From: morrison@vxprix.cern.ch
Subject: Bubble Chambers
Organization: At Home; San Jose, CA USA
Date: Mon, 11 Jan 1993 17:38:02 GMT

   Tom Droege raised the possibility that Terry's ideas could be tested by 
looking at bubble chamber photographs where the liquid used was deuterium
and he asked me for comments. I replied briefly to Tom privately - he found 
it too brief, so sent him a second longer private message, hoping to end the 
discussion. However Steve Jones raised the issue based on his own experience of
bubble chambers and Terry has also mentioned the possiblility. Hence enclose
a modified version (reference added) of what I sent to Tom.
                                               Douglas R.O. Morrison.


Dear Tom,                                                6 January 1993.
         Yes, you are right, I did work on Bubble Chambers, starting when 
I arrived in CERN where I spent the first four years building chambers.
Have been busy with them since, though after the Fermilab 15-foot chamber 
closed down have not done any running, but as spokesman for E632 am still 
busy with the analysis which is now being done in Russia so will go there 
in 10 days. There are no bubble chambers that I know of operating recently.
      Let me try and explain better about bubble chambers.
First you keep the chamber under pressure so that it cannot boil. Then you 
release the pressure so that it wants to boil but does not know where. But if a
charged particle passes, then it gives lots of low energy delta rays (electrons)
along its path. As these electrons stop, they make many collisions which give 
a local heating in a radius of about a micron. This is the nucleus that 
starts the bubble. In a microsecond, the bubble centre is formed and and then
the growth follows a (time) to the power one-half law as more liquid evaporates
into the bubble. Essentially there is a competition between the surface 
tension trying to crush the bubble and the evaporation of heated liquid. This
is described by Seitz in an easy to remember reference - Physics of Fluids,
Vol. 1 page 1. 
    Suppose the maximum movement (ie expansion) of the piston is 100 units. It 
is normally arranged that the particles enter at about 95 units and an equal 
time after the lowest pressure, the flash is triggered and this is long enough 
for the bubbles to be big enough to be photographed. This time is about 
1 to 10 milliseconds. Now suppose that the extraction of the beam from the 
accelerator is not too good and some particles escape early and enter the 
bubble chamber, say at 50 units. Then the chamber is not fully sensitive and 
only a few of the delta rays will have enough concentrated energy to heat the 
liquid locally to give a nucleus and then a bubble. Thus on these early
tracks, there will be very few bubbles per cm. But since they have much longer 
to grow before the flash, these bubbles will be bigger than the later ones.
Often the chamber is not quite perfect and there are currents that will show
as distortion on the early tracks. But there is another more striking effect.
As these very early bubbles are bigger than the good beam ones, they will be 
brighter. Now in almost all chambers it is not possible to illuminate all 
the volume of the chamber efficiently. So very high or very low in the 
chamber there are regions out of focus. If an track passes there, it will not 
be seen or only very poorly - unless it is an early one that is very bright when
it will appear as a broad diffuse track (especially if high and near the 
cameras as in the 15-foot chamber). No doubt this is what you were remembering.
   Another point, as you will have realised from my description, once the 
chamber is recompressed, the energy from the stopping of the delta rays will 
have been disappated long before it is possible to start a new cycle.
   The conclusion is that it would not be profitable to study old bubble
chamber photographs.

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