between the particle and sample. Thus the evolution of an initially mono energetic and mono directional beam leads to dispersion of energy and direction. The resulting statistical energy distribution or deviation from initial energy is called energy straggling. Energy straggling data are plotted as a function of depth in the material. Theories of energy straggling : Energy straggling distribution is divided into three domains depending on the ratio of ΔE i.e., ΔE /E where ΔE is the mean energy loss and E is the average energy of the particle along the trajectory. 1. Low fraction of energy loss: for very thin films with small path lengths, where ΔE/E ≤ 0.01, Landau and Vavilov derived that infrequent single collisions with large energy transfers contributes certain amount of loss in energy. 2. Medium fraction of energy loss: for regions where 0.01< ΔE/E ≤ 0.2. Bohr’s model based on electronic interactions is useful for estimating energy straggling for this case, and this model includes the amount of energy straggling in terms of the areal density of electrons traversed by the beam. The standard deviation Ω2B of the energy distribution is given by : Ω2B=4π((Z1e2)2NZ2∆x Where NZ2Δx is the number of electrons per unit area over the path length increment Δx. 3. Large fraction of energy loss: for fractional energy loss in the region of 0.2< ΔE/E ≤ 0.8, the energy dependence of stopping power causes the energy loss distribution to differ from Bohr’s straggling function. Hence the Bohr theory can not be applicable for this case. Various theoretical advances were made in understanding energy straggling in this case. An expression of energy for straggling is proposed by Symon in the region of 0.2< ΔE/E ≤ 0.5 is a function of momentums Mi( Mi = M1+M2 where M1 is stopping power, M2 is variation in straggling with depth of a stopping power) Tschalar et al. derived a straggling function solving the differential equation: d Ω2/dx = S(E) .d Ω2/dE The Tschalar’s expression which is valid for nearly symmetrical energy loss spectra, is Ω2 T = S2[E(x)]σ2(E) dE/S3(E) Where σ2(E) represents energy straggling per unit length (or) variance of energy loss distribution per unit length for particles of energy E. E(x)is the mean energy at depth x. Mass resolution In a similar way mass resolution is a parameter that characterizes the capability of recoil spectrometry to separate two signals arising from two neighboring elements in