Medical Radioisotopes Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on Mo

1.1 Radioisotopes in nuclear medicine Nowadays, many different stable and radioactive isotopes, each with unique physical and chemical properties, play significant roles in technological applications of importance to our modern society and are substantial to scientific research. One of the most common applications is the use of the radioisotopes in medicine. Medical radioisotopes are used to label some special chemical compounds to form radiopharmaceuticals. Radiopharmaceuticals are used extensively in the field of nuclear medicine in three main branches. The largest and the most common type involve diagnostic procedures in which a radionuclide in a chemically suitable form is administered to the patient, and the distribution of the radioactivity in the body is determined by an external radiation detector (Qaim, 2008). The results are in the form of image of the involved organ, which provides information about the functioning of person’s specific organs via emission tomography. The second branch of nuclear medicine deals with radionuclide techniques that are used for the analysis of concentration of hormones, antibodies, drugs and other important substances in samples of blood or tissues. The third branch is radiation therapy, which is the ultimate aim of all diagnostic investigations. Here the tissues or organs are treated with radiation and restored to the normal functions in the human body (Loveland, et al., 2006).


Introduction 1.1 Radioisotopes in nuclear medicine
Nowadays, many different stable and radioactive isotopes, each with unique physical and chemical properties, play significant roles in technological applications of importance to our modern society and are substantial to scientific research.One of the most common applications is the use of the radioisotopes in medicine.Medical radioisotopes are used to label some special chemical compounds to form radiopharmaceuticals. Radiopharmaceuticals are used extensively in the field of nuclear medicine in three main branches.The largest and the most common type involve diagnostic procedures in which a radionuclide in a chemically suitable form is administered to the patient, and the distribution of the radioactivity in the body is determined by an external radiation detector (Qaim, 2008).The results are in the form of image of the involved organ, which provides information about the functioning of person's specific organs via emission tomography.The second branch of nuclear medicine deals with radionuclide techniques that are used for the analysis of concentration of hormones, antibodies, drugs and other important substances in samples of blood or tissues.The third branch is radiation therapy, which is the ultimate aim of all diagnostic investigations.Here the tissues or organs are treated with radiation and restored to the normal functions in the human body (Loveland, et al., 2006).
Medical Radioisotopes Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on nat Mo 5 radiotherapy is a unique cancer treatment modality.It is systemic and non-invasive.The uptake and retention in the tumour can be assessed with a tracer study before administering a therapeutic dose to the patient.The major criteria for the choice of a radionuclide for endotherapeutic use are suitable decay characteristics and suitable biochemical reactivity.Concerning the decay properties, the desired half-life is between 6 hours and 7 days and the emitted corpuscular radiation should have a suitable linear energy transfer (LET) value and range in the tissue (Qaim, 2003;Sharp, et al., 2005).The ratio of non-penetrating to penetrating radiation should be high.The daughter should be short-lived or stable.The stability of the therapeutically pharmaceutical is demanded over a much longer period than that in the case of a diagnostic pharmaceutical.Thus, the choice falls on about 30 radionuclides.Most of them are -emitters but several of them are  emitters and Auger electron emitters.

Medical radioisotopes production
The main processes to produce the medical radioisotopes are neutron activation, nuclear fission, charged particles induced reactions and radionuclide generators.Mostly, chemical separation is needed to separate the required isotope from targets and any produced impurities before using in the labeling process.
The medical radioisotopes can be produced using nuclear reactors either by neutron activation or by nuclear fission.The first procedure depends mostly on the thermal neutron capture process (n, ).These isotopes will decay by means of -emission accompanied with some gamma rays and could be used in treatment or Single Photon Emission Computed Tomography (SPECT).The second procedure based on the fission of a heavy nucleus, from the fuel after thermal neutron absorption.Some of the produced fission fragments have found medical applications such as 99 Mo (used as 99 Mo/ 99m Tc generator), 131 I, and 133 Xe (Qaim, 2004).Charged particle accelerators are another tool for producing medical radioisotopes using charged particle induced reactions on some stable isotopes.The accelerators used for this purpose should deliver ion beam with enough energy suitable for the used nuclear reaction and high beam intensity for production of reasonable radioactive yield in a reasonable irradiation time.Usually cyclotron accelerators with energies in the range 10 to 50 MeV are suitable for this purpose.Cyclotron radionuclide production involves various constraints.First, a target has to be prepared, quite often from isotopically enriched material and energy should be carefully chosen to reduce, as much as possible, the impurities level.Second, the target should be stable in respect to ionizing radiation and heat generated by slowing down of the charged particles.Therefore, targets should be as thin as possible, just enough to degrade the incident energy to the required threshold energy, and they should display good heat conductivity to allow efficient cooling.After irradiation, the target is dissolved and various radiochemical operations are performed to isolate and purify the radionuclide.
The produced isotopes will usually be neutron deficient.This type of isotopes decay with + and/or EC accompanied with specific gamma rays and can be used for Positron Emission Tomography (PET) such as 11 C, 15 O, 13 N, and 18 F or SPECT such as 111 I, 67 Ga and 201 Tl (Lamberecht, 1979;Qaim, 2001).A number of isotopes as shown in Table 1 are technically available for use in medical applications (Troyer & Schenter, 2009).
Table 1.Common medical isotopes sorted by use category and production method (Troyer & Schenter, 2009)

Molybdenum and technetium in nuclear medicine
M o l y b d e n u m i s u s e d a s a t a r g e t m a t e r i a l f o r t h e p r o d u c t i o n o f m e d i c a l l y i m p o r t a n t radioisotopes, such as 99m Tc/ 99 Mo, 96(m+g) Tc and 94m Tc. 94m Tc (52min), has shown its applicability as a PET isotope (Rösch and Qaim, 1993;Nickles, et al., 1993;Sajjad and Lambrecht, 1993;Rösch, et al., 1994;Fabbender, et al., 1994;Qaim, 2000;Hohn, et al., 2008). 96Tc (4.28d) has been proposed for the use in prevention of coronary restenosis by Fox (2001).Despite of favorable moderate half-life, other isotopes of technetium, like, 93 Tc (2.75h), 94 Tc (4.883h) and 95 Tc (20.0h) are seldom discussed.Specially, radiological half-life of 94 Tc is ideal for diagnostic purposes. 95Tc (20.0h), due to its comparatively longer half-life is also promising for tracking long processes, like, metabolic pathways for brain and heart, studies with proteins, anti bodies, etc.Among short-lived radionuclides, 93 Tc (2.75 h) is another promising isotope for imaging as suggested by (Lambrecht and Montner, 1982).One of the most important medical radioisotopes is 99m Tc (T ½ = 6.01 h), which has a gamma ray energy of about 140 keV.The fact that both its physical half-life and its biological halflife are very short, as seen in Table 2, leads to a very fast clearing from the body after an imaging process.A further advantage is that the gamma is a single energy, not accompanied by beta emission, and that permits a more precise alignment of imaging detectors.

Isotope
Half-lives in days Table 2.The physical, biological and effective half lives for 99m Tc 99m Tc is a vital part of diagnostic tests for heart diseases and cancers; It accounts for over 80% of all diagnostic nuclear medicine procedures worldwide.According to the latest survey, the world demand for production of 99 Mo/ 99m Tc is estimated to be around 7 kCi/week and further growth is predicted (Takács, et al., 2003).Currently, only five nuclear reactors produce 99 Mo/ 99m Tc leading to a predicted shortage in covering the world demand.Consequently, many studies nowadays concentrate on producing 99 Mo generators with an alternative method using cyclotron accelerators ( Van der Marck, 2010;Gull, 2001).99m Tc is obtained from the decay of its parent isotope 99 Mo.It was discovered in 1937, and the first 99 Mo/ 99m Tc generator was invented at the Brookhaven National Laboratory in the U.S. in 1957.General usage of 99m Tc began in the early seventies when the Chalk River Laboratory established routine production of 99 Mo, its parent isotope (Tammemagi and Jackson, 2009;Ullyett, 1997).99m Tc is versatile and can be used to produce some 20 different compounds of radiopharmaceuticals.There are various technological options for the production of 99m Tc/ 99 Mo listed in Table 3.The usual production of 99 Mo for nuclear medicine depends on:

Reactors
1.The neutron induced fission of 235 U, which results in expensive but high specific activity 99 Mo (IAEA-TECDOC-1065, 1999), or 2. The (n, ) nuclear reaction with 98 Mo, 24% using natural Molybdenum, resulting in inexpensive but low-specific activity 99 Mo.Thus, for either method, at least one neutron is required for the reaction.Neutrons can be produced from accelerator reactions where the charged particles strike heavy atoms, also from alpha or gamma reactions with light atoms, such as beryllium or lithium.However, to produce the large quantities of neutrons needed for production of useful quantities of 99 Mo, the most effective source is a critical nuclear reactor operating at powers in the range of megawatts.Each fission process of an atom of 235 U produces an average of about 2.5 neutrons.In an operating reactor, these neutrons either are absorbed by materials in the reactor or escape from the boundaries of the reactor.One neutron must cause fission in another 235 U atom.Of the remaining 1.5 neutrons from each fission process in a critical reactor, some small fractions are available for production.The most appropriate target material for low specific activity 99 Mo production is molybdenum trioxide (MoO 3 ); neutron activation occurs via the reaction 98 Mo(n, ) 99 Mo.The potential use of accelerators for these purposes is another issue of current scientific and technological interest.Recently, a matter of concern has been the availability and supply of 99 Mo for the manufacturing of generators.These concerns arose from several factors including, amongst others, the shutdown of some nuclear reactors, uncertainty of reliable operating condition for radioisotope production and easy availability of enriched 235 U target materials.More recently, the utilization of charged particle accelerators, either LINAC's or cyclotrons, has been discussed as a potential alternative technology to the fission route.These discussions have been prompted by basic research concerns as well as the need to explore new production routes to offset the perceived situation of future problems with the availability of 99 Mo if no new dedicated reactors are licensed.The production of 99 Mo via the 100 Mo(p,pn) reaction was evaluated.A good agreement was found among the different excitation functions available.However, because of the rather low cross-section values found in these measurements, the production of 99 Mo via this potential process was found to be largely impractical.A significant limiting factor of this approach appears to be the need for a large inventory (tens of kg quantities) of enriched 100 Mo, the logistical considerations of its distribution and recovery, and the cost (2 US $/mg).Furthermore, proton accelerators delivering mA beam on target would be required including the development of high power targets.The production of 99m Tc via the 100 Mo(p,2n) reaction was also evaluated, and the cross section data available were found to be consistent and in good agreement.Extrapolating 99m Tc yields obtained from this data, using the operational conditions of the existing 30 MeV accelerator technologies, suggest that large-scale (kCi) production of 99m Tc is possible (Glenn, et al., 1997).

Nuclear data needs
The excitation function measurements of charged particle induced reactions are needed to improve and study the ideal way for medical radioisotope production.The optimization of nuclear reaction for the production of radioisotope at a cyclotron involves a selection of the projectile energy range that will maximize the yield of the product and minimize that of radionuclide impurities.The IAEA Coordinated Research program (CRP) which deals with all aspects of the production of medical radioisotopes that can be used for diagnostic and therapeutic purposes, requires a reliable database for production cross sections, not only for the main and the monitor reactions but also for the associated producing impurity reactions (IAEA-TECDOC-468, 2009).The program includes targetry (preparation, cooling and chemistry), yields, radionuclidic impurities, radiation dose from targets and target backings.By revising the database situation for 99 Mo & 94,95g,95m,96(m+g),96g,99m Tc production, it could be seen that the status of the present information is still not satisfactory for a detailed optimization of the production processes.Several authors (Kormali, et al., 1976;Takács, et al., 2002;Bonardi, et al., 2002;Uddin, et al., 2004;Khandaker, et al., 2006;Khandaker, et al., 2007;Uddin, et al., 2008) have reported a variety data for proton-induced reaction crosssections on molybdenum in the medium-energy range, but large discrepancies can be found among them.These discrepancies limit the reliability of data evaluations.

Experimental techniques
The reaction cross-section of the proton-induced reactions on molybdenum were measured, in this work, as a function of proton energy in the range from the respective threshold for each contributing reaction (E thr ) to about 40 MeV using the activation method and the wellestablished stacked foil technique combined with high resolution gamma-ray spectroscopy.

Stacked foil technique
By this method a series of thin target foils are put together to form the target as in Figure 1.Each target foil (Mo in this study) is followed by another material (mainly Al in our case) to www.intechopen.comMedical Radioisotopes Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on nat Mo 9 catch the ejected product nuclides (recoils) from the preceding Mo foil.This catcher foil is selected so that it does not produce any radioactive product by the given bombarding particle at the energy range used.The catchers should be also as low Z-material as possible to decrease the gamma attenuation during the activity measurements.Therefore, a pair of foils (Mo+Al catcher) will contain the total produced radioactive isotopes from the given Mo foil after the irradiation.The catcher Al foil contains only the ejected atoms (radionuclides) from the Mo implanted into it.The advantage of the stacked foil method is that one can get a whole excitation function curve using a lower number of irradiations.Another advantage of this method is that each target of the stack is irradiated with the same integrated beam charge.The main conceptual disadvantage of the staked foil technique is concerned with the energy straggling that is induced in the beam by passing through the stack of thin foils, recoil catchers and energy degraders (Zeigler, J.F., 1995).The inaccuracy of the foil thickness and surface roughness, which cause the accumulation of the error in energy calculations from the first to the last foil of the stack, which can be corrected by inserting some beam current monitor foils in different regions over the stack.The total charges collected by the Faraday cup have been integrated using current integrator circuit with good linearity at low current values.The target foils of 10 mm diameter were sufficiently larger than the proton beam diameter.Care was taken to ensure that equal areas of the monitor and the target foils intercepted the beam.The irradiation geometry used guaranteed that practically the whole beam passed through every foil.The secondary effect of the interactions of the secondary produce neutrons with the molybdenum targets was checked by placing some foils in the end of the stack far behind the range of the fully stopped proton beam followed by the measurement of its activities.

Targets and irradiations
Thin foils of molybdenum with natural isotopic composition were used as our main targets.
There are 35 known isotopes of molybdenum ranging in atomic mass from 83 to 117, as well The irradiations were performed using an external beam of accelerated protons with energy of about 40 MeV provided by K500 superconducting cyclotron at Texas A&M University, Cyclotron institute, USA.Two different sets of stacks were irradiated to cover the energy range from the respective threshold for each reaction up to 40 MeV.Each stack was made of several groups of targets; nat Mo (99.999% and 50 µm thickness) as the main target foils, nat Cu (99.98% and 125 µm thickness) were used as monitor foils that acted also as beam degraders and nat Al (99.999% and 50,100 µm thickness) as catcher foils, all foils were supplied by Goodfellow, Cambridge, UK.The set of foils was pressed together to avoid air gaps between them, which could have influence on the vacuum and particles stopping.The proton energy degradation along the stack was determined using the computer program SRIM-2003 assuming the incident energy was 40 MeV (Ziegler, et al., 1985).The irradiation conditions for each stack are shown in

Monitor reactions
To confirm the cyclotron beam intensity and energy, a thin copper monitor foil (50 m) was placed in the front of the stack (Al-Saleh, et al., 2006).Copper is an ideal target material with respect to its availability, physical, mechanical and chemical properties to be used in monitoring process.This Cu foil was irradiated simultaneously with the main target foils and then analyzed with the same gamma ray spectrometer in a comparable geometry.Thus, the ratio (α exp ) between the measured cross section values for the 63 Cu(p,n) 63 Zn and 63 Cu(p,2n) 62 Zn nuclear reactions can be calculated using equation (1) (Piel, et al., 1992):

Radioactivity measurements
The radioactivity of the residual nuclei in the activated foils was measured nondestructively using a HPGe -ray detector with 70% efficiency relative to a (3"x3") NaI detector, and energy resolution of 2.2 keV for the 1.332 MeV -line of the 60 Co standard source, a peak to Compton ratio of 58: l.The detector absolute efficiencies for various source-detector distances and photon energies were determined experimentally by using a selected set ofray standard sources ( 60 Co, 137 Cs, 133 Ba and 152 Eu), of known activities, to cover the whole energy range of the studied -rays.The detector-sample distance was kept large enough to ensure the point source geometry and to keep the dead time within 8% or less.In addition to the main characteristic -lines for each studied radioisotope, some other weaker -lines were also considered to minimize the relative errors due to counting statistics, wherever possible.
In the cases of the longer-lived radionuclides, activity measurements were carried out after sufficient cooling time, which is enough for the complete decay of most of the undesired short-lived isotopes, to avoid any possible interference of nearly equal energies -lines.The stack was dismantled and each foil was counted 2-3 times after different cooling times following the end of bombardment EOB to avoid disturbance by overlapping -lines from undesired sources and to evaluate accurately the cross-sections for cumulative formation of the corresponding longer-lived daughter radionuclide.Figure 6 presents an example of the calibrated measured -ray spectrum with identifiedlines covering the energy range up to 1350 keV.Table 6 shows the contributing reactions and the decay data of all the investigated radionuclides, which were taken from the Table of Isotopes (Firestone, 1998 andT-16, Nuclear Physics Group, LANL 1997). Fig.

Separation of interfered γ-lines
Some investigated radionuclides emit -rays that have very close energies, which were difficult to be separated using the HPGe spectrometer.
The individual activities of those overlapped -rays were analyzed using the difference in half-lives of the contributing nuclides by plotting the -ray emission rate as a function of time.Figure 7 shows the radioactive decay curve for the 140.5 keV -peak which resulted from the decay of the directly produced 99 Mo (65.94 h, 140.51 keV), the directly and indirectly produced 99m Tc (6.01 h, 140.51 keV), and 90 Nb (14.6 h, 141.2 keV).The radionuclides decay completely in the order of their half-lives, 99 Mo the longest-lived nuclide is the last to decay.After more than 14 days, the remaining activity was due to decay of the daughter nuclide 99m Tc in transient equilibrium with the parent 99 Mo radionuclide.The activities of the radionuclide; 99 Mo(A 2 ) → 99m Tc(A 1 ) at the end of bombardment (EOB) were estimated by using equation (2) (Uddin, et al., 2004): where t c is the respective cooling time, 1 and 2 are the decay constants of 99 Mo and 99m Tc, respectively, and A 1(EOB) is the activity of 99 Mo at the EOB.To separate the activities after the EOB of 90 Nb(A 3(EOB) ) and 99m Tc(A 4(EOB) ), we used the following equation ( 3): exp exp The daughter 99m Tc activity decreases from the maximum at a constant rate, which depends on the decay rate of 99 M. Then the directly produced 99m Tc completely decayed out before the measurement.The measured activity for the 140.5 keV -line was the sum of the -line from the daughter 99m Tc and from 90 Nb.We deduced the activities of 140.5 and 141.2keV -lines from the independent -lines of 99 Mo and 90 Nb, respectively; an excellent agreement was obtained when compared with the results of radioactive decay curve.

Cross section calculations and uncertainty
The reaction cross sections for the nuclear reactions nat Mo(p,x) were calculated using the activation formula as in equation 4 considering the decay data and rates of the radioactive isotopes produced, the detector absolute efficiency, and the measured beam intensity (Helus & Colombetti, 1980).
Whereas; M is the target molecular weight, Ze is the projectile charge, is the decay constant, T γ is the net area under each -peak, I γ is the gamma line intensity, Δx is the thickness of each target foil, N A is the Avogadro's number, f is the abundance of the isotope, ρ is the target density, I is the beam intensity, ε abs is the detector efficiency corresponding to each -line energy, t c is the cooling time and t m is the measuring time.The total experimental error was calculated by combining the individual errors as a square root of the sum of squares of the contributing relative errors, which are the lack of precision in: measuring the absolute detector efficiency of 3-6%, the calculation of the area under the photoelectric peak 1-4%, measuring the current intensity 4-7%, the calculation of irradiation time 2 %, determining the foil thicknesses and composition 1-4% and the nuclear decay data of 3%.The total experimental errors were obtained to be (8-12%).The total uncertainty in each energy point depends on the irradiation circumstances and the position of the foil in the stack.

Nuclear model calculations
All the measured cross sections over the whole energy range were simulated using TALYS (Koning, et al., 2008) code.A short description for the codes is given in the following:

TALYS code
We calculated the independent formation cross sections for both the ground and/or the isomeric states by using the TALYS code, which is a computer program that integrates all types of nuclear reactions in the energy range of 1 keV-200 MeV.TALYS incorporates modern nuclear models for the optical model, level densities, direct reactions, compound reactions, pre-equilibrium reactions, fission reactions, and a large nuclear structure database (Koning, et al., 2008).The database of this code is derived from the (Reference Input Parameter Library, http://www-nds.iaea.org/ripl2/).The pre-equilibrium particle emission is described using the two-component exciton model.The model implements new expressions for internal transition rates and new parameterization of the average squared matrix element for the residual interaction obtained using the optical model potential.The phenomenological model is used for the description of the pre-equilibrium complex particle emission.The contribution of direct processes in inelastic scattering is calculated using the ECIS-94 code (Raynal, 1994) incorporated in TALYS (Raynal, 1994).The equilibrium particle emission is described using the Hauser-Feshbach model.The default optical model potentials (OMP) which used in TALYS are the local and the global parameterizations for neutrons and protons.These parameters can be adjusted in some cases by the user.The present results of all the calculated excitation functions were evaluated using the default values of the code.

Result and discussion
The experimentally constructed excitation functions for the main investigated nat Mo(p,x) 99 Mo, 94g,95g,96(m+g),99m Tc nuclear reactions are shown in Figures 8-12 together with the results of the theoretical calculation using TALYS code and the previously published data.The numerical values of the present experimental cross-sections and their estimated uncertainties are presented in Table 7.

Proton Energy MeV
Reaction Cross-section (mb)  A comparison between our measured cross-sections and the previously reported data together with the theoretical calculations using TALYS code is presented in Figure 8. (Takács, et al., 2003) reported cross-section data up to 37 MeV and (Levkovskij, 1991) reported up to 29 MeV for 99 Mo production on the enriched 100 Mo isotope.Our measured values are consistent with the data presented by (Uddin, et al., 2004).The data reported by (Scholten, et al.,1999) are consist with our data in energy range lower than 22 MeV, although his results at the higher energies are scattered.Our results showed agreement with (Takács, et al., 2003) in low energy region.The data presented by (Levkovskij, 1991) are about 25% higher than our data.(Lagunas-solar, et al., 1991) reported numerical cross-section data that are much lower than our measured data and the other published data as well in the energy region above 20 MeV.A good agreement exists between the measured cross-sections and the TALYS code calculations within the experimental error and that fact confirms the reliability of our measured data.6. Mainly we used the 702.63 keV -line, which has no interference with any other -lines from any other produced isotopes in a cooling time of about 5 hours, to determine the cross section for 94g Tc production.The present experimental excitation function for the reaction nat Mo(p,xn) 94g Tc is presented in Figure 9 together with the previously published results and the calculated cross sections by the used nuclear model code TALYS.
A good agreement is found between our measured cross sections and the ones reported by (Bonardi, et al., 2002 andUddin, et al., 2004) over the entire energy range.There is a remarkable difference between the present results and the reported data by (Khandaker, et al., 2007) especially for the energies lower than 20 MeV and above 30 MeV.The measured cross sections by (Kormali, et al., 1976) show about 40% lower values than our data in the energy range from 11-20 MeV.The TALYS code calculation is about 50% higher than our measured data and higher than all the previously reported data sets.and the shorter lived ground state 95g Tc (T½ = 20 h, 9/2 + ).In this study, we report only the measured cross sections for 95g Tc due to the difficulty in measuring the interfering characteristic -rays for 95m Tc as shown in Table 6.The 95g Tc activity measurement was based on detecting the main -line at 765.79 keV.A comparison of the present measured data with some previously reported data and the TALYS code calculations is shown in Figure 10.The cross section is only measurable at 8 MeV, then increases gradually due to the 95 Mo (p,n) reaction.The contribution of the 96 Mo (p,2n) reaction appears as a little plateau starting at about 12 MeV, while the 97 Mo (p,3n) reaction contribution starts at about 20 MeV, creating another small peak.There is a good agreement between our experimental excitation function and the previously published data by (Bonardi, et al., 2002 andKhandaker, et al., 2007) within the experimental error, while the earlier presented study by (Khandaker, et al., 2006) shows 35% higher value than our experimental data at energies above 26 MeV.However, the data reported by (Birattari, et al., 2002) shows higher cross-section values in the proton energy range > 10 MeV.The presented data by (Uddin, et al., 2004) shows inconsistency with most of the other experimental data, , especially for the point at about 22 MeV.
The TALYS code calculation results are in good consistency with our experimental data within the experimental error, but there exists a small drop in the measured cross section values in the higher values of the energy range.).In this study we measured the crosssection of 96g Tc using the main characteristic -line 778.2 keV, while it was not possible to measure the characteristic isomeric transition 34.28 keV of the metastable state due to the intensive interfering of the X-rays.According to the short half-life and the high IT decay rate of the metastable state, we can consider the measured cross section as the total cross section of 96(m+g) Tc without measuring the metastable state independently.Figure 11  Very good agreement is found in the energy range above 9MeV between the present data and those reported by (Takács, et al., 2002;Uddin, et al., 2004& Khndaker, et al 2006,2007).


The results by (Bonardi, et al., 2002) overestimate the cross-section value in the energy range < 10 and >26 MeV.


The data by (Khandaker, et al., 2007) are somewhat low in the proton energy range below 10 MeV.An overall good agreement is found between the present experimental excitation function for 96(m+g) Tc formation and the calculated theoretical results by TALYS code and the recommended data (Takács, et al., 2002), within the experimental error.Possibly, the highest contribution is from the 100 Mo(p,2n) 99m Tc reaction (on the 9.63% 100 Mo present in the highly chemically pure Mo sample).Activity of 99m Tc was measured in this work by detecting the gamma peak at energy 140.5 MeV after the resolution of this peak as described before.The measured excitation function is compared with some earlier published data and the TALYS code calculations in Figurer 12.The data of (Takács, et al. 2003) and( Kandaker, et al. 2007) fit nicely our measured data specially in the low energy part up to 20 MeV.At higher energies (Kandakar, et al. 2007) data clearly over estimate our results.The results of (Challan, et al. 2007) agree with our results except the last two points.The cross section data for (Scholtan, et al. 1999) are clearly lower than our values over the hall energy range.The TALYS calculations over estimate the present results, especially in the energy range lower than 18 MeV , while they fit, within the experimental errors in the higher range.

Integral yield calculations
The integral yields, at the end of bombardment, for the production of the different isotopes were derived using the measured excitation functions for the production of these radioisotopes.The method was done by assuming the thick target as dividend to several thin targets each of an equivalent thickness of about 0.5 MeV.The cross section at each thin target is assumed constant, because of the small energy interval through the target.The number of target atoms/cm 2 was calculated using the target thickness, which reduce the proton energy by 0.5 MeV.The differential yield produced in each thin target was calculated using the following equation ( 5 Whereas, 購 博岫継岻 (mb) is the average cross section at a specific energy; N is the number of target atoms/cm 2 ; λ is the decay constant for the produced isotopes; P is the number of incident protons/sec for (1 A) and the irradiation time (t b= 1 h).We then calculated the integral target yield by summing up the differential yields.Figure 13 represents the values of the integral target yield for the studied reactions as a function of the proton energies.Obviously, the yields of the investigated radioisotopes increase with the proton energy and start to saturate at energy of about 30 MeV.The nearly saturation values for 99 Mo, 94g Tc, 95g Tc, 96(m+g)

23
For the production of 99m Tc via cyclotron, it is highly recommended to use an enriched target of 100 Mo to exclude all the other impurities by using the indirect 100 Mo(p,pn) 99 Mo and the direct 100 Mo(p,2n) 99m Tc nuclear reactions.From the present data we conclude that the optimum energy range for the production of 99m Tc directly and indirectly using protons is Ep= 35-18 MeV, the integral target yield amounting to to 412 MBq/ A.h to 1000 MBq/ A.h at saturation with respect to the half lives of both 99 Mo and 99m Tc.

Conclusion
99m Tc radioisotope is a very important medical radioisotope for diagnostic tests.In this work an alternative root of producing this isotope, either directly or through the generator 99 Mo ( 99m Tc ) , namely using cyclotrons, is introduced and discussed.The excitation functions for the different proton-induced nuclear reactions on nat Mo target are measured and compared with some previously measured data.This study aims to resolve some contradictions between the existing data, and to give a reliable data set for the production of 99m Tc and some other isotopes of importance in nuclear medicine beside some impurities.Monitoring reactions on Al and Cu targets are also measured and compared with the recommended IAEA data sets, in order to give high degree of consistency to our results.The present excitation functions confirm some previously measured sets, while contradict with others.Theoretical code calculations using TALYS code are performed and show a good consistency with the measured cross section values.The code calculations can be used for cross section estimations, when not enough experimental data exist.Furthermore, the integral or thick target yields are estimated based on the measured excitation functions for all the investigated reactions.Finally, it is well known that for medical uses, enriched targets have to be used in the production to avoid the secondary produced unwanted impurities.
While the studies on natural targets, gives an idea about the suitable energy range for maximum production of the wanted isotope and minimum of the impurities.

Fig. 2 .
Fig. 2. Schematic diagram of the target holder and the Faraday cup Fig.4.The energy calibration for the Proton beam using the exp ratio for σ 62Zn and σ 63ZnThe measured Cu monitoring reactions were also used for beam intensity calculations, using the reverse relation to the well-known reaction cross section values.The charge collected in the Faraday cup was registered, from which the average beam current was deduced.The two results generally agreed within 10%.The uncertainty of the proton energy along the stack was checked by inserting Al and Cu monitor foils into different points of the stack then by comparing the measured excitation functions for nat Al(p,x)22,24 Na and nat Cu(p,x)62,63,65 Zn monitor reactions with their recommended values(Tárkányi, et al., 2001), as shown in Figure5.The individual uncertainties of the contributing reactions were taken into account considering the cumulative effects.The total uncertainty for each energy point depends on the irradiation circumstances and the position of each foil in the stack.These are the uncertainties of the target homogeneity and thickness, the incident beam energy and the beam straggling.Typical uncertainty in the energy was (±0.3 MeV) at the beginning of the stack and (±1.2 MeV) at the end.Furthermore, the very good agreement with the recommended values for the measured cross-sections of the studied monitoring reactions confirms the reliability of our experimental setup.

Fig. 7 .
Fig. 7. Resolving the 140 keV -line which produced from three different radioisotopes 99 Mo, 99m Tc and 90 Nb 99 Mo is produced by proton activation on nat Mo target via the contribution of two reaction channels 100 Mo(p,pn) 99 Mo (Q= 8.3 MeV) and 100 Mo(p,2p) 99 Nb (Q= 11.14 MeV) through the - decay of the parent isotope 99 Nb(15 s).The highest cross-section value of about 160 mb corresponds to E p = 30 MeV.

Fig. 8 .
Fig. 8. Excitation function of the nat Mo(p,x) reaction (full red dots with vertical and horizontal error bars) compared to some previously published results and the TALYS code calculations (curve).

Fig. 9 .
Fig. 9. Excitation function of the nat Mo(p,x) reaction (full red dots) compared to some previously published results and the TALYS code calculations (curve).

Fig. 10 .
Fig. 10.Excitation function of the nat Mo(p,x) 95g Tc reaction compared to some previously published results and the TALYS code calculations.
illustrates a comparison between our measured cross sections and the available published data together with the TALYS code calculations.Some findings can be summarized from this figure as follows:  The first part of the curve is due to 96 Mo(p,n) reaction.It starts to increase rapidly to form a peak at 12 MeV.Then it decreases slowly and forms a plateau in the range 16-21 MeV due to the contribution of the 97 Mo(p,2n) and 98 Mo(p,3n) reactions which start at www.intechopen.comMedical Radioisotopes Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on nat Mo 21 11 and 19 MeV, respectively.The rapid increase in the cross-section values at energies higher than 22 MeV indicates the increasing contribution of the (p,3n) reaction.

Fig. 11 .
Fig. 11.Excitation function of the nat Mo(p,x) 96(m+g) Tc reaction compared to some previously published results and the TALYS code calculations.

Fig. 12 .
Fig. 12. Excitation function of the nat Mo(p,x) 99m Tc reaction compared to some previously published results and the TALYS code calculations.

Table 3 .
The various technological options for the production of 99m Tc/ 99 Mo

Table 4 .
(Audi, et al., 2003)s Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on nat Mo 11 as four metastable nuclear isomers.The seven stable isotopes are listed in Table 2(Audi, et al., 2003).All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.Most stable radioisotopes of molybdenum

Table 5 .
Irradiation conditions in the experiments relevant to cross-section measurements

Table 6 .
The contributing reactions and the decay data of the investigated radioisotopes 6.A calibrated Gamma ray spectrum with identified -lines

Table 7 .
Measured cross-sections for the proton-induced nuclear reactions on nat Mo. www.intechopen.com Tc, and 99mTc are equal to 110, 600, 310, 90 and 910 MBq/ A.h, respectively.Medical Radioisotopes Production: A Comprehensive Cross-Section Study for the Production of Mo and Tc Radioisotopes Via Proton Induced Nuclear Reactions on nat Mo www.intechopen.com