Exploring X-Ray Ionisation: Common Questions Answered

[Russell_B]

Hello,

I am fascinated by the properties of x-rays and their many uses, but i need to clarify a few things:

1) Can x-rays eject electrons from non-metals and possibly even gases, or is compton scattering constrained to metals only? (ie. photoelectric effect)

2) How do you synthesize x-rays?

3) How dangerous can x-rays be if used improperly?

 

[Bob S]

These are good questions.

First, visible light covers the wavelength range ~6500 Angstroms (~ 2 eV) to ~4000 Angstroms (~3 eV).
UV light extends from ~4000 Angstroms (~3 eV) to ~ 100 Angstroms (~120 eV). See chart in

http://en.wikipedia.org/wiki/Ultraviolet

X-rays cover the range 100 Angstroms (~120 eV) to 0.1 Angstroms (~120 KeV)

Photons above this energy are generally called gamma rays.

A photon exceeding the work function energy of a material can eject a photoelectron. Work function energies range from ~ 2 eV to ~ 6 eV. (Photoelectrons of a lower energy can knock electrons from the valence band to the conduction band in semiconductors.). See table in

http://en.wikipedia.org/wiki/Work_function

For isolated atoms, the ionization energy (energy to knock out an electron) ranges from ~ 3 eV (cesium) to 24 eV(helium). See table in

http://www.science.co.il/ptelements.asp?s=ionization

X-rays historically were the rays coming from atomic electron transitions, the highest being a K-shell x-ray from uranium. These were first generated by accelerating electrons and hitting targets made of elemental materials. The x-ray energy from a bare uranium nucleus capturing 1 electron is 13.6·Z² = 13.6·92² = 115 KeV.

In the photoelectric effect, a bound electron absorbs 100% of the photon energy. The electron must be bound to satisfy kinematic equations (conservation of energy and momentum). For deeply bound electrons in atoms (e.g., K-shell) this now called deep core photoejection.

For free electrons (and also bound electrons), a photon can inelastically scatter off an electron. At low energies this is called Thomson scattering; at higher energies Compton scattering.

X-ray sources include electron bremmstrahlung and x-ray targets and synchrotron radiation. Any photon from a nuclear decay is a gamma ray, irrespective of its energy.

X-rays from particle (e.g., electron) accelerators (including electrostatic machines) can be extremely dangerous. They can cause severe burns, organ damage, and chromosome damage.

Bob S

 

[Russell_B]

Thank you very much.

It would appear that producing x-rays is an energy intensive process that requires a lot of specialised equipment.

I have heard of oil molecules being ionised by x-rays, and you have pointed out that individual isolated atoms can be ionised, but that they will not need the energy of an x-ray (.12-120 keV) to lose an electron (ie. helium 24eV).

If i wanted to ionise an O2 molecule, for instance, would i really need x-rays, or could i use a lower energy photon to do the same job?

Russ

 

[ZapperZ]

Hello,

I am fascinated by the properties of x-rays and their many uses, but i need to clarify a few things:

1) Can x-rays eject electrons from non-metals and possibly even gases, or is compton scattering constrained to metals only? (ie. photoelectric effect)

X-ray Photoemission Spectroscopy (XPS) is a common technique. So yes, x-ray can eject electrons from non-metals, as well as metals, of couse.

2) How do you synthesize x-rays?

Depends on what you want to use it for. If you want x-ray to study other things, and the resolution, energy, etc. is very important to you, then you use a synchrotron center. The x-ray that you can get from, say, a doctor's office for diagnostic purposes can come from a particle accelerator that bombards some crystals.

3) How dangerous can x-rays be if used improperly?

It can kill you.

Zz.

 

[Bob S]

It would appear that producing x-rays is an energy intensive process that requires a lot of specialised equipment.

The voltages in the regular cathode-ray-tube (CRT) TV sets is ~ 10 kV, maybe more for color tV. The front window is thick lead glass to attenuate x-rays from the electrons hitting the back side of the glass. I have an old capacitor-discharge ignition circuit (Mark Ten B, sometimes available on eBay) that can generate 30,000 volt sparks from a 12 volt battery.

I have heard of oil molecules being ionised by x-rays, and you have pointed out that individual isolated atoms can be ionised, but that they will not need the energy of an x-ray (.12-120 keV) to lose an electron (ie. helium 24eV). If i wanted to ionise an O2 molecule, for instance, would i really need x-rays, or could i use a lower energy photon to do the same job.

A 13.6 eV photon is enough to ionize an oxygen atom. I think about 5 eV is required to break up an oxygen molecule into two neutral atoms.

Bob S

 

[Russell_B]

Zz, when you say 'particle accelerator', are you referring to an electron gun or equivalent?

I have an old capacitor-discharge ignition circuit (Mark Ten B, sometimes available on eBay) that can generate 30,000 volt sparks from a 12 volt battery.
Bob S

I've always wondered how capacitor-discharge ignition circuits work. I'll start a new thread on it, please contribute.
Can you use them to produce short bursts of high energy photons?

 

[Bob S]

I've always wondered how capacitor-discharge ignition circuits work. I'll start a new thread on it, please contribute.
Can you use them to produce short bursts of high energy photons?

CD ignition circuits (like the Mark Ten B circuit I have) have a high frequency toroidal coil driven by two NPN push pull transistors that charge a capacitor to about 300 volts. A signal from the points in the distributor triggers an avalanch transistor in series with the capacitor, which discharges into the primary of the automobile ignition coil. The 1:100 primary-secondary ratio gives a 30-kV spark. This will give a short burst of high-energy photons if discharged in vacuum. The photons are a combination of bremsstrahlung and characteristic x-rays from the material used in the discharge circuit. The characteristic K-shell x-ray of copper is 13.6 x 29² = 11.4 kV max, the characteristic K-shell x-ray of zirconium is 13.6 x 40² = 21.6 kV max.

Bob S

Correction: The Mark Ten B schematic shows the capacitor being discharged by a common-emitter NPN transistor firing a SCR.

 

[Russell_B]

Well that answers all my questions on this subject. Thanks again :)