NEUTRON AND GAMMA RAY SHIELDING PROPERTIES OF MODIFIED NI- TI- X MO MARAGING STEELS

MCNP5 program was used to investigate the neutron and gamma shielding properties of Ni-TiX Mo Maraging steels. The MCNP5 program experimental setup was designed using neutrons of 14.1 MeV from sealed neutron tube and 60 Co as a gamma rays source. Particular attention was given to the shielding properties of neutron and gamma for low nickel free cobalt titanium containing modified Maraging steel in correlation to the molybdenum as alloying element. Based on the shielding properties, Ni-TiX Mo Maraging steels with different molybdenum content show the same shielding properties for neutron and gamma rays when compared to the standard Maraging steel 18Ni(C250). In addition, the secondary gamma rays spectra due to neutron irradiation show no obvious difference between all steels under investigation. From economic point of view newly cobalt free low – nickel Maraging steel is cheaper than standard C250 Maraging steel as shielding materials.


INTRODUCTION
Today, nuclear radiation has a vital role in our recent live.It has applications in many of fields like medicine, industry, agriculture, archaeology, geology, academics and many others.Radiation shielding materials are required to protect population and equipments from harmful effect of radiation.For shielding design, neutrons and gamma rays are the main types of nuclear radiation, which have to be considered, since any shield that attenuates neutrons and gamma rays will be more effective for attenuating other radiations [1].Maraging steels belong to a new class of high strength steels with the combination of strength and toughness that are among the highest attainable in general engineering alloys [2][3][4][5][6][7][8].Maraging steel 18Ni(C250) with the highest strength are those containing cobalt.The use of steel with cobalt as a structural material in atomic applications is undesirable due to the high induced radioactivity under conditions of highenergy neutron bombardment.Expensive alloying additions such as Ni and Co increase the production cost for Maraging steels as cost has been the major roadblock for the widespread usage of these steels.The high production cost in association with dual-use marketing issues has motivated research worldwide for cheaper alloy design.cobalt-free Maraging steels with high strength, ductility, and toughness have been developed in recent years [9,10].The good combination of properties of Maraging steel can be utilized in nuclear applications where high reliability is the principal concern.This work concerns, study the radiation resistance (neutrons and gamma rays) of low nickel titanium containing Maraging steel without cobalt and with different amounts of molybdenum in comparison with standard Maraging steel 18Ni(C250) through MCNP5 code.MCNP5 code is a general-purpose Monte Carlo radiation transport code for modeling the interaction of radiation with matter.It utilizes the nuclear cross section libraries and uses physics models for particle interactions and gives required quantity with certain error [11,12].The neutron source used in the code is a point source which emit neutrons with energy 14.1 MeV.The two gamma energies used in the code are 1173 and 1332 keV emitted from 60 Co as a source of gamma rays.EXPERIMENTAL WORK

Material design
In this work new developed free cobalt low nickel Maraging steel with 12 mass percent nickel as base metal were designed.12 % Ni-Ti new developed Maraging steel with different amounts of molybdenum were investigated.To study the radiation resistance (neutron and gamma) of new developed Maraging steels, the authors also study famous Maraging steel 18Ni(C250)) for the sake of comparison.Tab. 1. shows the chemical composition of steels under investigations.
Tab. 1 The chemical composition of different steels under investigation.

MCNP5 experimental setup
To calculate the linear attenuation coefficient of gamma rays and the fast neutron removal cross section for Maraging steels under study, two narrow beam transmission geometries were done.The first geometry, for gamma, Fig. 1.a shows the 60 Co house box with dimensions 15 x 30 x 30 cm 3 made of lead and coated by 0.5 cm thickness of steel.a cone lead collimator was putted between the source and the investigated material with 15 cm length and end opening of 0.5 cm radius and also coated with 0.5 cm of steel.The materials under investigation were a cylinders of 3 cm radius and thicknesses 2, 5, 10 and 15 cm .The gamma detector was putted on the same line in front of gamma source at distance 44.5 cm from it.the detector shield face was 39.5 cm from the source and it have opening radius 0.5 cm.It was assumed that, the gamma detector is an ideal detector.Tally type 4 was used to estimate gamma photons registered in the detector per MeV.cm . The number of histories used in the code is 20x10 6 . The statistical error in calculated data not exceed 0.05% for each gamma energy.The second geometry, for neutron, Fig. 1.b.shows a lead box of 3 x 3 x 3 m 3 dimensions as a neutron source housing with a cylinder collimator of 1 cm radius.The detector was putted on the same line at a distance 61 cm from neutron source.The detector collimator have an aperture of 1 cm radius.The investigated materials were putted between the source and the detector at a distance 40 cm from the source and have the same dimensions as in gamma geometry.The same tally type 4 was used to count the number of neutrons inter the detector per MeV.cm

Neutron and secondary gamma rays shielding properties
Neutron attenuation through the investigated materials have been calculated by performing the transmission experiment in narrow beam geometry shown in Fig. 1.a Fig. 2 shows the neutron flux transmitted to the detector in case of no material and the neutron flux transmitted through 2, 5, 10, 15 cm thicknesses of the different shielding materials under investigation.The figure shows that, the fast neutrons were moderated and moves from its higher group to a group of less energy.The number of moderated neutrons were increased with shielding materials thickness increased.The important is that, the neutron moderation ratio for the different steels and for the compared steel C250 is the same.This result was insured by calculating the effective fast neutron removal cross section (cm -1 ) which obtained by the equation where I and I 0 are the neutron intensity with and without the material between the neutron source and the detector respectively and is the material thickness.Therefore, plotting each versus , the slope of the straight line obtained is the value of the material.The obtained effective fast neutron removal cross section (cm -1 ) and the mass removal cross section (cm 2 /g) for the investigated materials are tabulated in Tab. 2 with the values of standard steel for comparison.As shown in the table, the produced steels have approximately the same values of effective and mass removal cross-section and as the values of the standard material C250.This means that, our produced steels work as neutron shielding material and have the same neutron attenuation in comparison with the standard.Interaction of neutrons with elements constitute the material produce secondary gamma-rays.Fig. 3 shows the spectra of gamma-rays produced due to neutrons interactions with 2 cm thickness of steels under investigation including the standard.There are no appear signals for gamma coming from cobalt present in the standard sample C250 as expected.This can be explained by, the number of neutrons which moderated and arrived to thermal nuetrons is very modest number due to the small thicknesses used in the study.In addition, the self shielding of the material make it absorb some of produced gamma rays specially low energy gamma like that produced from neutron interaction with cobalt (1173 and 1332 keV).Also, the figure shows an increase in secondary gamma for the standard steel in the energy range from 5.5 to 8 MeV more than others.Fig. 4 shows the integral flux of secondary gamma versus the material thickness for all investigated materials.As shown in fugure the integral flux decrease exponentially with material thickness increase.This because, the most number of neutrons interact through the first layers of shielding materials and produce secondary gamma rays which shielded from the residual layers of the material itself.An empirical equation for the emitted secondary gamma rays versus material thickness can be deduce in the form where, is the integral flux of secondary photons and is the material thickness and a, b are constants for the material type.
Fig. 2 Transmitted neutron spectra versus neutron energy at different thickness of the investigated steels in comparison with neutron spectra without sample (black line).

Gamma rays shielding properties
Narrow beam geometry shown in Fig. 1.b was used to study gamma rays attenuation through our materials and through the standard one.Fig. 5 shows 60 Co gamma rays spectra transmitted through 2, 5, 10 and 15 cm thickness of the investigated materials with gamma spectra transmitted directly without material between the source and the detector.As shown in figure, there is no obvious difference between all materials in the value of gamma attenuation.Also, when material thickness increase gamma intensity decrease.Intensity of the two gamma lines 1173 and 1332 KeV which collected under energy bins 1200 and 1350 KeV respectively were calculated versus the materials thicknesses.As in the case of neutron, the gamma linear attenuation coefficient µ (cm -1 ) of the two gamma lines for materials can be calculated from the equation where I, I 0 are the gamma line intensity with and without the material and is the material ) and mass attenuation coefficient µ (cm 2 /g) of the materials for the two gamma lines are summarized in Tab. 2. The tabulated data show the obvious approximation between the investigated materials in attenuation of gamma rays.Also, there is no difference between standard steel and the investigated materials in gamma attenuation.) and mass removal cross section Σ R /ρ ( cm 2 /g) with gamma linear attenuation coefficient µ (cm -1 ) and mass attenuation coefficient µ/ρ ( cm 2 /g) for steels under study.

CONCLUSIONS
• The produced steels have the same ability to moderate neutrons and to attenuate gamma rays in comparison with C250 Maraging steel.
• The problem of secondary gamma rays produced due to neutron interactions with steel can be solved with higher material thickness, where there is an exponential relation between secondary gamma and the material thickness due to the material self shielding.
• From economicl point of view, newly modified Maraging steel with previous shielding properties are prmosing material in shielding applications.Future work should be done to study neutron shielding properties and neutron reflection characteristics at higher thicknesses and at higher temperature for the produced steels to be used as a nuclear reactor vessel.

Fig. 1
Fig. 1 Experimental setup for measuring gamma linear attenuation coefficient (a) and effective fast neutron removal cross-section (b).
thickness.Hence, plotting each versus for each gamma line, the slopes of the two straight lines obtained are the values µ of the material for these two energy lines.The values of gamma linear attenuation coefficient µ (cm -1

Fig. 3 Fig. 4 Tab. 2
Fig. 3 Secondary gamma rays spectra arising from neutrons interactions with under investigation steels Fig. 4 Integral flux of secondary gamma varsus shielding material thickness for all studied materials.