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52
5 Production ofRadionuclides
Many other nuclides besides those mentioned above are also produced. For
99
Mo production, high enriched uranium (HEU) is commonly used for irradiation,
which contains more than 90% U-235. The irradiation is carried out for 5–10 days.
Nowadays, many investigators are using low enriched uranium (LEU) targets that
contains less than 20% U-235. This technique requires a longer irradiation for production of sufcient Mo-99 and is less expensive.
5.2.2 Neutron Capture or (n, γ) Reaction
In neutron capture reaction, the target nucleus captures one thermal neutron and
emits γ-rays to produce an isotope of the same element. The radionuclide so produced is therefore not carrier-free as it is embedded in the target molecules,and its
specic activity is relatively low. This reaction takes place in almost all elements
with varying probability. Some examples of neutron capture reactions are; 98Mo
(n, γ) 99Mo,
ral molybdenum. 99Mo so produced is called the irradiated molybdenum as opposed
to the ssion molybdenum described earlier and contains a large number of target
atoms making it a low specic activity product and rendering it less useful for
99
Mo–
sis of trace elements in various samples including forensic material.
196
Hg(n, γ)
99m
Tc generators. The (n, γ) capture method is commonly used in the analy-
197
Hg, and 50Cr(n, γ)51Cr. 98Mo is only 24.3% enriched in natu-
5.3 Nonuranium Production of99Mo
Many companies are deeply committed to nding ways to produce nonuranium
99
Mo and some of them have been successful, but low yield of 99Mo has been a
major concern. Northstar Medical Radioisotope LLC (NMR), a Wisconsin State
nuclear medicine company in the USA, has developed a proprietary method of
using nonuranium based
generate
99m
Tc. The NMR employs two methods to produce 99Mo: (1) 98Mo (n, γ)99Mo
reaction in the University of Missouri Research Reactor (MURR); (2) irradiation of
100
a
Mo target in an electron accelerator. In essence, the electron beam in the accelerator impinges on a tungsten target to produce bremsstrahlung (x-rays), which are
directed to strike the
The method of production and various characteristics of radionuclides com-
monly used in nuclear medicine are presented in Table5.1.
99
Mo, which is utilized in a novel RadioGenix® system to
100
Mo target causing the
100
Mo (γ, n)99Mo reaction.
5.3 Nonuranium Production of99Mo
53
Table 5.1
Nuclides
3
H
1
11
C
6
13
N
7
14
C
6
15
O
8
18
F
9
32
P
15
51
Cr
24
52
Fe
26
57
Co
27
58
Co
27
59
Fe
26
60
Co
27
62
Cu
29
64
Cu
29
67
Cu
29
62
Zn
30
67
Ga
31
Characteristics of common radionuclides
Physical
half-life
Mode of
decay (%) γ-Ray energya (MeV)
12.3 year β−(100) – –
20.4min β+(100) 0.511 (annihilation) 200
10min β+(100) 0.511 (annihilation) 200
5730 year β−(100) – –
2min β+(100) 0.511 (annihilation) 200
110min β+(97) 0.511 (annihilation) 194
EC(3)
14.3 day β−(100) – –
27.7 day EC(100) 0.320 9
8.3 h β+(56) 0.165 100
EC(44) 0.511 (annihilation) 112
271 day EC(100) 0.014 9
0.122 86
0.136 11
71 day β+(14.9) 0.511 (annihilation)
EC(85.1)
0.811
45 day β−(100) 1.099 56
1.292 43
5.2 year β−(100) 1.173 100
1.332 100
9.7min β+(97) 0.511 (annihilation) 194
EC(3)
12.1 h β+(17.9)
−
β
(39)
511 (annihilation) 36
EC(43)
2.6 day β−(100) 0.185 49
0.92 23
9.3 h β+(8)
EC(92)
0.511 (annihilation)
0.548
0.597 26
0.093 40
78.2 h EC(100) 0.184 20
0.300 17
0.393 5
Abundance
(%)
30
99.5
16
15
Common production
method
6
Li(n, α)3H
10
B(d, n)11C
14
N(p, α)11C
12
C(d, n)13N
16
O(p, α)13N
13
C(p, n)13N
14
N(n, p)14C
14
N(d, n)15O
15
N(p, n)15O
18
O(p, n)18F
32
S(n, p)32P
50
Cr(n, γ)51Cr
55
Mn(n, 4n)52Fe
50
Cr(α, 2n)52Fe
56
Fe(d, n)57Co
55
Mn(α, n)58Co
58
Fe(n, γ)59Fe
59
Co(n, γ)60Co
62
Ni(p, n)62Cu
9.3h
62
64
67
63
68
62
Zn Cu
EC
,
Ni(p, n)64Cu
Zn(n, p)67Cu
Cu(p, 2n)62Zn
Zn(p, 2n)67Ga
(continued)
54
66 h
nI
5 Production ofRadionuclides
Table 5.1
Nuclides
68
Ga
31
68
Ge
32
82
Rb
37
82
Sr
38
89
Sr
38
90
Sr
38
90
Y
39
89
Zr
40
99
Mo
42
99m
Tc
43
(continued)
Physical
half-life
Mode of
decay (%) γ-Ray energya (MeV)
68min β+(89) 0.511 (annihilation) 178
EC(11)
270.8 day EC(100) – –
75 s β+(95) 0.511 (annihilation) 190
EC(5) 0.776 13
25.5 day EC(100) – –
50.6 day β−(100) – –
28.6 year β−(100) – –
2.7 day β−(100) – –
78.1 h β+(23)
EC(77)
0.511 (annihilation) 46
0.909 100
0.181 6
66 h β−(100) 0.740 12
0.780
6.0 h IT(100) 0.140 90
Abundance
(%)
4
Common production
method
68
Zn(p, n)68Ga
66
Zn(α, 2n)68Ge
25.5days
82
85
Rb(p, 4n)82Sr
Sr Rb→
82
EC
Mo(p, spallation)82Sr
88
Sr(n, γ)89Sr
235
U(n, f)90Sr
89
Y(n, γ)90Y
90 90
.
28 6Sry
89
Y(p, n)89Zr
98
Mo(n, γ)99Mo
235
U(n, f)99Mo
99
Mo
99m
Y
Tc
111
49
113m
123
53
124
53
125
53
131
53
133
54
137
55
2.8 day EC(100) 0.171 90
In
100min IT(100) 0.392 64
In
49
13.2 h EC(100) 0.159 83
I
4.2 day β+(23) 0.511 (annihilation) 46
I
60 day EC(100) 0.035 7
I
8.0 day β−(100) 0.284 6
I
EC(77)
0.245 94
X-ray (0.027–0.032) 140
0.364 81
0.637 7
5.3 day β−(100) 0.081 37
Xe
30.0 year β−(100) 0.662 85
Cs
¹¹¹
112
Sn(n, γ)
113
121
Sb(α, 2n)
124
Te(p, n)
124
Xe(n, γ)
125
130 130
235
U(n, f)
131 131
235
U(n, f)
235
U(n, f)
235
U(n, f)
¹¹¹
Cd p,
117days
Sn In→
Xe I→
EC
17 hours
EC
n
113
Sn
113m
123
I
124
I
125
Xe
125
Te n, Te
131
Te
25
min
Te I
131
I
133
Xe
137
Cs
(continued)
EC
5.4 Target andIts Processing
55
Table 5.1
Nuclides
a
Gamma rays with abundance less than 4% are not cited; d, deuteron; EC, electron capture; f, s-
153
Sm
62
153
64
177
71
186
75
188
74
201
81
223
88
(continued)
Physical
half-life
Mode of
decay (%) γ-Ray energya (MeV)
1.9 day β−(100) 0.070 5
0.103 28
241.6 day EC(100) 0.097 28
Gd
6.65 day β−(100) 0.113 7
Lu
0.103 20
0.208 11
3.8 day β−(92) 0.137 9
Re
69.8 day β−(100) 0.155 15
W
73 h EC(100) 0.167
1T
EC(8)
X-ray
(0.069–0.083)
11.4 day α(95.3)
Ra
β−(3.6)
Abundance
(%)
9.4
93
Common production
method
152
152
176
177 177
185
186
187
203
201 201
226
153
Sm(n, γ)
Gd(n, γ)
Yb(n, γ)
Yb Lu
Re(n, γ)
W(n, γ)
W(n, γ)
Tl(p, 3n)
Pb
Ra n, Ra
227 227
223
Sm
153
Gd
177
Yb
. h
19
186
Re
187
W,
188
W
201
Pb
9.3h
227
Ac Th
Ra
T1→
sion; IT, isomeric transition; n, neutron; p, proton; Data are obtained by web browsing
5.4 Target andIts Processing
Various types of targets have been designed and used for both reactor and cyclotron
irradiation. In the design of targets, primary consideration is given to heat deposition in the target by irradiation with neutrons in the reactor or charged particles in
the cyclotron. In both cases, the temperature can rise to 1000°C, and if proper material is not used or a method of heat dissipation is not properly designed, the target is
likely to be burned or melted. For this reason, water cooling of the cyclotron probe
to which the target is attached is commonly adopted. In the case of the reactor, the
core cooling with heavy water is sufcient to cool the target. Most often, the targets
are designed in the form of a foil to maximize heat dissipation.
The common form of the target is metallic foil, for example, copper, aluminum,
uranium, vanadium, and so on. Other forms of targets are oxides, carbonates,
nitrates, and chlorides contained in an aluminum tubing, which is then attened to
maximize the heat loss. Aluminum tubing is used because of its high melting point.
In some cases, compounds are deposited on the appropriate metallic foil by vacuum
distillation or by electrodeposition, and the plated foils are then used as targets. In
specic cases, high-pressure gases (e.g.,
(e.g., H
18
O for 18F production) are also used.
2
124
Xe for
123
I production) and liquid targets
56
A
A IN .
N
WK
w
23
5 Production ofRadionuclides
5.5 Equation forProduction ofRadionuclides
While irradiating a target for the production of a radionuclide, it is essential to know
various parameters affecting its production, preferably in a mathematical form, to
estimate how much of it would be produced for a given set of parameters. These
parameters are therefore discussed below.
The activity of a radionuclide produced by irradiation of a target material with
charged particles in a cyclotron or with neutrons in a nuclear reactor is given by
IN et1 (5.2)
where
A = activity in disintegrations per second of the radionuclide produced
I = intensity or ux of the irradiating particles (number of particles/(cm2s))
N = number of target atoms
σ = formation cross section (probability) of the radionuclide (cm2); it is given in
1/2
−24cm2
(s−1)
units of “barn,” which is equal to 10
λ = decay constant given by 0.693/t
t = duration of irradiation (s)
Equation (5.2) indicates that the amount of radioactivity produced depends on
the intensity and energy (related to the cross section σ) of the incident particles, the
amount of the target material, the half-life of the radionuclide produced, and the
duration of irradiation. The term (1 − e
−λt
) is called the saturation factor and
approaches unity when t is approximately 5–6 half-lives of the radionuclide in question. At that time, the yield of the product nuclide becomes maximum, and its rates
of production and decay become equal. For a period of irradiation of 5–6 half-lives,
Eq. (5.2) becomes
(5.3)
A graphic representation of Eqs. (5.2) and (5.3) is given in Fig.5.2.
The intensity of the irradiating particles is measured by various physical techniques, the description of which is beyond the scope of this book; however, the
values are available from the operator of the cyclotron or the reactor. Normally, the
14
neutron ux is of the order of 10
neutrons/s ⋅ cm2 and the proton beam runs around
the orders of 1012 protons/s ⋅ cm2. The formation cross sections of various nuclides
are determined by experimental methods using Eq. (5.1), and they have been compiled and published by many investigators. The number of atoms N of the target is
calculated from the weight W of the material irradiated, the atomic weight Aw and
natural abundance K of the target isotope, and Avogadro’s number (6.02×1023) as
follows:
60210
A
.
(5.4)
Duration of irradiation (half–lives)
8
96.9 %
Activity (arbitrary unit)
NoofUatomsN
235 23
3 235 60210
76910
/.
.
5.5 Equation forProduction ofRadionuclides
93.7 %
87.5 %
57
0.8
0.6
0.4
0.2
Fig. 5.2
Production of radionuclides in a reactor or a cyclotron. The activity produced reaches a
maximum (saturation) in 5–6 half-lives of the radionuclide
75%
50%
123
4 5
6 7
After irradiation, isotopes of different elements may be produced and therefore
should be separated by the appropriate chemical methods. These radionuclides are
identied and quantitated by detecting their radiations and measuring their halflives by the use of the NaI(Tl) or Ge(Li) detectors coupled to a multichannel pulse
height analyzer. They may also be assayed in an ionization chamber if the amount
of radioactivity is high.
Problem 5.1
Calculate the activity of 99Mo produced when 3 g of
3days in a nuclear reactor, given that neutron ux is 1014 neutron/(s⋅cm2), t
of 99Mo is 66h, and the formation cross section for 99Mo is 30 barn.
Answer
Neutron fluxI scm10
14 2
n /
21
Formation cross-sectionfor Mo=30 barn
-242
30 10 cm
--232
310
cm
235
U is bombarded for
99
1/2
(continued)
58
Ip
42
Cross sectioncmcm200 10 210
27 2252
Activity A dps dps10 37 10 37 10
10 11
..
Time of bombardmentst 3246060
2 592 10
.
Decay constant forMo
99
0 693 66 60 60
./
A
..
5 Production ofRadionuclides
Problem 5.1 (continued)
5
61
TBq
s
6
e
10
ctivity of Mo
99 14 21 23 292102592
29210
.
10 76910310 1
.
12210122
329 7
13
..
.
Ci
Problem 5.2
How long a target of 2 g of 18O-water needs to be bombarded to produce 10
curies of 18F by the 18O(p, n)18F reaction, given that the proton beam intensity
is 50 μA/cm2, 18O enrichment is 75%, the formation cross section is 200
mbarn and t
of 18F is 110min.
1/2
Answer
Since 1 ampere (A) is equal to 1 coulomb (C)/s and 1 C equals to
6.25×1018 protons, the number of protons in 50μA/cm2 is
6181
.. /
rotonscm50 10 625103125 10
5
Effective molecular weight of enriched water is 0.25×18+0.75×20=19.5
No of target atomsN
Decay constant
./..
0752 19 5602 10
41
s
1
s
18
22
.ooms of O
46310
at
0 693 110 60
./
10510
.
0 000105
.
Now,
11 14 22 25 0 000105
7103125 10 46310210 1
.. .
28
12 0 000105ext.
.
99101
23
.ext
(continued)
10128
5.6 Radionuclide Generators
59
Problem 5.2 (continued)
0 000105
xt
.
e
e
0 000105
.
t =
=
=
xt
0 1370
0 000105
.
1304
21 7
0 8720
.
.
s
.min
.
5.6 Radionuclide Generators
Radionuclide generators provide the convenient sources of short-lived radionuclides that are very useful clinically. The basic requirements for a generator are that
a parent radionuclide has a longer half-life than that of the daughter radionuclide,
and the daughter can be easily separated from the parent. In a generator, a long-lived
parent radionuclide is allowed to decay to its short-lived daughter radionuclide, and
the latter is then chemically separated. The importance of radionuclide generators
lies in the fact that they are easily transportable and serve as sources of short-lived
radionuclides in institutions without cyclotron or reactor facilities.
A radionuclide generator consists of a glass or plastic column tted at the bottom
with a fretted disk. The column is lled with adsorbent material such as ion exchange
resin, alumina, and so forth, on which the parent nuclide is adsorbed. The parent
decays to the daughter until transient or secular equilibrium is established (Eqs.
(3.17) and (3.18)) in several half-lives of the daughter. After equilibrium, the daughter appears to decay with the same half-life as the parent. Because of the differences
in chemical properties, the daughter activity is eluted with an appropriate solvent,
leaving the parent on the column. After elution, the daughter activity builds up again
and can be eluted repeatedly.
A schematic diagram of a radionuclide generator is shown in Fig.5.3. The vial
containing the eluant is rst inverted onto needle A, and an evacuated vial is then
inverted on the other needle B.The vacuum in the vial on needle B draws the eluant
from the vial A through the column and elutes the daughter nuclide, leaving the parent nuclide on the column. In some commercial generators, a bottle of eluant is
placed inside the housing, and aliquots of eluant are used up in each elution by the
evacuated vial.
An ideal radionuclide generator should be simple and sturdy for transportation.
The generator eluate should be free of the parent nuclide and the adsorbent material.
Several radionuclide generators are available for ready supply of short-lived
radionuclides:
(271days)– 68Ga (68 min); 82Sr (25.6days)– 82Rb (75s); 81Rb (4.6h)–
Of these, the 99Mo–
medicine.
99
Mo (66h)–
99m
99m
Tc (6h);
113
Sn (117days)–
113m
In (100 min); 68Ge
Tc generator is the workhorse of nuclear pharmacy in nuclear
81m
Kr (13s).
60
A
tt
0
..
Fig. 5.3 Typical
radionuclide generator
system. Vacuum in vial B
draws the eluant from vial
A through adsorbent
material and the daughter
is collected in vial B
5 Production ofRadionuclides
5.6.1 99Mo–
The 99Mo–
99m
Tc Generator
99m
Tc generator is constructed with alumina (Al2O3) loaded in a plastic or
glass column. The 99Mo activity is adsorbed on alumina in the chemical form
24−
MoO
(molybdate) and in various amounts. The amount of alumina used is about
5–10 g depending on the 99Mo activity. Currently, all generators use ssion- produced
99
Mo. The growth and decay of
3.5 of Chap. 3. The
99m
Tc activity is eluted with 0.9% NaCl solution (isotonic saline)
and obtained in the chemical form of Na
Considering that only 87% of 99Mo decays to
99m
Tc along with the decay of 99Mo is shown in Fig.
99m
TcO4.
99m
Tc, the
99m
Tc activity ATc can be
calculated from Eq. (3.15) as follows:
Tc Mo
Ae e
0 957
.
0 0105 0 1155
(5.5)
where (AMo)0 is the 99Mo activity at t = 0, λMo=0.0105h−1, and λTc=0.1155h−1. The
time t has the unit of hour. At transient equilibrium (from Eq. (3.16)),
0 1155
t
AAe
.
Tc Mo
0 957
.
.
0 957
0
A
Mo
t
(5.6)
Upon elution with saline, approximately 75–85% of the total activity is eluted
from the column. After about 4 half-lives, the
Problem 5.3
A 2.5-Ci 99Mo–
99m
Tc generator calibrated for Thursday noon was received on
Wednesday afternoon. What would be the
on Friday, assuming a transient equilibrium between 99Mo and
99m
Tc activity again reaches maximum.
99m
Tc activity, eluted at 8:00 a.m.
99m
Tc at the
time of elution, and 85% elution yield?
(continued)
= 2026mCi
99
0 957 2026 085
1648
m
Tc activity
mCi
..
5.7 Cyclotron Production of
99m
Tc
Problem 5.3 (continued)
Answer
99
Mo activity on Thursday noon = 2500 mCi
The time from Thursday noon to 8:00 a.m. on Friday = 20h
Therefore, 99Mo activity at 8:00 a.m. Friday=2500×exp(−0.0105×20)
Using Eq. (5.5) at transient equilibrium,
61
The 99Mo activity is likely to be eluted in trace quantities along with
99m
Tc activity.
This is called the 99Mo or Moly breakthrough. The presence of 99Mo gives unnecessary radiation dose to the patient. According to the Nuclear Regulatory Commission
(NRC) regulations, the permissible limit for 99Mo breakthrough is 0.15μCi (5.5
kBq) per millicurie (37 MBq) of
99m
Tc at the time of injection. The 99Mo breakthrough is determined by the detection of high-energy photons (740 keV and 780
keV) of
99
Mo in a dose calibrator after stopping 140-keV photons of
99m
Tc in a lead
container (6-mm thick).
Aluminum also is likely to be eluted from the Moly generator along with
99m
Tc
activity and must be checked. The presence of aluminum interferes with the preparation of
99m
Tc-labeled sulfur colloid by forming larger particles, which are trapped
in the lungs. It also agglutinates the red blood cells during labeling. Its acceptable
limit is 10μg/ml of
99m
Tc solution. Aluminum ion (Al3+) is checked by the colorimetric test using a paper strip impregnated with a coloring agent and comparing the
intensity of the color developed by the sample solution with that by a standard test
solution (10μg/ml). If the sample color is more intense than the standard color, the
99m
Tc sample is not acceptable for use.
5.7 Cyclotron Production of
99m
Tc
In the past decade and in recent years, there had been a severe shortage of ssionproduced 99Mo due to the closing of major reactors—some for ageing and some for
maintenance—in several countries. This led to an acute shortage of
99m
Tc in nuclear
medicine communities throughout the world causing great difculty in patient care.
Investigators have been looking for alternative sources of
99m
Tc can be produced in a medium-energy cyclotron by the
99m
Tc and suggested that
100
Mo (p, 2n)
99m
Tc
reaction in quantities sufcient to supply daily to several hospitals for all necessary
nuclear imaging studies. It has been shown that at 24MeV proton energy and a
99m
beam current of 500 μA, nearly 74.3Ci (2.75 TBq) of
Tc can be produced for