Proton Number Density:
The proton areal density using the effective target length and LH2 density (below) is
n = L * density * Navogadro * (2 protons/molecule)/GMW
n = 3.941 cm * 0.07230 g/cm^3 * 6.022^23 molecules/mole * 2 atoms/molecule / 2.016 g/mole
n = 1.702^23 proton/cm^2
with an asymmetric error bar of -.7%+1.1%. Due to some fortuitous cancellations (actual density higher but actual thickness lower), this
agrees well with Tanja's working values.
missing: everything listed below in red, plus a correction
for the density reduction due to beam heating
Target Thickness Calculations:
For the LH2 target in Loop 2, using Meekins's measurements (below), and
subtracting the window thicknesses, the inner diameter is
D = (4.013 +- .008 cm) - .01384 cm - .01270 cm = 3.986 +- .01 cm
which is 0.35% shorter than a nominal 4 cm thickness.
missing: what is the correction for thermal contraction?
The beam spot was clearly off to one side of the cylindrical cell by an amount large enough
to significantly affect the target thickness calculation. (See photos below.) Assuming the
beam passed thru parallel to a diameter, the effective target length is
L = 2 sqrt( (D/2)^2 - dx^2 )
where D is the inner diameter from above and dx is the beam offset. Averaging by the fast raster has been
neglected. For offsets of 2mm, 3mm, and 4mm, the effective target length is reduced by
0.5%, 1.1%, and 2.0%, respectively. Assuming for now that the offset was 3 mm, then from the
above formula the effective target length is estimated to be
L = 3.941 cm
with asymmetrical error bars of -.5%+1%. Thus our target was actually about 1.5% shorter than
the nominal value of 4 cm.
missing: need a more accurate value for the offset, and the effect of
fast raster averaging should be included.
Target Density:
Using the value for the LH2 density for 19K and 24 psi (Jim Dunne, T20
target memo),
density = 0.07230 g/cm^3
with about 0.5% uncertainty.
Target Thickness References:
Meekins's March 27, 2003 memo
erratum: Loop 1 was the spare, Loop 2 contained LH2.
Target Thickness References:
Meekins's March 27, 2003 memo
erratum: Loop 1 was the spare, Loop 2 contained LH2.
Target Gas Purity References:
A sample was taken of the H2 gas in early 2004 for G0. The biggest concern for
FpiII was probably the D2 contamination; this appears to be negligible. Any air ices are probably
cold trapped in the target so another measurement must be made when the target is warmed up.
Preliminary Gas Analysis
Close up photos of cells:
Close-up upstream view of cryogenic cells.
Radiation damage has eroded some of the super-insulation.
Close-up upstream view of the loop 2 (LH2) cryogenic cell. The beam scar is the light patch. Walking over to the EEL to eyeball, the beam scar is at least 3-4 mm beam left of the center of the target. We will therefore require a significant correction for the target thickness. The target survey below could account for 1.25mm of this offset. (Thanks, Greg.) The rest may be due to a discrepancy between the surveyors DIMAD (0,0,0) and the spectrometer centers of rotation, or perhaps we just set the beam spot wrong.
Close-up upstream view of the loop 3 (LD2) cryogenic cell. The beam scar is not as obvious as in loop 2 but it is similarly misaligned.
February 19, 2004, by Dave Mack
