Roy and Niels

Roy and Niels

Monday, January 31, 2011

Making Bubbles: The Particle Way

BTI BubbleTech Industries manufacture neutron detectors intended for personal dosimetry. Theses devices contain a polymer gel holding very small droplets of a superheated gas. When a neutron interacts with these superheated droplets, a phase transition happen from liquid phase to gas phase expanding the volume dramatically - a bubble appears.

The video below demonstrates how such a detector responds to a (weak) Americium-Beryllium neutron source:

The activity of the AmBe source was 2.64E+4 neutrons per second. The detectors we had were calibrated against ICRP-60 in terms of dose equivalent, according to BTI. The sensitivity of the particular detector shown in the video above was about 0.7 bubbles / µSv dose equivalent.

The detectors come with an integrated piston which repressurizes them, so they can be reused, however not indefinitely. We used our detectors rarely, and kept them refrigerated. However, after two years the encapsulation/pressurization system leaked.

The bubble detectors can be bought with varying sensitivity ranges and BTI even offers a set of detectors which are sensitive above a varying energy threshold. Deconvoluting the counts in each detector of this set will yield a coarse energy spectrum, in e.g. 6 energy bins.

We have used the these bubble detectors in our antiproton beam line at CERN in order to get a coarse measure of the amount of fast neutrons emitted from the antiproton annihilation. We used both the personal dosimeter type and the BDS spectrometer.

The picture above show how multiple personal dosimeter detectors are places at a certain distance from the annihilation vertex.

Unfortunately we had some trouble interpreting the results from the spectrometer. The BDS spectrometer counts seemed to be simply unphysical and a spectrum could not be deconvoluted. The readings from the personal dosimeter also seemed to be off by an order of magnitude.

After some investigations we started to suspect that these bubble detectors were not only sensitive to fast neutrons, but also to charged particles, such as protons. From the antiproton annihilation we do get a similar amount of protons and a threefold multiplicity of energetic pions, which have a long range, far beyond the position of the bubble detectors.

A paper in NIM B, which was published a few years ago by us, lists our findings. Basically we conclude that the sensitivity (# of neutrons per bubble) is quite comparable to that for protons, and perhaps a bit less for pions. The proton part we could test at our storage ring, ASTRID, which we have in the basement of our Physics Department in Aarhus.

In the video you are about to see, we extract a few million protons at about 50 MeV from the synchrotron. The bubble detector here is immersed in a water bath.

The range of the protons are clearly visible. A distinct Bragg peak does not really form, the effect is primarily related to nuclear interaction cross sections.

Friday, January 14, 2011

Open access, medical physics, and

If you read research papers, chances are you’ve heard the term open access. In this post I’m going to talk about what open access is, the state of open access in medical physics, and what medical physicists can do if they want to make their work open access using sites such as The quick summary is: the primary obstacle to open access in medical physics is adoption by authors. Most journals are already on-board in important ways. If you want to make your medical physics publications open access, you probably can and I encourage you to do so.

According to our friends at Wikipedia, open access is “unrestricted online access to articles published in scholarly journals”. Open access is generally placed into two categories: “Green” open access and “Gold” open access. Green access is defined as author “self-archiving”, when the author places a copy of a paper on their own site or on an e-print server. Gold access is free access provided directly by journals.

No-fee access has many benefits for researchers. For medical physics, these benefits are potentially greater than for other fields, due to the fact that medical physicists are found in a wide variety of settings with varying levels of paid journal access (i.e. universities, community hospitals, small clinics, etc). Even being located at a large university with a large medical center, I have personally run into access barriers. For example, I can only access Medical Physics with my personal subscription; for a time the library subscribed to the Red Journal, but not the Green Journal; the university has no love for Radiation Protection Dosimetry whatsoever. In the current economic climate I’m not optimistic that institutional subscriptions will be on the increase. Ultimately, open access offers availability of information to all regardless of institutional affiliation or budget. (Also, I hate messing with proxies... :) )

While open access is strongly established in some disciplines, particularly physics, computer science, math, and earth science, medical physics seems to have lagged behind the curve greatly, especially in self-archiving.

“The availability of gold and green OA copies by scientific discipline. The disciplines are shown by the gold ratio in descending order, rather than in alphabetical order.” CCA 3.0. Björk B-C, Welling P, Laakso M, Majlender P, Hedlund T, Gudnason G. doi:10.1371/journal.pone.0011273

The above plot from Björk et al. shows the percentage of publications that are made open access in different disciplines. In some sub-disciples of physics, such as high energy physics, the rate of self-archiving is up to 100%. For reasons unknown to me, medical physicists have not embraced open access, despite the supportive polices of medical physics journals (see lists below). I suspect that medical physicists are largely ignorant of the journals’ policies. If medical physicists want to provide open access to their work, they have options to both self-archive (green) or publish in open access journals directly (gold).
In 1991, high energy physicists began self-archiving their publications on a site called the arXiv (that X is supposed to be like a Greek “chi”). Since then, the arXiv has expanded to cover all of physics, as well as other fields, such as mathematics and computer science. As the leader in physics self-archiving, the arXiv is a logical destination for medical physicists to post their papers. In fact, the arXiv has a medical physics category. Currently, the medical physics category of the arXiv has very low activity relative to the number of medical physics articles published that are eligible to be posted. (I plan to investigate the posting rates in a future blog post.) While the activity is low, it is encouraging to see prominent medical physics researchers, such as Steve Jiang (UCSD) and Thomas Bortfeld (MGH/Harvard), posting articles. (Thanks!) In fact, one tiny area of medical physics that seems to be very well covered on the arXiv is GPU based calculations in medical physics. That’s probably due to Jiang’s group leading the way in posting their publications.

Medical journals
While medical physics journals all allow self-archiving to servers such as, medical journals related to medical physics seem to be much less enthusiastic about open access (see the list for details). The Elsevier journals allow pre-prints and self-hosted archiving, but the main radiology journals have open access “hostile” policies. The one thing that has seemed to crack journals such as those from RSNA is government mandates. One example is the recent rule requiring NIH funded research publications to be made available as open access on PubMed Central within 12 months of initial publication. This rule has had a wide ranging effect on journals and led to much discussion. Funding agencies in other countries have instituted similar rules.

What does this all mean for you?
  1. If you’ve published in medical physics journals, you can probably make your work open access right now by posting your articles to
  2. If you are planning to submit an article to a journal, you should read the journal’s copyright policy before submitting and before posting a pre-print (or post-print). Some journals have very strange policies, unfortunately, and this has to be taken into account when submitting for publication.
I encourage authors to strongly consider making their work open access, either by self-archiving to the arXiv or publishing in one of the gold access journals. Ultimately, the arXiv is just an example of an e-print repository, but it seems to be the best choice for now. If, for example, a dedicated medical physics repository were created and critical mass were achieved, the papers on the arXiv could be stored there as well. I haven’t discussed the concerns some people have with open access (see the Wikipedia entry). If there is interest, I might talk about that in another post.

Journal policies
Below I will list the current (Jan. 2011) policies of the journals (as far as I can tell, UAYOR, YMMV, IANAL, etc). You can find out more information about the open access policies of these journals and others by using the SHERPA/RoMEO tool.

The state of open access in medical physics journals:
The state of open access in medical journals related to medical physics:

Wednesday, January 5, 2011

libdEdx version 1.0 released

Version 1.0 of libdEdx is now available at

List of features:
  • ICRU 49 date tables for protons and Helium ions (PSTAR, ASTAR)
  • MSTAR for heavy ions
  • ICRU 73 with and without the erratum for water target
  • A Bethe implementation for any ion, including Linhard-Sørensen equation for low energies
  • Support for 278 ICRU target materials, i.e. the complete ESTAR material table, all with default I-values for the Bethe equation
  • I-values can be overridden for elements
  • Automatic application of Bragg's additivity rule, if requested target material does not exist in default table for e.g. MSTAR.
  • Detailed documentation, and multiple example files
  • CMAKE based installer, with uninstall target.
  • getdedx as a frontend command line program for querying the library
  • Two modes of operation: simple for lazy programmers and fast for e.g. MC codes.
  • GPL license (non-GPL versions available upon request)

How to use libdEdx, simplest possible example.

Demonstration of command line program getdedx. 100 MeV protons using PSTAR on water:

Usage: getdedx program_id  Z icru_target_id energy.

bassler@kepler:~$ getdedx 2 1 276 100
100.000000 MeV/u HYDROGEN ions on WATER target using PSTAR
dEdx = 7.286E+00 MeV cm2/g

Carbon ions on alanine target, using ICRU 73:

bassler@kepler: getdedx 5 6 105 400
400.000000 MeV/u CARBON ions on ALANINE target using ICRU73
 Bragg's additivity rule was applied,
 since compound ALANINE is not in ICRU73 data table.
dEdx = 1.068E+02 MeV cm2/g

For reporting bugs and feature request, you can use our trac ticket system or drop us an email.