X-ray radiation from RHEED and from XPS

It is mandatory to measure the radiation dose at view ports when you construct a UHV chamber equipped with RHEED, X-ray tube, etc.

When a high speed electron suddenly stops, X-ray is generated. This X-ray generation mechanism is known as "Bremsstrahlung". In the case of RHEED, the high speed electrons stop at the sample, the fluorescent screen, and the inner wall of the UHV chamber. Thus X-ray is generated there. The spectrum of the X-ray generated in this way has continuous spectrum. If the electron speed (or kinetic energy) is high enough to excite the core electron of the element atom composing the target, the characteristic X-rays are additionally generated.

In our setup, X-ray dose at the RHEED view port (view port itself is normal glass, but a lead glass plate covered with fluorescent powder is attached inside of the view port ) measured in the unit of Sv (dose equivalence) was

RHEED acceleration voltage: Dose equivalence

15keV : 0.5μSv/h

20keV : 20μSv/h (reduced to 0.2μSv/h using Pb doped PMMA protective board at the screen)

*at the surface of CRT monitor (used as PC monitors in ancient time...): 0.6μSv/h

In the case of Mg/Al twin anode X-ray tube (Perkin-Elmer Φ for XPS, X-ray dose at the view port (normal glass) in front of the X-ray tube in our system was

Acceleration voltage Dose equivalence

10kV : 0.35μSv/h

13kV : 200μSv/h

In the case of our apparatus, we are safe when the RHEED is operated at 15keV, and X-ray tube is operated at 10kV. Though there are some X-ray, the amount is less than at the surface of CRT monitors. If we need to increase the RHEED acceleration voltage to 20keV, though we use RHEED with 15keV most of the time, we need to attach Pb doped PMMA protective board to the RHEED screen.

200μSv/h, which we got when accelerating the X-ray tube with 13kV, is too high for a normal experimental room. By a law, a space where the X-ray effective dose exceeds 1.3mSv per 3 months have to be defined as the radiation controlled area. By standing in front of our UHV chamber for 30 hours, effective dose exceeds the upper limit of the domestic safety rule for female (5mSv / 3 months; see note 2.). Upper limit of the X-ray Acceleration voltage allowed in our chamber is currently 10kV.

Note 1.

The attenuation length of the photon varies with its energy and the transmitting material. The relationship among the photon energy, the mass attenuation coefficient (MAC) and the attenuation length (AL) where the strength of the X-ray becomes 1/e of the original in the air1) and in the pyrex (borosilicate) is as follows1), 2).

air (1.293×10-3 g/cm3 ) pyrex ( 2.23 g/cm3 )

photon energy MAC AL MAC AL

20keV 7.779×10-1 cm2/g 10 m 2.297 cm2/g 0.2 cm

15keV 1.614 cm2/g 500 cm 5.217 cm2/g 0.1 cm

10keV 5.120 cm2/g 150 cm 17.05 cm2/g 0.03 cm

Normal view port for UHV chamber uses 7056 glass. It is Alkali Borosilicate and the density is 2.29g/cm3.

Note 2.

The limit of the radiation dose defined by laws in Japan is

100mSv /5 years

50mSv/ 1 year

5mSv / 3 months (female)

1mSv (pregnant female)

For eyes, skin, the abdominal region of the pregnant female, additional rules exist.

Eye: 150mSv/ year

Skin: 500mSv/ year

the abdominal region of the pregnant female: 2mSv

For comparison, X-ray dose from medical X-ray investigation

Head X-ray CT(computed tomography): 1350 mGy・cm (2.8 mSv)

1) http://henke.lbl.gov/optical_constants/atten2.html

2) http://physics.nist.gov/PhysRefData/FFast/html/form.html