[Isotope-prod] Meeting notes

[Pavel Degtiarenko]

Hi George and All,

The second important engineering question, the answer to which could 
severely limit our options, is:

What is the maximum beam current that the vacuum window, separating the 
beam vacuum and the irradiation setup, could let through without 
breaking the vacuum. The beam size should be small (~0.2x0.2mm max) and 
the beam should not be rastered.

Beam current is the right parameter here, as for thin windows the energy 
loss is only weakly dependent on energy. For crude estimates it can be 
assumed to be ~2 MeV/(g/cm^2) - and can be calculated precisely if 
needed. Beam cannot be rastered because Radium target is small (and I 
guess it will be quite difficult to move the target in sync with the 
rastered beam).

Beryllium water-cooled vacuum window comes to mind. Still, I would 
expect it's a challenge to hold milliampere-range currents in these 
conditions. This limitation may force us to lower the beam currents and 
go to higher energies to provide for the activation power in the target.

One "crazy" option that may have to be considered, is to have the window 
itself move in a circular motion, together with the last straight 
section of the beamline, that would have to be mounted on a flexible 
vacuum bellows... "Rastering" the window itself...

Regards,
Pavel


On 09/12/2015 10:01 PM, Pavel Degtiarenko wrote:
> Hi George and All,
>
> I have set up a toy FLUKA model for the Radium irradiation, that can 
> simulate Ac-225 production. That is all extremely preliminary and 
> would require independent verification. But one immediate engineering 
> question came along, and in any case it will have to be answered 
> before we could propose a solution for the Radium irradiation problem.
>
> The question is: what is the maximum heat power that could be 
> deposited safely and continuously into the Radium target.
>
> In any realistic setup, power deposition in the target will be not 
> small. The figure of merit, "Ac-225 hourly production per power in the 
> target" in (mCi/hour)/kW, varies, depending on the setup, from about 6 
> mCi/h/kW in the case when 150 MeV beam strikes directly at the target, 
> - to about 22 mCi/h/kW at 75 MeV and with a 2 rad. lengths radiator in 
> front of the target. The setup with thin radiator and sweeping magnet 
> before the target can produce 13 mCi per hour in the target 10 cm away 
> from the radiator at the full beam 150 MeV, 1 mA (150 kW), and the 
> power in the target is evaluated to be about 700 W. The term "hourly 
> production" here means that the target is irradiated for one hour, 
> parent Ra-225 is produced, and then, in 15-20 days, Ac-225 daughter is 
> "harvested".
>
> One gram (one Ci) of Radium is a small target, I used a cylinder 1 cm 
> length and 0.48 cm diameter.
>
> Seems like for any setup we would need a reliable engineering 
> evaluation of the methods of taking heat off the small target. That 
> would be one of the critical parameters that set limits on possible 
> solutions.
>
> One approach could be to investigate the option to use a beryllia (for 
> example, see http://www.sanjosedelta.com/beryllium_oxide.shtml) 
> vessel, holding Radium, cooled from the outside (by water?..). It 
> should be shaped appropriately (with cooling fins attached, or such). 
> The irradiation could happen through the entrance beryllia window.
>
> Radium boils at 1140 C, and beryllia can be operated at up to 1800 C. 
> So I guess the option to remove Radium from one such vessel into 
> another without a loss could be the distillation process again: heat 
> everything above 1140 C and attach a fresh vessel with a colder bottom.
> Actinium should mostly stay, as it boils above ~3000 C.
>
> Best regards,
> Pavel