeV gammas can
reduce their energy to 1/e of initial via pair production in 1.6cm (as
above, Eqn 2.16); this too would lead to visible gammas within the 
physical thicknesses of cathodes being used, assuming a cascade mechanism
took place.  I havn`t even tried to address 4, and really can`t since
I lack the experience and knowledge.  In fact we already know that 4 is 
important based on neutron detection studies done by the Frascati group and
verified by others (including LANL).

In summary, we can account for a reduced gammas emission by invoking 
high energy alpha emission as part of the energy loss occuring during CNF.
Other effects should be detected if this is occurring.  We cannot
account for a near complete absence of gammas by this mechanism.

Mike Brooks/Stanford Electronics Lab (solid state)/SU
MIT astronomer Walter Lewin: "Absence of evidence should never be
mistaken for evidence of absence."