**1. Introduction**

TLD method is considered an important technique as it can store radiation in trap centers for long period. Glow peaks of thermoluminescence dosimeters are later measured and discussed based on some models related to the physical changes in the band structure of dosimeter because of ionizing radiation exposure. A wide range of substances exhibits thermoluminescence (TL) phenomena after being exposed to nuclear radiation such as activated LiF and CaSO4. Thermoluminescent dosimeter (TLD) emits light when heated up after being irradiated. Due to this special property, TLD could be used as a radiation dosimeter. TLD has many advantages and sensitive to different types of radiation. A dosimeter of higher TL response to thermal neutrons is most commonly used in mixed radiation fields (neutron and gamma ray). The sensitivity of TLD to neutrons depends on the detector compound type, environment and neutron energy. For neutron dosimetry purposes, the neutron and gamma ray dose contribution must be separated by using two different detector types of TLD. The first one should be sensitive to gamma and the other should be sensitive to neutrons plus gamma (as LiF-700 and LiF-600) [1, 2].

The response of fast neutrons depends on the cross-section for the interaction in TLD material and the relative TL efficiency, which depends on the linear energy transfer (LET) of the reaction products in the first place. The response to intermediate-energy depends mainly on the cross-section of the reaction, which may take place with the composite material of the TLD.

#### **1.1 TLD applications in neutron and gamma ray dosimetry**

Generally, there are three types of TLD used for neutron dosimetry as follow:

#### *1.1.1 Albedo neutron dosimeter*

A considerable fraction of intermediate and fast neutrons can be slowed down to epithermal neutron energy and backscattered in the human body, interacting with the sensitive TL material. An albedo neutron dosimeter is a type of neutron monitor and is typically used in the neutron energy range of 0.2 eV to around 0.5 MeV. The slow neutrons interact with TL material, usually through <sup>6</sup> Li (n,α) 3H reaction, and the resulting induced charged particles to stimulate the TL material. Recently, some of albedo TLD dosimeters depend on 10B (n, α) 7Li reactions. Because neutron TL sensitive material responds to gamma radiation, and neutrons are accompanied by this gamma radiation, another TLD is usually utilized in conjunction with TLD with a gamma ray.

The neutron albedo dosimeter measures (a) direct fast neutrons, (b) direct thermal neutrons, and (c) albedo neutrons reflected from the body. This type of dosimeter uses Lexan polycarbonate and/or CR-39 foils, as well as two 10B (n, γ) <sup>7</sup> Li converters in a cadmium cover, to efficiently measure the three neutron dosage components independently [3–5]. Fast neutron dose is assessed in CR-39 by counting proton recoil tracks, while thermal neutron dose is determined by counting α particles created during the process. Because the albedo dosimeter has a sensitivity range of 0.3– 30 mSv, it is advised that it be used as a backup dosimeter to assist in the assessment of high dose values in the event of accidents or patients receiving neutron therapy.

In another application, the 10B (n, α) <sup>7</sup> Li reactions with the backscattered albedo neutrons employed with Electret's ionization chamber proposed by Seifert et al. [6, 7]. In this chamber, induced <sup>7</sup> Li from the ionization of the gas in the chamber worn on the body's surface in the above reaction instance. Under saturation conditions, produced charge carriers with the corresponding polarity travel to the surface of the electret. As a result, the change in the electrets voltage is a direct measure of albedo neutron fluence and an indirect estimate of primary neutron fluence. In general, the advantages of albedo TLD dosimeter are: they are relatively inexpensive and can be reused, easily fabricated, lightweight to wear, Readout is simple and can be automated, Insensitive to humidity.

While their disadvantages are: Some of TLD exhibit fading, TLD is sensitive to gamma-ray, they must be worm properly or serious errors can be resulted, the

*Thermoluminescence Dosimetry Technique for Radiation Detection Applications DOI: http://dx.doi.org/10.5772/intechopen.102728*

measured values of TLD does not give permanent record as the track detectors, their sensitivity is highly dependent on the angle and energy of incidence radiation.

#### *1.1.2 Hydrogenous radiator TLDs*

In this type of dosimeters, the fast neutrons knock out protons from hydrogenous material mixed with the phosphor, and the protons dispel their energy in the dosimeter. In this method, the hydrogenous substances are called proton radiators [8]. This technique has demonstrated that TL materials mixed with hydrogenous material can detect fast neutrons, but the sensitivity needs to be improved by one order of magnitude before using in personnel neutron dosimetry.

#### *1.1.3 LET-dependent deep trap TLD glow peaks*

The fast neutron interacts directly with the TL material as calcium fluoride (CaF2: Tm) which is commercially called TLD-300. This type has a glow curve with two glow peaks and the peak temperature Tm centered 150 and 250°C, respectively. The higher temperature peak (250°C) has a greater response to the fast neutrons. TLD-300 dosimeter CaF2: Tm (0.35 Mol. %) showed a lower detection limit of about 0.3 mSv from 241Am-Be source.

### **2. Characteristic of TLD phosphors**

#### **2.1 The glow curve**

The term "Glow curve" refers to the graph of TL as a function either of temperature or of time as shown in **Figure 1**.

Glow curves have the following features:-

• The glow curve of a certain phosphor probably best characterizes that phosphor. For example, the appearance of glow peaks only at low temperatures implies that the phosphor loses its stored TL with time, and therefore would be unsuitable for long-term measurements.

**Figure 1.** *TLD glow curve and time–temperature profile (TTP).*


The following factors may affect the shape of the glow curve:
