Last update: 5/6/1997 (Original: 4/3/1997) Update (5/6/1997): Correct tables II and III: a) Uncertainties were in the wrong column. b) Simulations were re-done: minor changes, same conclusions. See tables II and III.
See E895_music.html for general information on MUSIC analysis.
We describe current progress on measuring the beam energy at E895 I using the MUSIC detector for the 4 and 2 A GeV beams. This is a working document, intended only to conveniently describe work in progress.
1. Method
Please see "E895 MUSIC energy I" for a full description of the method.
Briefly, the idea is to use the measured position of the beam in MUSIC to extract the Au incident beam energy.
A GEANT simulation for the 1 T MPS magnet tell us that for a 4 A GeV 197Au beam, the beam spot should be at z=10.3 m, and x=-1.2 m (that is, a deflection in the horizontal plane of 1.2 m at a distance of about 10 m).
The simulation gives the following sensitivity
dE (A GEV) / dx (mm) = 0.1/25.8 (A change in beam energy of 0.1 A GeV results in a change of the beam spot in MUSIC of about 2.5 cm).
dE (A GEV) / dx (mm) = 0.1/60.4 (A change in beam energy of 0.1 A GeV results in a change of the beam spot in MUSIC of about 6 cm)
Therefore, the method depends critically on the ability to know the absolute position of the beam spot in MUSIC during the experiment. If, for example, we can locate the beam to within 1 inch, then the uncertainty in the energy measurement is +/- 0.1 A GeV at 4 A GeV, and +/- 0.04 A GeV at 2 A GeV.
In fact we need to know fairly accurately (to better than 1 inch):
In addition, we need to know fairly accurately the MPS magnetic field profile. See "E895 MUSIC energy I" for a discussion of those three topics.
2. Results at 4 and 2 A GeV
Here we report additional analysis done at 4 A GeV based on an independent determination of To and the drift velocity. We also report results for 2 A GeV.
In the present analysis, we verified also that cage 4 is within 2 mm of the survey value at both energies.
The EOSGX simulations were done assuming multiple scattering and energy loss in the target and in the media after the target. We assume a point (perfect) spot.
The observed widths (Table I) are consistent with a Gaussian beam spread at the target of about sigmax = 15 mm , sigmay = 4 mm.
The number in parenthesis is the expected energy (from EOSGX) assuming that the nominal beam was degraded by the material upstream the target, as measured during the E895 II run. See log book E895 II, p. 162.
We need to:
Table II.- Measured position in MUSIC. The position values are in the MUSI coordinate system. For the vertical position (y), the x and z values correspond to the center of MUSIC. For the horizontal position (x), the y and z values correspond to the center of the cathode in cage 4. The last column is the measured energy of the beam just before the target. This was obtained from an EOSGX simulation that reproduces the measured horizontal position. We used the measured values for To (-5.69 bucket) and Vd, as shown.
Note: s=std. dev; r=rms value
a) 4 A GeV. MUSIC positioned according to survey. Field map corresponding to full field (1 T).
b) 2 A GeV. MUSIC positioned according to survey. Field map corresponding to 3/4 field (.75 T).
Table III.- Same as Table II, but using Vd=3.44 mm/bucket (5.51 cm/usec) and obtaining To (-1.4 bucket) by adjusting the vertical position corresponding to the 4 A GeV run, as in the earlier report. The last column gives the calculated (EOSGX) energy of the beam just before the target.
3. Comments, suggestions are welcome