A list of physical parameters of the two-mile Stanford Linear Accelerator (Fig.4) is given in Table II. Although the example chosen is at present the highest-energy linac, most of its parameters are quite representative of many other
travelling-wave accelerators if the differences related to its great length (multiplicity of sections and therefore of beam energy and power) are taken into account. The particularly unique features of the SLAC facility are the interlaced-multiple-beam capability, the ability to accelerate also positrons and polarized electrons to high energies, and a special facility for extremely short (10 ps) beam pulses.
Tables III, IV and V provide an overview of three classes of linear accelerator installations. It is seen that the development in medical accelerators within the past decade (Table III) has been toward a capability for isocentric therapy using
11
FIG.4. Aerial view of the Stanford Two-Mile Accelerator (SLAC), a modern high-energy facility for elementary-particle research. The Research Area with multiple-beam capability
is in the foreground. The electron-positron storage ring SPEAR is to the lower right. The 360 beam pulses accelerated per second are shared by as many as six different beam paths, each with separately adjustable energy, current and pulse length.
(Reproduced with kind permission of the Stanford Linear Accelerator Center and the Energy Research and Development Administration.)
both electrons and photons [4], The maximum useful energy appears to be
approximately 40 MeV. The radiation characteristics at each energy are surprisingly similar among these modern facilities, reflecting a general consensus among
manufacturers and users.
Accelerators for industrial radiography are surveyed in Table IV. The very high outputs of these machines may pose a great potential hazard to operating personnel in industrial settings.
Table V contains an abbreviated list of physical parameters of representative operating research and special-purpose installations. There is great variety in the capabilities of these installations, reflecting the purposes to which they are applied.
Figure 5 illustrates the general rise in beam power with accelerator energy.
Text continued on p.24
TABLE II. PHYSICAL PARAMETERS OF THE STANFORD TWO-MILE ACCELERATOR (SLAC)
Accelerator length 1 0 0 0 0 f t ( 3 0 4 8 m )
Length between feeds 1 0 ft ( 3 . 0 4 m)
Number of accelerator sections 960
Number of klystrons 245
Peak power per klystron 20 - 40 MW Beam pulse repetition rate 1 - 360 p u l s e s / s
RF pulse length 2.5 ixs
Filling time 0.83 us
Electron energy, unloaded 22. 8 GeV (max) Electron energy, loaded 21.5 GeV
Electron peak beam current 70 mA (max) Electron average beam current 40 fj,A (max) Electron average beam power 800 kW (max) Electron beam pulse length 10 ps - 1. 6 jxs Electron beam energy spread (max) 0.5%
Positron energy 15 GeV (max)
Positron average beam current cl 0.5 fxA
Multiple beam capability 6 interlaced beams with independently ad-justable pulse length, energy, and current
Operating frequency 2856 MHz
For 140 kW of incident electron beam power at positron source located at one-third point along accelerator length.
13
TABLE III. RADIATION PARAMETERS OF MEDICAL ELECTRON LINEAR ACCELERATORS INTRODUCED SINCE 1965
A p p r o x .
See footnotes at end of table.
TABLE III. (cont.)
TABLE IV. RADIATION CHARACTERISTICS OF ELECTRON LINEAR ACCELERATORS FOR INDUSTRIAL RADIOGRAPHY
Manufacturer Model
Nominal beam energy (MeV)
RF power source (magnetron or klystron)
Maximum X-ray output (unflattened)
( G y m2 -s"1) ( r a d - m2- m i n ' ) Maximum field size (at 1 m) (cm)
Nominal photon leakage radiation (per cent of useful beam at 1 m )
CGR MeV Neptune 6 6 M 0.13 7 5 0 50 (dia.) 0.1
CGR MeV Neptune 10 10 M 0.33 2 0 0 0 50 (dia.) 0.1
Efremov LUE-15-1.5 15 M 1.7 10 000 3 0 (dia.) 1.0
Efremov LUE-10-1D 10 M 0.30 1 800 22 (dia.) 1.0
Efremov LUE-10-2D 10 M 0.83 5 0 0 0 25 (dia.) 1.0
Efremov LUE-15-15000D 15 M 2.5 15 0 0 0 4 0 (dia.) 1.0
Efremov LUE-5-500D 5 M 0.08 500 35 (dia.) 0.2
EMI Therapy Radiograf 4 4 M 0.08 500 26 X 35 0.5
Mitsubishi ML-1 Rll 0.95 M 0.003 2 0 30 (dia.) 0.1
Mitsubishi ML-1 RIII 0.45 M 0.00025 1.5 3 0 (dia.) 0.1
0.95 0 . 0 0 2 5 15 0.1
Mitsubishi ML-3R 1.5 M 0.01 50 3 0 (dia.) 0.3
Mitsubishi ML-5R 3 M 0.05 3 0 0 3 0 (dia.) 0.3
Mitsubishi ML-5RII 4 M 0.06 3 5 0 3 0 (dia.) 0.3
Mitsubishi ML-1 OR 8 M 0.33 2 0 0 0 30 (dia.) 0.2
Mitsubishi ML-15RII 12 K 1.2 7 0 0 0 3 0 (dia.) 0.1
TABLE IV (cont.)
. , , „ „ Maximum X-ray output Maximum Nominal photon Nominal beam RF power source , . „ .. , . . , . „
„ , , , (unflattened) field size leakage radiation Manufacturer Model energy (magnetron , ^ , . . ^ ,
. . . . | , \ (at 1 m) (per cent of useful (MeV) or klystron) 2 - 1 , , j 2 • -k / . ,
( G y m s ) (rad-m min ) (cm) beam at 1 m) Radiation Super X 6 0 0 4 M
Dynamics
Radiation Super X 2 0 0 0 8 M Dynamics
Radiation Super XX 12 K Dynamics
Varian Linatron 200 2 M Varian Linatron 4 0 0 4 M Varian Linatron 2 0 0 0 8 M Varian Linatron 6 0 0 0 15 K
0.1 6 0 0 3 0 (dia.) 0.1 0.33 2 0 0 0 30 (dia.) 0.1 1.0 6 0 0 0 3 0 (dia.) 0.1 0.03 175 77 X 77 0 . 0 2 0.07 4 0 0 3 9 X 39 0.1 0.33 2 0 0 0 55 (dia.) 0.1 1.0 6 0 0 0 27 (dia.) 0.1
TABLE V. RADIATION PARAMETERS OF RESEARCH AND SPECIAL-PURPOSE ELECTRON LINEAR ACCELERATORS
Machine use Special capabilities
N u m b e r a n d
t o o TABLE III. (cont.)
I n s t a l l a t i o n
See footnotes at end of table.
t o TABLE III. (cont.)
N o m i n a l T y p i c a l h i g h - p o w e r o p e r a t i o n ( a p p r o x . )3
I n s t a l l a t i o n p e a k N u m b e r a n d ^ p
Machine use Special c a p a b i l i t i e s t y p e of c Peak _ Pulse D u t y Electro;
l o c a t i o n energy K K b s o u r c e Energy Tp - .
Vv s e c t i o n s c u r r e n t ® v rate f a c t o r p o w e r
K 6 ' ( M A ) ( M e V ) (ms) (Hz) (%) (kW) 6 5 White S a n d s WSMR 4 8 N u c l e a r e f f e c t s N a n o s e c o n d pulses 2 TW (S) 2 K ( 2 0 ) 6 0 0 4 8 10 1 2 0 0 . 1 2 3 5 6 5
66 W i n f r i t h A E E IS R a d i a t i o n r e s e a r c h 1 TW (S) 1 K ( 1 0 ) 2 0 0 14 4 . 5 2 0 0 0 . 0 9 2 . 5 6 6
6 7 Yale 7 0 N u c l e a r p h y s i c s n(0—20 MeV) S TW ( L ) 7 0 0 4 0 4 . 5 2 5 0 O . U 3 0 6 7
6 8 Yerevan 4 8 0 N u c l e a r p h y s i c s I t e r a t i v e a c c e l e r a t i o n 13 TW (S) ! 3 K ( 2 0 ) 1 5 0 0 1 2 0 8 1 0 0 0 . 0 8 1 4 0 6 8 e+( 0 - 2 0 0 MeV)
(a) P a r a m e t e r s s i m u l t a n e o u s l y achievable h i g h e s t e l e c t r o n b e a m p o w e r u n d e r c o n t i n u o u s o p e r a t i o n . ( b ) TW = travelling wave, SW = s t a n d i n g wave, S = S - b a n d , L = L - b a n d o p e r a t i n g f r e q u e n c y .
(c) N u m b e r of k l y s t r o n s ( K ) , m a g n e t r o n s (M) or a m p l i t r o n s ( A ) . Peak p o w e r p e r u n i t (MW) given in p a r e n t h e s e s .
I01 I 02 I 03 I04 I 05
MAXIMUM ENERGY (MeV)
FIG.5. Beam power (kWj of representative electron linacs plotted against beam energy (MeV).
The line represents the typical but arbitrary average current of 100 ^lA (corresponding to, e.g., /peak = 100 mA, DF = 0.1%).
The parameters which most directly affect radiological safety are:
(a) Electron beam energy E0 (b) Average beam power P
(the product of E0 and the average beam current I)1.
The most important derived quantities of radiation protection, such as dose rate or shielding thickness, are generally not simple functions of energy E0, and the complete information needed for radiation protection over a broad range of energies requires an extensive set of tables or graphs.
Many quantities are relatively simpler functions of energy when normalized to average beam power rather than to average current, and therefore are so presented in this manual. At a given energy E0, the dose rate or exposure rate is directly proportional to average beam power P. The required shielding thickness at a given distance, and for a given beam energy E0, is approximately proportional to the logarithm of average beam power.
1 The average power may be obtained from E0I because Eo, when specified in eV, is numerically equal to the potential difference (V) effectively used to accelerate each particle.
Potential difference (V) times current (A) is equal to power (W).
The beam is not continually accelerated but comes in short pulses of typically TP= 1 - 3 /us duration. Where desired, TP can be made as short as 10 ps.
The pulse repetition rates may range between 1 and 1440 Hz, but most are in the range of 60 to 360 Hz. The duty factor DF is the fraction of operating time during which the linac is actually producing radiation, which is generally in the range
1CT4 to 10"3. It is the product of pulse repetition rate p (in Hz) and pulse length TP (in seconds):
DF = p • Tp (1) This small duty factor is a disadvantage in some research applications but is
unimportant in the most common applications such as radiotherapy and industrial radiography. Special large-duty-factor and continuously operating (CW) accel-erators have also been developed. Very short pulses (usually 5 —10 ns) are used to advantage in pulse radiolysis and neutral-particle spectrometry where precise timing of the reactions studied is crucial.
In radiological protection, the duty factor is important insofar as it may affect radiation measurements; some measurements may be rendered completely useless or even dangerously misleading by duty-factor effects. Any measurement involving the counting of discrete events must be carefully evaluated to ensure that these effects are kept small or properly corrected for. Geiger-Miiller and proportional counters are particularly susceptible to saturation, owing to their long dead times. Procedures for correction are explained in Section 5.
REFERENCES TO SECTION 1
[1] See, for example, papers in: LAPOSTOLLE, P.M., SEPT1ER, A.L. (Eds ), Linear Accelerators, North-Holland Publishing Co., Amsterdam (1970).
[2] See, for example, papers in: Proc. 1975 Particle Accelerator Conf., Accelerator Engineering and Technology, held in Washington, DC, 1 2 - 1 4 March 1975, IEEE Trans. Nucl. Sci.
NS-22 (1975).
[3] See, for example, papers in: Proc. IXth Int. Conf. High Energy Accelerators, held at Stanford Linear Accelerator Center, 2 - 7 May 1974, CONF-740522, UC-28-Accelerators (TID-4500, 60th ed.), National Technical Information Service, Springfield, Virginia (1974).
[4] KARZMARK, C.J., PERING, N.C., Electron linear accelerators for radiation therapy:
history, principles and contemporary developments, Phys. Med. Biol. 18 (1973) 321.
[5] ALMOND, P.R., Some applications of particle accelerators to cancer research and treatment, Phys. Rep. 17C 1 (1975).
[6] See, for example, papers in: Proc. Conf. Particle Accelerators in Radiation Therapy, held at Los Alamos Scientific Laboratory, 2 - 5 Oct. 1972, USAEC Rep. LA-5180-C, Technical Information Center, Oak Ridge, TN (1972).
[7] BLY, J.H., High energy radiography: 1 - 3 0 MeV, Mater. Eval. (Nov. 1964) 1.
[ 8 ] See, for example, papers in: Large Radiation Sources for Industrial Processing (Proc.
Symp. Munich, 1969), IAEA, Vienna (1969).
25
[9] See, for example, papers in: Radiation Preservation of Food (Proc. Symp. Bombay, 1972), IAEA, Vienna (1973).
[10] INTERNATIONAL ATOMIC ENERGY AGENCY, Manual on Radiation Sterilization of Medical and Biological Materials, Technical Reports Series No.149, IAEA, Vienna (1973).
[11] See, for example, papers in: BERMAN, B.L., Ed., Proc. Int. Conf. Photonuclear Reactions and Applications, Asilomar, California, 2 6 - 3 0 March 1973, Lawrence Livermore Laboratory and USAEC Rep. CONF-730301, Office of Information Services, Oak Ridge, TN (1973).
[12] FULLER, E.G., "Photonuclear Physics 1973: Where we are and how we got there", Proc. Int. Conf. Photonuclear Reactions and Applications, Asilomar, California,
2 6 - 3 0 March 1973 (BERMAN, B.L., Ed.), Lawrence Livermore Laboratory and USAEC Rep. CONF-730301, Office of Information Services, Oak Ridge, TN (1973) 1202.
[13] See, for example, papers in: KIRK, W.T., Ed., Proc. 1975 Int. Symp. Lepton and Photon Interactions at High Energies, held at Stanford University, 21—27 Aug. 1975, Stanford Linear Accelerator Center, Stanford, CA (1975). ,
[14] CHODOROW, M., GINZTON, E.L., HANSEN, W.W., KYHL, R.L., NEAL, R.B., PANOFSKY, W.K.H., Stanford high-energy linear accelerator (Mark III), Rev. Sci.
Instrum. 27 (1955) 134.
[15] NEAL, R.B., Gen. Ed., DUPEN, D.W., HOGG, H.A., LOEW, G.A., Eds, The Stanford Two-Mile Accelerator, Benjamin, New York (1968).
[16] BORGHI, R.P., ELDREDGE, A.L., HELM, R.H., LISIN, A.F., LOEW, G.A., NEAL, R.B.,
"Design, fabrication, installation, and performance of the accelerator structure", Ch. 6, Stanford Two-Mile Accelerator (NEAL, R.B., Ed.), Benjamin, New York (1968).
[ 1 7 ] KNAPP, E.A., KNAPP, B.C., POTTER, J.M., Standing wave high-energy linear accelerator structures, Rev. Sci. Instrum. 39 (1968) 979.
[18] ONO, K., TAKATA, K., SHIGEMURA, N., A short electron linac of side-coupled structure with low injection voltage, Part. Accel. 5 (1973) 207.