**7. Technique problems**

316 Radioisotopes – Applications in Physical Sciences

**DraftReport** 

0.05 Brain 0.01

Remainder: adipose tissue, adrenals, connective tissue, extrathoracic airways, gall bladder, heart wall, lymphatic nodes, muscle, pancreas, prostate, small intestine wall, thymus, uterus/cervix

Salivary glands 0.01

0.10

factor, *wT* Tissue or organ Tissue weighting

factor, *wT*

**ICRP 1991 ICRP 2005** 

Gonads 0.20 Gonads 0.05 Bone marrow (red) 0.12 Bone marrow (red) 0.12 Colon 0.12 Colon 0.12 Lung 0.12 Lung 0.12 Stomach 0.12 Stomach 0.12 Bladder 0.05 Bladder 0.05 Breast 0.05 Breast 0.12 Liver 0.05 Liver 0.05 Oesophagus 0.05 Oesophagus 0.05 Thyroid 0.05 Thyroid 0.05 Skin 0.01 Skin 0.01 Bone surface 0.01 Bone surface 0.01

Kidneys 0.01

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

Tissue or organ Tissue weighting

Remainder: adrenals, brain, Lower Large Intestine, Upper Large Intestine, Kidneys, muscle, pancreas, spleen, thymus, uterus

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 it.

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.*  **1973).** 

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 irradiation so this worry vanishes.

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

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 abundance of the emitting nucleus in the body.

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 are given as follows:


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

Body Composition Analyzer Based on PGNAA Method 319

The protons which produce the interfering reaction with oxygen originate as the result of elastic collisions between neutrons and hydrogen nuclei, the most numerically abundant element in the human body. There are many other minor reactions which also interfere,

The vast majority of gamma rays induced by the inelastic scattering and capture of neutrons by atomic nuclei in the human body are emitted within a few microseconds. The abundance of the emission, at all energies up to 11 MeV, makes it difficult to distinguish gamma rays of similar energies from different elements unless a high energy resolution detector is employed, such as the semiconductors Ge(Li) or hyperpure Ge. Otherwise NaI(Tl) crystal scintillation detectors, with an optically coupled photomultiplier tube, are usually employed, since they have a larger sensitive volume and greater stopping power for gamma

The reason for the better energy resolution of the semiconductor detectors is that it requires the deposition of only approximately 3 eV of energy from the gamma ray in the detector's depletion layer to produce an electron–hole pair, whereas it requires around 100 times as much energy to be deposited in the NaI(Tl) crystal to produce one photoelectron at the photocathode of the photomultiplier due to losses of light in the crystal. The total number of electrons released in each type of detector is a measure of the amount of energy absorbed. Although the signal is amplified many times in the photomultiplier tube attached to the NaI(Tl) crystal, the anode current reflects the fluctuations in the number of electrons emitted from the photocathode. Therefore, for the detection of gamma rays of a given energy, there is a greater statistical variation in the signal from a NaI(Tl) detector than a semiconductor, so

If the energy resolution of a Germanium semiconductor detector is 2 keV, the corresponding energy resolution of a NaI(Tl) crystal scintillator is around 80 keV. The semiconductor detectors suffer the disadvantage of having to be cooled with liquid nitrogen when in use,

Another problem associated with prompt-gamma neutron activation analysis is neutron irradiation of the detectors themselves, which in the case of semiconductors produces dislocations in the crystal lattice, and in NaI(Tl) crystals activates both the sodium and the iodine nuclei, from which the resulting gamma rays are counted with great efficiency. This increases the background in the gamma ray spectrum upon which the characteristic emissions of body elements are superimposed. Therefore suitable neutron shielding of the

Bismuth germinate scintillation detectors have a greater stopping power for gamma rays than sodium iodide, and therefore may improve the signal to background for nitrogen, but this advantage has not been realized in practice due to activation of germanium. Multiple small NaI(Tl) crystals were found to give a better signal to noise ratio than a few larger

producing positron annihilation radiation at 0.511 MeV.

**9. Prompt-gamma neutron activation analysis** 

that the latter is used for high resolution gamma spectroscopy.

and, in the case of Ge(Li) detectors, cooled continuously.

detectors is necessary.

crystals.

rays.

The minor elements magnesium, copper, iodine and iron may also be determined from the delayed emission of gamma rays.

The reaction with oxygen has been successfully employed, where the subject was transferred (within 30 s) from the irradiation facility to a whole-body counter. The reactions with nitrogen, oxygen and phosphorus only occur with fast neutrons above an energy threshold: 11 MeV for the reactions with oxygen and nitrogen and 2 MeV in the case of phosphorus. Two configurations of the delayed gamma neutron activation analysis system have been shown in Figures 2. And 3.

Fig. 2. Pu-Be neutron source arrangement for the Delayed Gamma Neutron Activation Analysis

Fig. 3. 241Am-Be neutron source arrangement for the Delayed Gamma Neutron Activation Analysis

The reaction with nitrogen suffers from the disadvantage that the positron annihilation radiation (0.511 MeV) is common to many nuclear reactions, and it is not possible to distinguish this from another reaction which produces the same daughter nuclide which decays with the same half life:

The minor elements magnesium, copper, iodine and iron may also be determined from the

The reaction with oxygen has been successfully employed, where the subject was transferred (within 30 s) from the irradiation facility to a whole-body counter. The reactions with nitrogen, oxygen and phosphorus only occur with fast neutrons above an energy threshold: 11 MeV for the reactions with oxygen and nitrogen and 2 MeV in the case of phosphorus. Two configurations of the delayed gamma neutron activation analysis system

Fig. 2. Pu-Be neutron source arrangement for the Delayed Gamma Neutron Activation

Fig. 3. 241Am-Be neutron source arrangement for the Delayed Gamma Neutron Activation

The reaction with nitrogen suffers from the disadvantage that the positron annihilation radiation (0.511 MeV) is common to many nuclear reactions, and it is not possible to distinguish this from another reaction which produces the same daughter nuclide which

delayed emission of gamma rays.

have been shown in Figures 2. And 3.

Analysis

Analysis

decays with the same half life:


The protons which produce the interfering reaction with oxygen originate as the result of elastic collisions between neutrons and hydrogen nuclei, the most numerically abundant element in the human body. There are many other minor reactions which also interfere, producing positron annihilation radiation at 0.511 MeV.

#### **9. Prompt-gamma neutron activation analysis**

The vast majority of gamma rays induced by the inelastic scattering and capture of neutrons by atomic nuclei in the human body are emitted within a few microseconds. The abundance of the emission, at all energies up to 11 MeV, makes it difficult to distinguish gamma rays of similar energies from different elements unless a high energy resolution detector is employed, such as the semiconductors Ge(Li) or hyperpure Ge. Otherwise NaI(Tl) crystal scintillation detectors, with an optically coupled photomultiplier tube, are usually employed, since they have a larger sensitive volume and greater stopping power for gamma rays.

The reason for the better energy resolution of the semiconductor detectors is that it requires the deposition of only approximately 3 eV of energy from the gamma ray in the detector's depletion layer to produce an electron–hole pair, whereas it requires around 100 times as much energy to be deposited in the NaI(Tl) crystal to produce one photoelectron at the photocathode of the photomultiplier due to losses of light in the crystal. The total number of electrons released in each type of detector is a measure of the amount of energy absorbed. Although the signal is amplified many times in the photomultiplier tube attached to the NaI(Tl) crystal, the anode current reflects the fluctuations in the number of electrons emitted from the photocathode. Therefore, for the detection of gamma rays of a given energy, there is a greater statistical variation in the signal from a NaI(Tl) detector than a semiconductor, so that the latter is used for high resolution gamma spectroscopy.

If the energy resolution of a Germanium semiconductor detector is 2 keV, the corresponding energy resolution of a NaI(Tl) crystal scintillator is around 80 keV. The semiconductor detectors suffer the disadvantage of having to be cooled with liquid nitrogen when in use, and, in the case of Ge(Li) detectors, cooled continuously.

Another problem associated with prompt-gamma neutron activation analysis is neutron irradiation of the detectors themselves, which in the case of semiconductors produces dislocations in the crystal lattice, and in NaI(Tl) crystals activates both the sodium and the iodine nuclei, from which the resulting gamma rays are counted with great efficiency.

This increases the background in the gamma ray spectrum upon which the characteristic emissions of body elements are superimposed. Therefore suitable neutron shielding of the detectors is necessary.

Bismuth germinate scintillation detectors have a greater stopping power for gamma rays than sodium iodide, and therefore may improve the signal to background for nitrogen, but this advantage has not been realized in practice due to activation of germanium. Multiple small NaI(Tl) crystals were found to give a better signal to noise ratio than a few larger crystals.

Body Composition Analyzer Based on PGNAA Method 321

2.224 MeV gamma-rays, the inner wall of the valley, made above the neutron source (Figure 7), was lined by Pb sheet of 2cm thickness. By this way, a rectangular neutron-beam aperture measuring 40 cm length (perpendicular to the paper sheet) and 20 cm (width) at

Fig. 4. A typical schematic representation of PGNAA setup based accelerator. In this setup

Fig. 5. Schematic of a conventional machine used to measure the Total Body Nitrogen (TBN)

the sample location is defined.

the D2O is used as moderator.

A third problem associated with prompt-gamma neutron activation analysis is the high count rate encountered. Since the output pulse from a detector is of a finite length (typically with a rise time of 0.25 \_s and a fall time of up to 10 \_s), any radiations being detected within this interval may be added to the original event, producing a pulse of greater amplitude. This process of random summing at high count rates has the effect of increasing the background in the gamma ray spectrum further. The statistical uncertainties in the determination of the abundance of any element in the body from the number of events in the corresponding full-energy peak in the spectrum are increased by the contribution from the underlying background. It is necessary to minimize this background. One method to reduce the random summing background to nitrogen is to electronically suppress the counting of events below 5 MeV for the major part of the measurement, and only count the whole spectrum (including the 2.223 MeV peak from hydrogen) for a short interval .

This increases the nitrogen signal to background by 18%. Since many (inelastic or non-elastic scattering) reactions (e.g. with carbon, oxygen) have an energy threshold of several megaelectron-volts, the optimum signal for a given dose is achieved when the subject is irradiated with monoenergetic neutrons at 14.4 MeV from a D–T neutron generator. These neutron generators, or alternatively cyclotrons, can be temperamental to operate, so that often neutron sources, comprising an alloy of beryllium and an alpha emitting radionuclide, are preferred.

These sources (241Am/Be, 238Pu/Be) produce a 4.439 MeV gamma ray per neutron which may interfere with the determination of carbon and add significantly to the problem of random summing of gamma rays in the detectors unless the source is well shielded.

Moreover, it is possible to improve the signal to background ratio in operating a neutron generator or cyclotron in a pulsed or cycled mode by counting short-lived induced activity between pulses of neutrons, thereby reducing the lower limit of the target element that can be measured.
