Tuesday, November 11, 2008

Technetium-99

99TcWhen nuclear physics hits the headlines, it's usually in unpleasant contexts, such as leakage of radioactive substances from reactors, the unsolved problem of disposal of nuclear waste, or the proliferation of nuclear weapons.

So, I was surprised by I story I had heard on the radio a few weeks ago, and read about again a while later: It was about the temporary shortage in the supply of a radioactive isotope heavily used in medicine, Technetium-99.

Technetium-99, a nucleus made up of 43 protons and 56 neutrons, has an excited state which decays with a half-life of about 6 hours. Similar to an excited atom emitting a photon of visible light when converting back to the ground state, the decay of the exited nucleus comes along with the emission of a γ photon. This γ photon has an energy of 140 keV – about 100.000 times the energy of a photon of red light. Here is schematic representation of the energy levels and the transitions involved:



To create nuclei of Technetium-99 in the excited state – also called Technetium-99m, where "m" stands for "metastable" –, one resorts to another isotope, Molybdenum-99. This isotope undergoes a β decay with a half-life of 66 hours, thereby ending up as Technetium-99 in the excited state. Like all isotopes of Technetium, Technetium-99 isn't stable either and finally ends up, following another β decay, as Ruthenium-99.

Technetium-99 thyroid uptake scans. Scan (A) shows the normal, healthy result. (from Petros Perros: Thyrotoxicosis and Pregnancy, PLOS Medicine 2(12): e370)
Now, what is this good for? As it came out from investigations at the Brookhaven National Laboratory done in the 1960s, the decay reaction of Technetium can be adapted as a very elegant and practical tool for medical diagnosis. To this end, "freshly produced" Tc-99m is extracted chemically from a probe of Mo-99, bound to suitable large molecules, and administered intravenously to the blood circuit. Then, the 140 keV γ photons emitted at the decay of Tc-99m map from inside the body the distribution of blood. They trace regions of lacking blood supply, for example after a stroke, or highlight spots with enhanced metabolisms, which could be tumours. Energy of the γ photon and lifetime of Tc-99m are just so that such an exploration does not produce to high a radiation exposure, and can be done in a very reasonable time.

To use this technique, Molybdenum-99 is needed, and this is where the current shortage comes from: Molybdenum-99 is created in nuclear reactions, either by bombarding more common isotopes with neutrons, or by fissioning of U-235 in highly enriched uranium targets. This second source, which is the most important one, is of course highly linked to weapon-grade stuff, so there are only a handful of civilian reactors in the world that produce Molybdenum-99. In Europe, the main source is a reactor in Petten in the Netherlands, which is currently shut down for maintenance and inspections. It seems, however, that the consequential shortage of Molybdenum-99 and Technetium-99m for medical purposes is not critical.

While trying to get some background on this news story, I realised that the decay scheme of Molybdenum-99 and Technetium-99m involves a few interesting questions:

Why does the decay of Mo-99 not end up in the ground state of Tc-99? Why is the lifetime of the γ decay of Tc-99m so long? Usually, γ transitions happen within fractions of a second. And finally, what a strange element is technetium in the first place, as without stable isotopes, it marks a gap in the middle of the periodic table? And as it comes out, the answers to these questions touch upon a few concepts very central to nuclear physics.

But this will be the stuff of another post.

17 comments:

Phil Warnell said...

Hi Bee,

That was quite the explanation and I ‘ll be curious where you are leading to on this. This whole medical isotope shortage thing first began in Canada last year when a safety inspector ordered the shutdown of the reactor in Chalk River Ontario were they produce the material you explain here for all of North America and subsequently heads rolled because of the decision. I wasn’t aware of what was going on in Europe but it seems to relate to a similar incident in Petten in the Netherlands. Oh yes, interestingly enough Chalk river is also famous for a first which was unfortunately the first reactor meltdown in 1952. Luckily it was mostly contained within the building and dealt with rendering little exposure of the surrounding populous.

Best,

Phil

Bee said...

Credits go to Stefan...

Plato said...

Stefan:what a strange element is technetium in the first place, as without stable isotopes, it marks a gap in the middle of the periodic table?

I'm wondering. Because I have a special interest with the elements and the periodic table developed by Dmitri Mendeleev.

Your using the periodic table then to question "the nature of the element" in relation to that gap. Photogenically measured?

If your then able to see the nature of this element based on that "predictability," then to have it validated in the periodic table as a marker for consideration? The validation of questions go beyond this element in a new framework?:)

"Dyson, one of the most highly-regarded scientists of his time, poignantly informed the young man that his findings into the distribution of prime numbers corresponded with the spacing and distribution of energy levels of a higher-ordered quantum state." Mathematics Problem That Remains Elusive—And Beautiful While I think too, of the Fibonacci numbers in expression. I can't help but think decay from the original event are possible in a descriptive format highlighted here?

One, which I am always question with a "certain resonance in principal" and brought to my mind by Riemann's consideration of primes in relation to the Ulam spiral.

Hope that made sense.

Best,

Neil' said...

Yes, technetium is weird because of being unstable even as element 43 and way below polonium (the first all-radioactive element, unless you say that all Bi has a very long HL, or does it?) I am sure this is understood in terms of nuclear shell theory. Few people "on the street" have ever heard of it.

But Tc is also important for chemical reasons, the latter being less well-known. Some Tc compounds, like "pertechnetates" are good as anodic corrosion inhibitors. Too bad Tc is all radioactive. (Heh, maybe since Bush was "43" that was bad luck?)

BTW, when are they ever going to make some of that cool "island" isotope stuff like Uuq (#114) with half-life around 1,000 years? We could do a good physical/chemical properties study with stuff like that.

Uncle Al said...

Neutrinoless double beta-decay of Mo-100 is also a barrel of fun! Safe for the kids: t_1/2 = 4.6x10^23 years (NEMO3). A clever approach uses scintillator crystal CaMoO4 with Mo-100. The source is also the detector.

Fewer than 2.2x10^14 technetium atoms/cm^2, about 55 ppm of KTcO4 in water, lends remarkable corrosion resistance to mild carbon steel. Alas, that is about 3 x 10^6 Bq/g for the protectant or 1700 Bq/liter for its solution.

changcho said...

Very interesting post on Technetium, thanks Stefan.

CapitalistImperialistPig said...

Ah yes, Tedhnitium 99 m and I are better acquainted than I would prefer. I had wondered how one kept the stuff for shipment, since it won't exactly stay fresh in the fridge. The MO I guess, is the Mo, which lives long enough for shipment.

Kaleberg said...

It's interesting how "exotic" elements seem to pop up more and more in every day life. Smoke detectors use americium, batteries use lithium, and cell phones use tantalum. I grew up in a world of wood and metal, but we are in a golden age of materials science. Who knows how soon the typical home with have the whole periodic table?

Phil Warnell said...

Hi Stefan,

“But this will be the stuff of another post.”

I must apologize for addressing the wrong author.Also,as mentioned I am curious of what your follow might be to this. To be truthful I was expecting you to perhaps announce the demise of Phoenix with it finally falling victim to the frigid temperatures it's subjected to in the region of Martin North Pole.

Best,

Phil

stefan said...

Hi Phil,

thanks for pointing out the Canadian version of the story! In the radio feature I head heard, it was mentioned that replacement for the Mo missing from Petten actually is coming from a 53 years old reactor in Canada, so that must be Chalk River. Interesting that it had been shut down also just earlier this year!

Chalk River goes back to Manhattan Project, doesn't it? And I guess the Petten facility is also quite old already...


Hi Neil, Uncle Al

Ah, I didn't know that Tc has useful chemical and technological applications - thanks for the hint.

The shell model will feature in the next part (illustrations are done already ;-)), though the most simple form doesn't show Tc is unstable. I am still searching for a good explanation...


Hi CIP,


oh, I hope you are fine... Yes, it's the Mo-99 that's transported from the reactor to the hospital - with a half life of about 3 days, that's OK... although you better wouldn't bring the stuff by ship from Canada to Europe or the other way round ;-). The Tc compounds used for the screening are then extracted chemically form the Mo at the hospital just before the exploration, so they are always fresh, so to say.


Hi kaleberg,

thanks - very interesting comment! I didnt know that cellphones need tantal. But lithium has hit the news these days.


Cheers, Stefan

Phil Warnell said...

Hi Stefan,

Yes Chalk River was one of the facilities set up for and during the Manhattan Project after the signing of the Quebec Agreement between the U.S., Britain and Canada. Canada’s initial contribution was in the mining and processing of Uranium along with related nuclear reactor research. After the war Britain and Canada went their separate paths with Britain developing their own weapons capability and Canada concentrating solely on civilian reactor development and industry.

It for many years has been a world leader in this area yet within recent times has been eroding which I would contribute primarily due to the negative perception that spread after the U.S.S.R.’s Chernobyl incident despite the clear difference in technologies involved. For me this has always represented to highlight the problem with the misunderstanding and with it mistrust of the general public about science in general and the rapid rise in the power and decisions made as the result in the rise of pseudo science. The way I look at it is that unfortunately between the environmental activists and big oil the world has been put in a box in respect to being able to solve both the energy crisis and legitimate environmental issues.

Best,

Phil

Neil' said...

For the record: I said " Yes, technetium is weird because of being unstable even as element 43 and way below polonium (the first all-radioactive element, unless you say that all Bi has a very long HL, or does it?)" Well Tc itself is the first ARE. But it is true that Bismuth is the next ARE, not Po, since all isotopes are radioactive (see http://en.wikipedia.org/wiki/Isotopes_of_bismuth.) But since the longest-lived is the common 209Bi at 1.9 x 10^19 years, it might as well be stable.

But even then, someday it will essentially all be gone (unless "someone" replenishes it.) Sniff. How will they treat indigestion then? (Oh, but in plenty of MWs it lasted indefinitely longer, just as it all already decayed in others ...)

ZoloftNotWorking said...

Technitium 99 is used for heart function measurements too. The test itself typically takes about 20 minutes and the preparatory work of removing a blood sample, tagging it with the 99 Tc marker, and reinjecting the blood takes about another 30 minutes. Except for the apparently nail-sized needle used to extract the large blood sample the procedure is painless!
A highly multipoint detector is placed close to the chest and EKG pads are attatched. The data from the detectors and the EKG are collected and computer correlated to measure both the expanded volume and the contracted volume of the heart. The ratio of the two is known as the ejection fraction. From 58% to 70% is considered normal. Below this range is considered to be heart failure.
Interestingly the ability of a patient to function is not so tightly correlated to the ejection fraction as you'd think. Patients with EJs in the 30% to 40% range, such as myself, can be debilitated while conversely people with EJs in the teens can function nearly normally. Luckily I'm very highly functioning with a great deal of stamina.

Jasper said...

I used to live close to Petten. I've been there once to look at their reactor. It looked really old and almost all the buildings around main reactor building were deserted. One marvelous thing I saw that I will never forget was that you could see the Cerenkov radiation as you looked down through the water. I know they did a lot of medical experiments but I never knew they produced Molybdenum-99. Can't wait for the post about Technetium!

Best,
Jasper

Anonymous said...

Why isn't U-238 used instead of U-235?

eagle52997 said...

@Neil' - don't forget about Promethium. It and Technetium both have no stable isotopes, and they are both lower in Z than Bismuth.

The idea of stability raises an interesting point - at what point do we consider something to be stable? What if something could decay, but its half-life is so long and it is present in such small amounts that observation of decay during our lifetime is not probable? Is that material still radioactive?

brian said...

@eagle

It's possible for all elements to eventually decay. I do not know how anyone would be able to analyze the decay of something that has a trillion year half life- it would appear to be "stable" to us.

There is also the fact that spacial fields may mediate decay patterns which would allow for different decay rates. Introduction to the concept: http://phys.org/news202456660.html
The sun would be modifying spacetime though its postulated to be a neutrinos emission mediated effect that is independent from space time structure (I dont believe things are independent of space time geometry though).