**5. Sources**

In applying this technique many kinds of neutron sources have been used and suggested. The compact and portable neutron sources such as 252Cf and 241Am-Be are commonly used in the PGNAA method because of their high flux and reliable neutron spectrum. Also Anderson et al. (1964) and then Cohn et al. (1972) suggested using the fast neutron reaction 14N(n, 2n)I3N. This proved unsuitable because of interferences from other reactions and because of problems in maintaining a uniform fast (>11.3 MeV) neutron flux. The Birmingham group (Harvey et al. 1973), however, have shown that a suitable nitrogen measurement can be made by using the thermal neutron capture gamma rays from the reaction 14N(n,y )15N\*. 15N is stable but in this reaction is formed in the excited state, 15N\*; 15% of the time de-excitation results in the release of a 10.83 MeV gamma ray.

NaI(Tl) scintillation detectors are more suitable for this measurement than semiconductor detectors because of their greater stopping power. The most precise measurement of TBN reported is 1.6% for a neutron dose of 0.45 mSv (Ryde *et al* 1989) using a 4 GBq 252Cf fission source. A commonly employed technique in the measurement of body nitrogen is to measure the ratio of the emissions from nitrogen and hydrogen. This ratio is much less sensitive to variations in body size, neutron fluence and detector characteristics, which affect the signal from each element alone. It also permits the determination of TBN from partialbody irradiation (of the torso and thighs, thereby minimizing the radiation dose to radiosensitive tissues such as the eyes) assuming that hydrogen comprises one-tenth of body weight (Vartsky *et al* 1979). This requires correction since the proportion of body weight due to hydrogen has been estimated to vary from 9.5 to 10.8% in a large population

Chlorine may be determined from its emission at 8.57 MeV after deduction of the underlying background noise due to random summing and scattered gamma rays from nitrogen (Mitra et al. 1993). It may also be determined from its prominent emission at 6.11 MeV, but a high-resolution semiconductor detector (Ge(Li) or hyper pure Ge) must be employed to distinguish this emission from the emission from oxygen at 6.134 MeV.

In according to the latest recommendations of international institutes of radioprotection, an increasing attention must be paid to the patient protection during cancer radiotherapy. Therefore one of the primary attempts should be protection of the patient from hazardous radiation and minimizing un-useful doses. Designing a Body Chemical Composition Analyzer (BCCA) in order to use for cancer therapy while having the lowest gamma and neutron dose equivalent rate in the soft tissue is desirable. The Design of the BCCA need to be modeled by Monte Carlo N-particle general code (MCNP) (Briesmeister, 2000) before the construction. By this way we can assess all the geometry and material's effects and other parameters affect the dose received by the patient and the personnel. Also if we have an improving idea we can investigate its subsequent role in simulation design before the real

In applying this technique many kinds of neutron sources have been used and suggested. The compact and portable neutron sources such as 252Cf and 241Am-Be are commonly used in the PGNAA method because of their high flux and reliable neutron spectrum. Also Anderson et al. (1964) and then Cohn et al. (1972) suggested using the fast neutron reaction 14N(n, 2n)I3N. This proved unsuitable because of interferences from other reactions and because of problems in maintaining a uniform fast (>11.3 MeV) neutron flux. The Birmingham group (Harvey et al. 1973), however, have shown that a suitable nitrogen measurement can be made by using the thermal neutron capture gamma rays from the reaction 14N(n,y )15N\*. 15N is stable but in this reaction is formed in the excited state, 15N\*; 15% of the time de-excitation results in the release of a 10.83

**3. Detectors** 

of patients.

structure.

**5. Sources** 

MeV gamma ray.

**4. Simulation and advantages** 

In another works we see that viable signal/background ratio can be obtained using Pu-Be neutron sources and heavy shielding of both sources and detector. (Mernagh et al. 1977)

#### **6. Absorbed dose quantities and attentions**

Absorbed dose, D, is the energy imparted by ionizing radiation to matter per unit mass at a point given in units of J kg-1 (commonly called the Gray, Gy) (Alpen, 1998).

$$D = \frac{dE}{dM}$$

The effective dose, *E*, which is a summation of differing risks to organs in the human body in units of Sieverts (Sv), is given by (Clark et al., 1993).

$$E = \sum\_{\mathcal{T}} w\_{\mathcal{T}} H\_{\mathcal{T}}$$

Table 1 lists all the tissue weighting factor based on two reports.

Because of biological effects and absorbed dose don't always have one-to-one correspondence, so another factor called quality factor is introduced.

And HT is the equivalent dose (in Sv) in tissue or organ, T, and is given by (Clark et al, 1993).

$$H\_T = \sum\_R w\_R D\_{T,R}$$

Where wR is the radiation weighting factor (or quality factor) due to radiation of type R (for example neutron, alpha etc.) and DT,R is the absorbed dose averaged over a tissue or organ, T, due to a radiation of type R.

Radiation weighting factors (wR) for neutrons, according to ICRP Publication 60 can be chosen from either a step function or a continuous function to avoid discontinuity. The following formula (ICRP 60,1991) is used to calculate the wR continuous values:

$$\omega\_R = 5.0 + 17.0e^{\frac{-\left[\ln\left(2E\_n\right)\right]^2}{6}}$$

where En is the neutron energy in MeV. Another set of new wR data, is also released from ICRP Publication 103 (ICRP 103, 2008). The new radiation weighting factors function was expressed as:

$$w\_R = \begin{cases} 2.5 + 18.2 \, e^{\frac{-[\ln(E\_n)]^2}{6}}, E\_n < 1 MeV \\ 5.0 + 17.0 \, e^{\frac{-[\ln(2E\_n)]^2}{6}}, 1 MeV \le E\_n < 50 MeV \\ 2.5 + 3.25 \, e^{\frac{-[\ln(0.04 E\_n)]^2}{6}}, E\_n > 50 MeV \end{cases}$$

Body Composition Analyzer Based on PGNAA Method 317

One of the disadvantages of the neutron sources is that they don't generate only neutron but also they emit high-intensive gamma-rays. When using PGNAA method for medical purposes, the sample is a human body so these gamma-rays can cause destructive effects on

Another major problem of this technique is thermal and epithermal neutron capture by the iodine in the detecting crystal (NaI(Tl)), plus pile-up of gamma-rays from lower energy reactions or from the source of the neutrons. The Birmingham group has largely solved this problem by the use of a pulsed neutron beam and gated circuits (Harvey *et al.* 

Note that the activation of gamma detector is only in prompt gamma technique but in the delay gamma neutron activation analysis since the detection of delayed gamma rays is after

When the body is irradiated with neutrons, penetrating gamma rays are emitted both during irradiation (prompt) and for some time afterwards (delayed). These gamma rays originate from atomic nuclei which have absorbed energy from the neutrons or captured the neutrons themselves, and the energies of the gamma rays are characteristic of the nucleus which emits them. Therefore energy sensitive detectors may identify the emitting nucleus and the number of gamma rays detected at a given energy may be used to determine the

The majority of gamma rays are emitted during irradiation, but the elements sodium, chlorine, calcium, nitrogen and phosphorus may be determined after irradiation, if the subject is transferred from the irradiation facility into a whole-body counter within a short period, typically 5 min. Sodium and chlorine are extracellular ions from which the extracellular fluid space of the body may be determined. Calcium is contained almost entirely within the skeleton, comprising 34% of bone mineral. Phosphorus occurs mainly in the skeleton but is also found in lean soft tissue, in association with the energy metabolism. Nitrogen is uniquely a constituent of protein, 16% by weight, so that measurement of total body nitrogen (TBN) is used to determine total body protein (TBPr). These nuclear reactions

Where E denotes the energy of the characteristic gamma rays emitted and t1/2 is the half

**8. Delayed-gamma-emission neutron activation analysis** 

**7. Technique problems** 

irradiation so this worry vanishes.

are given as follows:

life of the induced activity.

abundance of the emitting nucleus in the body.

it.

**1973).** 


Table 1. ICRP 60 (1991) and ICRP 2005 proposed tissue-weighting factors.
