[Tpe] TPE - primary beam on target

[Larry Weinstein]

Dear TPEers,

At some point, we want to run the primary electron beam thru the chicane 
and onto the LH2 target.  There are two main concerns,
  1) radiation in the hall due to the beam hitting vacuum and Helium windows
  2) radiation damage to the Kapton target windows

There is a possibility that Hall C will be down tomorrow (Friday) during 
the day shift.  If so, we could do the test then with minimal impact on 
the other halls (since it will take time to tune the beam to us).

*Are we confident enough that the test is safe that we should proceed 
for tomorrow or do we need to study this?*

Here is my analysis of the energy deposited by radiation on the Kapton 
windows at 1 nA:
6e9 e/s * 2 MeV/(e-g/cm^2) * 1 g/cm^3 * 1 cm = 12*10^15 eV/s = 2*10^-3 J/s
(I used a 1 cm thick window, but this will divide out when we calculate 
the energy deposited per mass.)

Now we need the beam size on the windows.  The beam passes through a 75 
um Al window about 15 m upstream of the target.  t = 75 um * 2.7 g/cm^3 
= 0.02 g/cm^2
The radiation length of Al is 24 g/cm^2 so this is 10^-3 RL.

The multiple scattering for a 2.2 GeV beam is
sigma = (13.6 MeV/2200 MeV)*sqrt(1e-3) = 0.2 mrad
Over a distance of 15 m, this is a spread of 3 mm.

A block of Kapton that is (3 mm)^2(1 cm) has a mass of 0.1 g.  Thus, the 
energy deposited amounts to a radiation dose of
R = 2e-3 J/s / 0.1 g = 2e-3 J/s / 1e-4 kg = 20 J/kg-s = 20 Gray/s

Kapton (according to the manufacturer's data) can withstand 10^6 Gy with 
no change in mechanical properties.  At 10^7 Gy, it loses 25% of its 
tensile strength and 50% of its elongation.

10^6 Gy corresponds to a time of 10^6 / 20 = 5*10^4 s = 14 hours.

To reduce this we can a) run at lower energy (which will double the beam 
spot size, reducing the radiation by a factor of 4) or b) run at lower 
beam current (subject to the minimum current that the accelerator can 
monitor).

-- 
				Sincerely,
				Larry

-----------------------------------------------------------
Lawrence Weinstein
University Professor
Physics Department
Old Dominion University
Norfolk, VA 23529
757 683 5803
757 683 3038 (fax)
weinstein at odu.edu
http://www.lions.odu.edu/~lweinste/

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