Transcript Document

KEELE UNIVERSITY
WORKING WITH X-RAYS
Dr D E Dugdale
October 2012
ROTATING ANODE GENERATOR
X-RAY SPECTRUM
Emax = eV
DOSE RATES
• Can be very high
• For anode of atomic number Z, operating voltage
V, current I
• Unfiltered dose rate = D  ZVI/d2
at a distance d
• Typical figures for V = 50 KeV, I = 10 mA
@d=1m
@ d = 10 cm
D - 1 Sv per minute
D - 100 Sv per minute
• Radiation burns ( 3 Sv ) in a time of
•
@ 1m ~ 3 mins.
•
@ 10 cm ~ 2 secs.
• Using Ni filter or monochromator D  D/100
• To have radiation level below maximum allowed
( 7.5 micro Sv/hr ) requires shielding of > 108
X-RAY SHIELDING
I = Io e- x
=linear attenuation
coefficient
H
1/2
= 0.69/
For 50 keV X-rays
H1/2 - 10 m in air
H1/2 - 3 cm in body tissue
H1/2 - 0.1 mm in Pb
For Pb @ 50 keV X-ray energy
I/Io = 1/10 for x = tenth value thickness = 0.23 mm
For x = 2.3 mm therefore I/Io = 10-10
X-RAY LEAKAGE - Typical Arrangement Of X-ray
Generator And Camera
Leakage at joints must be less than 7.5 mSv/hr - but ALARA
principle demands much less since extra shielding is reasonably
achievable for X-rays
Perspex Shielded Enclosures
Leaded Perspex outer shield
MINI MONITORS
• Soft X-rays ( < 30 keV ) do not easily penetrate Geiger -Muller ( GM )tube walls.
• Ideally tubes with Be end windows should be used - but even cheap general purpose
GM tubes have some response down to 9 keV ( Cu K lines used in crystallography)
• For GM tube MX-168 @ 10 keV
1 c/s ~ 1 micro Sv/hr ( background ~ 0.5 c/s )
• 1 micro Sv/hr is only ~ 2 mSv/yr but nevertheless more shielding should be used
if any count rate above background can be detected
PERSONAL MONITORS
• Film badges and thermoluminesent detectors can
detect doses > 0.05 mSv (~ 1/40 annual background)
• Film badges are changed every month ( every 3
months for TLD’s)
• Not likely to detect accidental exposure to collimated
beams but should detect any diffuse scattered radiation
• Wear at chest height with serial number facing
outward
WORKING PROCEDURES
• Register each year as a radiation worker
(so that dose and training records can be kept )
• Follow the general rules of the University and the local rules
of the Department for specific equipment and specific
procedures
• Local rules and local training are given by research
supervisors and RPS
• Never overide interlocks
• Never operate if warning lights fail or if equipment is
malfunctioning
• Report all faults to the responsible technician immediately