From: Brown, RM (Bob) - PPD [bob.brown@stfc.ac.uk] Sent: 21 February 2008 17:27 To: Brown, RM (Bob) - PPD; Bell, Ken Cc: Cockerill, DJA (David); Kennedy, BW (Bruce); Shepherd-Themistocleous, CH (Claire); Sproston, M (Martin); Barbara Camanzi; Hobson, Peter Subject: RE: VPTs Attachments: anomalous_1600_addendum.pdf I finally found a copy of Bruce's report (no Bruce couldn't find it either). It goes by the catchy title: 'anomalous_1600_addendum' Bob. -----Original Message----- From: Brown, RM (Bob) - PPD Sent: 21 February 2008 14:47 To: Brown, RM (Bob) - PPD; Bell, Ken Cc: Cockerill, DJA (David); Kennedy, BW (Bruce); Shepherd-Themistocleous, CH (Claire); Sproston, M (Martin); 'Barbara Camanzi'; 'Hobson, Peter' Subject: RE: VPTs OK, so I understand a bit more now. There is a report by Bruce, dated 20/9/2002, which I have in my files, but which I have so far failed to locate on the EE web pages. This comapres the results obtained at RAL and Brunel, for 2 anomalous tubes: 1133 and 1118. At RAL, both tubes showed noisy behaviour over a range of angles. At Brunel, both tubes showed noisy behaviour at very localised angles. These angles are precisely those at which there are (normal) dips in response. Thus one cannot make the simple statement that the 'bad angles' scale with field. Since at 1.8T, the 'bad angles' are hard to identify. However, the fact that at 4T the 'bad angles' are precisely the 'dip angles' strongly suggests that some scaling with field is to be expected. Note that the dips can be explained by the v x B force. When the tube is tilted in a magnetic field, there is a displacement of the photoelectrons and secondary electrons ORTHOGONAL to the plane of the tilt. Dips in response occur when the sum of the displacements of the photo-electrons and secondary electrons (they have opposite signs but different magnitudes) is equal to an integral number of mesh steps. Note that at the dip points, secondary electrons, passing through holes in the grid, will deccelerate in the anode/cathode space, then retrace their trajectories back to the point of origin on the dynode (in the limit that they were originally emitted with no transverse velocity). They could then produce additional secondary electrons. However, I am not convinced that this might induce instability. Particularly since the tube response DIPS at the magic angles. So, is it safe to use anomalous tubes at 4T and if so at what angles? Answer - I don't know. Indeed, I wonder if tubes that PASSED at 1.8T might not show instabilities at 4T at the magic angles? Note that the standard Brunel test angle of 15 deg just misses a magic angle at 13/14 deg...... Bob. -----Original Message----- From: Brown, RM (Bob) - PPD Sent: 21 February 2008 11:04 To: Bell, Ken Cc: Cockerill, DJA (David); Kennedy, BW (Bruce); Shepherd-Themistocleous, CH (Claire); Sproston, M (Martin); 'Barbara Camanzi'; 'Hobson, Peter' Subject: RE: VPTs > Dave remembers that an early Discharge tube was taken to the B=4T > rig at Brunel, and that we were dismayed to find that the discharge > angle was indeed approximately halved at 4T. > So we FEAR your initial reaction this morning (B_t being important) > was indeed correct, contrary to this afternoon's conclusion. > Albeit statistics of unity or at least very small... I am, as ever, completely confused. In his Thesis, Omar describes angle scans performed at Brunel on two tubes: 1133 and 1118. He says that these are 'Suspect' VPTs, identified in tests at RAL. His plots concentrate on 1118. Dips in response are seen at angles of 3, 8, and 13 deg. This is what is expected from scaling the RAL scan from 1.8T to 4T. Furthermore, the width of the LED peak increases by a factor 2-3 at precisely these angles. However, it is not clear if this increase has anything to do with the increase in width due to instabilities, observed at RAL. The RAL width increases were in general much larger and did not show a tight correlation with angular dips in response. I have not found the RAL plots for this particular tube. Although Omar says that 1133 was 'suspect' at RAL, in several talks, this tube is used to demonstrate typical NORMAL behaviour. See for example: 'Addendum to Observation of noise in VPTs 1-3200 giving extra details', at: http://hepwww.rl.ac.uk/CMSecal/VPTs.htm HOWEVER In the talk: 'VPT performance, DPG meeting, CMS week, Dec 4 2002, R M Brown ppt' filed at the above URL; The behaviour of 1133 at 9 deg and 4T is shown as a demonstration of instability at 4T !!!!! Can anyone explain???? Bob.