Return-Path: wrnrp@SLAC.Stanford.EDU Return-Path: Received: from smtpserv1.SLAC.Stanford.EDU (SMTPSERV1.SLAC.Stanford.EDU [134.79.16.136]) by aeh.engin.umich.edu (8.8.7/8.8.7) with ESMTP id AAA24643 for ; Thu, 28 Jan 1999 00:17:42 -0500 Received: from rpdice.SLAC.Stanford.EDU (RPDICE.SLAC.Stanford.EDU [134.79.82.27]) by smtpserv1.slac.stanford.edu (PMDF V5.2-29 #34068) with SMTP id <0F690046U7W1IE@smtpserv1.slac.stanford.edu> for alex@aeh.engin.umich.edu; Wed, 27 Jan 1999 21:14:25 -0800 (PST) Date: Wed, 27 Jan 1999 21:14:24 -0800 (PST) From: Walter Ralph Nelson Subject: Photos for your book To: Alex Bielajew Message-id: MIME-version: 1.0 Content-type: TEXT/PLAIN; charset=US-ASCII Content-transfer-encoding: 7BIT Hi Alex - Here is some information on two photos that you can fetch. If you use these photos, please give credit to Dieter Walz (SLAC), not me. Ralph On the anonymous ftp site (ftp.slac.stanford.edu) there is a subdirectory called /groups/techpubs/outbox/wrnrp that currently contains the two files: end_stopper.jpg stopper.jpg The file called "end_stopper.jpg" is a photo of a copper beam stopper inside a chamber that is normally under vacuum (but not so in this test experiment). The stopper was intentionally struck with a high-power electron beam (maybe > 50 kW) having an energy of 18 GeV. If you look carefully, you will see that the stopper has melted out (at the top left) and that there is material in the bottom of the vacuum chamber. The highest energy deposition occurs within the stopper at approximately shower-maximum depth, and this leads to a molten-metal "core". The core gets larger, both radially and longitudinally, as the beam is kept on, until eventually there is a "breakthrough" of the molten metal at the weakest (thinnest?) location. In the case of this stopper, the sides are more vulernable than the front or back surfaces. But also, since heat tends to rise in liquids along isothermal flow lines, the breakthrough occurs "off axes" near the top of the stopper as shown in the photo. Whether it goes to the left or right is purely chance in perfectly symmetric situations. The second photo (stopper.jpg) is this same stopper, viewed from the side after it was removed from the chamber. A simple calculation of shower maximum can be done using the EGS-designed formula (see Particle Physics Booklet) Tmax (rl) = ln(E/Ecrit) - 0.5 with Ecrit = critical energy = 24.8 MeV and X0 = radiation length = 1.44 cm for copper (from Swanson's book). At 18 GeV this gives a Tmax of 6.1 cm, or 2.4 inches, which is approximately what you see in the photograph. As a side note, after the induced radioactivity (mostly Cu-62 and Cu-64) died away, we sliced this stopper in half along, and symmetrically about, its cylindrical length. We observed three "caverns"---due to three cores---each depicting their own peaks at the shower maximum. For the second and third core, the "weakest" point was now the BACKWARDS direction, where a significant temperature rise had already taken place due to core #1. But, core #1 had already broken through the side of the cylinder, and the molten metal from cores #2 and #3 simply flowed out the hole. If I can ever find this photograph, I will scan it in for you. Hope this is of use. Ralph (PS - Yoshi is sitting by me and says hi)