Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 158

Table 4.2 Heavy ions for biological research in RIKEN Accelerator Research Facility (RARF).
(Modified from Kazama et al. 2008 )
Heavy ions Charges Energy LET (keV/μm) Range in water

(mm)

MeV/u GeV

12 C

+ 6 135 1.62 22.5 43

14 N

+ 7 135 1.89 26.3 34

20 Ne

+ 10 135 2.70 61.1 23

40 Ar

+ 17 95 3.80 280.0 8

56 Fe

+ 24 90 5.04 624.0 4

Typical heavy ions used for irradiation on biological samples are neon-20, nitro-
gen-14, carbon-12, lithium-7, argon-40, iron-56 (Table 4.2 ).
They have different energy levels and linear energy transfer (LET), ionization
densities which correlate to the complexity of DNA damage, and different ranges
of penetration (Fig. 4.3 ).
LET is the energy deposited to target material when an ionizing particle passes
through it. Once an accelerated particle encounters any substance, it gradually los-
es its own energy (i.e., the same amount of energy is transferred to the substance
causing damage.) and eventually stops at the point where the maximum energy
loss is observed (Fig. 4.4 ). In this figure, an ionizing particle gradually loses its
own energy as it slows down in the target material. LET refers to this energy loss,
which is deposited to the material. In this cartoon, LET is represented by wavy
lines. LET reaches its maximum just before the ionizing particle stops. Immediately
after this peak, LET plunges to zero. LET is usually expressed in kilo electron volts
per micrometer (keV/mm), which represents the average amount of energy lost per
unit distance. Ion beams have a relatively high LET (around 10-1,000 keV/µm or
higher), while X-rays, γ-rays and electrons have low LETs (around 0.2 keV/µm).
Therefore, ion beams are able to cause more severe damage to living cells than
other forms radiation, resulting in the high relative biological effectiveness (Blakely
1992 ; Lett 1992 ). It is possible to modulate the treatment of plant material with one
species of ion at different LETs by passing the ions through a combination of ab-
sorbers—since changes in the LET of ion species occur as they pass through matter
(Kazama et al. 2008 ).
Studies have shown that the biological effect of ion beam radiation is dependent
on absorption doses and LET values but independent of ion species (Kazama et al.
2008 ), which means that the treatment of carbon-12 would produce similar biologi-
cal effect on rice seeds as neon-20 if the same dose (say 50 Gy) and same LET (say
30 keV/μm) is applied. DNA double strand breaks are believed to be the most im-
portant consequence of ion beam radiation. Very complex repair mechanisms have
been unveiled but are prone to errors due to double-strand breaks and lead to dele-
tions, insertions, inversions and translocations. Studies on the mutant gene alleles
induced by ion beam radiation showed that most mutations are deletions and that
the size of DNA deletion is LET-dependent. Most complex DNA damage caused
by the intricate set of effects of heavy ion beams (HIB) escapes the repair efforts