Bremsstrahlung & X-Rays

bremsstrahlung - x-rays - radiative energy loss

What it shows:
In 1895, Wilhelm Röntgen discovered that a beam of cathode rays could create a new type of radiation (x-rays) when allowed to impinge upon an obstacle such as the glass of the tube itself. The present demonstration recreates that discovery. A Maltese Cross CRT is the source of cathode rays as well as target; a modern Geiger-Mueller Counter detects the x-rays.

How it works:
When fast electrons interact with matter, they may lose energy by Coulomb interactions as well as radiative processes. Energy is mainly lost through repeated collisions with atomic electrons (coulomb interactions), causing excitations and ionizations. Electron-nuclear interactions, which can abruptly change the electron direction, can sometimes occur. Classically speaking, the strong acceleration of the electron as it is deviated from its straight-line path gives rise to radiation, and some of the electron's energy is lost due to this electromagnetic radiation (radiative processes known as bremsstrahlung). The fraction of the electron energy converted into bremsstrahlung increases with increasing electron energy and is largest for absorbing materials of high atomic number. The bremsstrahlung energy spectrum is a continuum with photon energies extending as high as the initial electron energy, but low energy photons predominate in number by orders of magnitude and the average energy is a small fraction of the incident energy.

In this experiment a Maltese Cross CRT is energized by an induction coil whose output is about 40 kV. The Maltese Cross should be tilted down so as not to obstruct the cathode rays -- we wish the electrons to strike the CRT glass. If the lecture hall lights are turned down a bit, the glass will be seen to fluoresce a beautiful eerie green color.



A Geiger-Mueller (G-M) tube 1   is held up to the face of the CRT to detect the x-rays being given off. One can demonstrate that these are low energy or "soft" x-rays by interposing some additional glass as an absorber. The glass of a Pyrex baking dish is thick enough to effectively stop all the x-rays. 2

Setting it up:
The entire setup occupies very little space and can sit on the lecture bench or be on a separate cart. Use a 6 volt Gel Cell battery to power the primary of the induction coil. This is important because the interrupter in the primary circuit of the induction coil produces a considerable back emf which can destroy the semiconductor components of a DC power supply -- we have learned this the hard way. The induction coil should not be operated continuously for more than 10 minutes at a time (overheating).

The radiation level is approximately 4 mR at a distance of 1 meter and therefore quite safe for the audience and instructor. 3   To minimize exposure, keep away at least an arm's length; the demonstration should take less than a minute.

Comments:
We have measured the energy spectrum of the emanating radiation and, as expected, the intensity of bremsstrahlung falls off rapidly with increasing energy. The characteristic x-rays from the glass dominate by an order of magnitude in number over the bremsstrahlung. The NaK x-rays at 11.7 keV are the most prominent. Below this energy the bremsstrahlung background rises exponentially so that the Si line (at 7.0 keV) is barely detectable and Ca (3.4 keV) is completely buried in the background. 4 An interesting point to make the audience aware of is that the visible electromagnetic radiation (light) is not attenuated by the thick glass whereas the glass is quite opaque to the invisible (x-rays). Alternatively, a sheet of black paper can be interposed between the CRT and Geiger tube. Now one has the opposite situation -- the paper is opaque to the visible light but transparent to the invisible radiation. This can be a nice lead-in to the question of "what is transparency?" Rating ***

References:
W.R. Leo, Techniques for Nuclear and Particle Physics Experiments - A How-to Approach, 2nd revised edition, (Springer-Verlag, NY, 1994).
G.F. Knoll, Radiation Detection and Measurement, 2nd edition, (John-Wiley, NY, 1989).

1 Ludlum Measurements, Inc. model 177 ratemeter with model 44-7 G-M tube
2 And this is the reason why the glass of television picture tubes is so thick.
3 Note that, for x and gamma ray measurements in the energy range from a few keV to a few MeV, values of exposures in roentgens can be considered numerically equal to absorbed doses in rads to tissue, or to dose equivalents in rem. The rad is the traditional unit of absorbed dose. The gray was adopted as the unit in the International System and 1 gray = 100 rad. The limit for occupational exposure of whole body is 1,250 mrem/qtr; 5,000 mrem/yr. On the average, occupational exposure should not exceed a few mrad per hour. The limit for nonoccupational exposure (including exposure of minors) is 125 mrem/quarter.
4 Glass is composed of mainly quartz, or silica (SiO2), mixed with soda (Na2O) and/or lime (CaO). We were not able to resolve the Kα and Kβ lines with our spectrometer.