The Pulse-Height Defect

Heavy-ion spectroscopy is different from that of lighter charged particles because of the pulse-height defect (PHD), which makes the energy calibration equation mass dependent. 

The measurement of particle energy with any type of detector is based on the assumption that the charge collected at the output of the detector is proportional to the energy of the incident particle. The assumption is valid if all the particle energy is lost in ionizing collisions and all the charge produced is collected, i.e., no recombination takes place. 

The nuclear defect is due to nuclear collisions. As a result of such collisions, the moving ion imparts energy upon other nuclei. The recoiling nuclei lose their energy partly in electronic ionizing collisions and partly in nuclear nonionizing ones. The nuclear defect has been calculated based on the work of Bohr and of Lindhard et al. (see also Chap. 4). 

The recombination defect arises from incomplete collection of the charge produced in the detector. A heavy ion is a strongly ionizing particle. It creates a dense plasma of electron-hole pairs along its path, a plasma that reduces the electric field established by the external bias applied to the detector. The reduction of the electric field intensity hinders the drifting of the electrons and holes and thus increases the probability of recombination. The calculation of this defect is not so easy as that of the nuclear one, but an approximate calculation was performed by Wilkins et al.  

The window defect is due to energy loss in the dead layer (window) of the front surface of the detector. It can be obtained from the thickness of the window and the stopping power of the ion. The thickness of the window can 

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For heavy ions, a phenomenon called the pulse height defect (PHD) seems to have an important effect on energy calibration. As a result of the PHD, the relationship between pulse height and ion energy is mass dependent. In semiconductor detectors, experiments have shown that the PHD depends on the... 

Figure 13.18 The dependence of the pulse height defect on ion energy (from Ref. 33). The numbers correspond, approximately, to uranium ions.

This chapter discusses the subjects of energy loss and straggling, pulse height defect, energy calibration methods, and source preparation, from the point of view of their effect on spectroscopy. All the effects are not equally important for all types of particles. Based on similarity in energy loss behavior, the charged particles are divided into three groups, as in Chap. 4 ... 

The lack of nuclear collisions for channeled ions is not the only phenomenon that affects the pulse height. It is known that the electron density is much reduced along the channel. As a result, the electronic stopping power is lower and, consequently, so is the charge density produced by the heavy ion. Thus, not only the nuelear but also the recombination defect is reduced for the channeled ions. 

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