[Isotope-prod] Gallium irradiation

[Andrew Hutton]

Dear Jamal, Sundaresan

I thought it might be helpful to try and clarify everyone’s understanding of gallium so that we are all on the same page.  

Gallium is a solid up to it’s melting temperature of 29.76°C; in other words the gallium we sent you was in a solid form at the temperature in your laboratory.  

When the gallium is irradiated, short and medium lived isotopes are created, one of which (Cu67) we want), but all of which create internal heating of the gallium.  The irradiated sample we will send you will therefore be in a liquid form.  Our first sample of gallium we irradiated a year ago remained liquid for weeks, so we can be certain that you will receive liquid gallium.  

As I suggested on the phone, we should practice the sending, transport and delivery process ahead of time.  The mock-up will be a sealed boron nitride crucible containing unirradiated gallium, shipped in a Class A radiation container, even though it will not be radioactive.  Prior to opening the crucible, it should be heated to 35-40 °C to liquify the gallium, mimicking the conditions that you will have when the radioactive gallium arrives at VCU.  I think it would be useful for us to come to VCU to observe the procedures - we will all profit from learning each other’s skills!  

In the meantime, you might like to use the remaining sample of gallium that we sent you to learn how to separate a part of the gallium (it tends to flow in a connected stream like mercury, not like water).   I would also imagine that it would not be too difficult to freeze it so you would be dealing with a solid sample, but I am not sure the temperature that would be needed.  

Another property of gallium is its high surface tension (~720 dynes/cm), even higher than mercury (~480 dynes/cm).  Gallium wets many materials, but not boron nitride.  So when you look at gallium in the crucible, it has strongly rounded edges, exactly like you would see with mercury.  This also means that liquid gallium will not leak through small holes in the crucible.  

Now let’s move to irradiating the gallium.  We understand that you would like as high a specific activity of Cu67 as possible in the gallium.  We had assumed 20 MeV electron energy for this test.  At this energy, the concentrations of Cu67 and Ga67 are roughly equal, so that we can radio-assay the quantity of Cu67 we produce before sending it to you.  Is this measurement important to you?  

We will look into making the target shorter (this will increase the specific activity) and to increasing the electron energy.  The ratio of Ga67 to Cu67 will increase with electron energy, plateauing somewhere in the 25-30 MeV region.  The Ga67 production rate is expected to be significantly higher than that of Cu67 because it is produced via a gamma-4n reaction rather than a gamma-alpha reaction.  This will increase the total radioactivity in the gallium - is this a problem for you?  Also, the high concentration of Ga67 makes the radio-assay of Cu67 inaccurate as one of the Ga67 peaks overlays the Cu67 peak.  

We will do some quantitative studies to parameterize the energy and target depth, your input would be welcome and we will make sure that you are involved in the final decision.  

One last thing.  I think the primary goal of the next irradiation is to demonstrate that we can produce Cu67 in a form that would be useful for R&D purposes, and even clinical trials.  Would you agree with this?  I think it is important that we all have a common vision for the next test.  

All the best

Andrew