[labnetwork] Hazard due to x-ray production in e-beam evaporators?

[Sweeney, John]

Aaron,
I have asked your question to one of Harvard’s EHS health physicist’s and she gave the following answer:

That's a good question and I'm glad people are thinking about this. So the general answer to this safety concern is that we can tell the dose rate outside an electron beam device is low and at a safe level because we measure it. You can take a look at the answer given by the Health Physics Society: https://hps.org/publicinformation/ate/q12299.html. Basically calculating the dose rate from an electron beam is very difficult because of differences in geometry, scatter, filtration, distance, interaction probabilities, etc, so the dose rate is almost always measured empirically. Also very little empirical data on x-ray dose below 50kV exists because it is too low for traditional x-ray applications (medical x-rays and other imaging) plus it's so low that detector efficiency drops. I survey electron microscopes every two years to check for leaks; so far I've never had a measurement above background although ~15 years ago there was one leak detected from a faulty gamma camera.

What we can do is look up the mass attenuation coefficients from the NIST website (https://www.nist.gov/pml/x-ray-mass-attenuation-coefficients) to get an idea of the x-ray attenuation through a glass viewport. Let's assume the max 10kV electron beam translates into a 10kVp x-ray source (untrue, but it's the most conservative estimate). The average energy of a typical x-ray source is 1/3 of the peak voltage, or 3.3kV.  The mass attenuation coefficient for 3.3 kV x-rays for borosilicate glass is roughly 400. If the glass is 0.25cm thick and its density is 2.23 g/cm^3 (given in NIST table 2), then the intensity drop from this shielding is given by:
  I/Io = e^(-u/p*p*x) = 1.4*10^-97
So the penetration of 3.3kV x-rays through .25cm glass is already really small, and that's assuming it's an x-ray beam pointed directly at glass with no other obstructions. Even if the x-ray dose rate reaches what we know of for x-ray fluorescence devices that can reach >3000 rem/hour on skin contact, this becomes negligible through the glass shielding.

This doesn't say anything about leakage however. That's why we measure.


From: labnetwork-bounces at mtl.mit.edu <labnetwork-bounces at mtl.mit.edu> On Behalf Of Joseph E. Palmer
Sent: Thursday, May 28, 2020 10:31 AM
To: Aaron Hryciw <ahryciw at ualberta.ca>
Cc: labnetwork at mtl.mit.edu
Subject: Re: [labnetwork] Hazard due to x-ray production in e-beam evaporators?

Aaron,

     It has been some years since I worked at the Nuclear Physics Lab at Rutgers University (with an 8MeV Van De Graff particle accelerator), but I will relay what I know.  10 keV x-rays are barely able to make it past the chamber walls.  Of course, this depends upon the proximity of the gun to the chamber walls, their thickness, and the material they are constructed from.  It is always good to confirm this with a Geiger Counter.

     Over years that I have been here at Princeton, occasionally someone from our own EHS is sent over to measure the amount of radiation being produced by our tools, and they have never been able to get a reading over background.  You would get a larger dose of radiation while standing in front of the bananas (K40) at Shop Rite, sleeping next to someone (C14), living in Denver, Co, or flying in an airplane.  I don't mean to belittle their concern; when involved in any research that involves the possibility of radiation exposure, keeping track of your cumulative dosage is important.  In short, I am generally not concerned with anything lower than 10keV.

Regards,

Joe Palmer
On 5/27/2020 11:21 PM, Aaron Hryciw wrote:
Dear colleagues,

One of the users in our open-access facility recently expressed concern about x-ray production in electron-beam evaporation systems.  Since the typical acceleration voltage in our e-beam systems is 7–10 kV, he was concerned that bremsstrahlung and characteristic x-rays would be generated during deposition, with a maximum energy of 7–10 keV (soft to hard x-rays), and that these x-rays would pose a health and safety hazard to an operator standing next to the viewport for ~30 minutes.

A literature search yielded a few reports describing radiation damage to sensitive devices from x-rays produced during an e-beam metallization step, but I did not find any mention of related health and safety considerations.  While some x-rays are undoubtedly produced, presumably there are reasons why they are not hazardous to an operator (e.g., perhaps total x-ray output is very small, majority of x-ray spectrum is low energy, x-rays do not penetrate stainless steel chamber walls or viewport windows, etc.).  I would like to answer this user with specific physical arguments as to why the hazard is insignificant, however, so any advice you could offer to this end would be greatly appreciated.  Many thanks.

Cheers,

 – Aaron



Aaron Hryciw, PhD, PEng

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