Amino acid decomposition induced by keV ion irradiation

Nuclear Instruments North-Holland

AMINO

in Physics

ACID DECOMPOSITION

A.M. FOTI, Dipartimento

and Methods

F. MILAN0

Research

361

B46 (1990) 361-363

INDUCED

BY keV ION IRRADIATION

and L. TORRISI

dr Fisicu, Unioersifd di Catania, Corm Italia, 57, 95129 Catania, Italy

Molecular emission of H,, NH,, CO, and dimer hydrocarbons has been detected during keV ion bombardment of alanine. 200 keV helium and argon beams produce high yield emission up to lo3 molecules/ion for all the species. The time (or fluence) dependence of molecular emission has been investigated in the ion current range 10 nA to 1 PA over a 3 ~3 mm* spot, with a quadrupole mass analyzer. We have observed a prompt emission of H,, while for heavier masses an incubation ion fluence is required (-10 I3 ions/cm’ ). The observed effects have been correlated with the EPR signals, detected in the irradiated amino acids, mainly in alanine. The good linearity between the energy deposition and EPR signals allows to use such material to build dosimeters in the high fluence regimes.

1. Introduction Amino acids are the basic organic compounds of proteins; alanine is a neutral amino acid with chemical composition CH,-CHNH,-COOH containing the acid group (-COOH) and amino group (-NH,) with low molecular weight [l]. Commercially available alanine is a molecular solid of low density (0.98 g/cm3). The interaction of high energy ions with molecular solids such as H,O, CH, and C,H, produces a fast decomposition of the target with a prompt emission of hydrogen molecules at a high rate of about 10’ molecules/ion. Moreover, specific molecules such as 0, for H,O or C,H, for methane and benzene can also be observed by increasing the ion fluence [2,3]. Molecular decomposition occurs in a small region ( - 10 A) along the ion track where the energy density is very high [4]. The aim of this work is to investigate the ion beam effects induced by keV ion beam irradiation in alanine. This amino acid has been chosen because it is a medium equivalent to tissue where primary events, such as molecular formation induced by high LET radiation such as keV ions, can be easily investigated with direct correlation to biological effects [5].

mm2 was maintained constant during all the experiments. During the bombardment time the ion beam was swept over the target in order to have uniform implantation. A thin thermocouple embedded in the alanine pastille was used to measure the temperature during the irradiation to avoid target heating above 30 o C. The temperature was increased intentionally up to 300 o C with an ohmic system to investigate the molecular emission during the pyrolysis of alanine. A quadrupole spectrometer was used to detect the masses of emitted particles during the irradiation. The quadrupole response, given in mV, is proportional to the partial pressure in the analysis chamber for a given molecule in units of atomic mass (amu). Molecular weight analysis was done up to 300 amu with a mass resolution of less than 1 amu. To calibrate the mass quadrupole spectrometer, several spectra were recorded after the admission in the scattering chamber of well known amounts of pure gases like CH,, H,, NH,, CO, and CO. Absorbed doses (MGy) were calculated from the energy loss (keV/pm) in alanine of helium and argon ions and from the ion fluence (ions/cm2). The energy losses in alanine were evaluated from the Ziegler tables [6] and through the Bragg rule, knowing the stoichiometric composition C,O,NH,.

2. Experimental 3. Results Amino acid pastilles of 5 mm in diameter and 2 mm in thickness were prepared by compression of crystalline powder (10 ton). The targets were bombarded with 100-300 keV helium and argon beams. The ion current ranged between 10 nA and 1 PA and the spot size of 9 0168-583X/90/$03.50 (North-Holland)

0 Elsevier Science Publishers

B.V.

Ion irradiation of alanine produces a strongly molecular emission of hydrogen, ammonia and carbon dioxide, as shown in fig. 1 (upper section). The full mass spectra have been recorded during 200 keV helium VI. POLYMERS

/ ORGANICS

A.M. For/et al.

362

200 KeV

2

F

0 t

Pyrolysis

He+ lrradlatton

(22O’C

1

L-LA.-

2

10

20

Mass

30

(a mu

)

Fig. 1. Mass spectra as quadrupole signals (mV) vs detected masses (amu) for 200 keV helium irradiation (upper section) and during pyrolysis at 220°C (lower section) of alanine pastilles.

irradiation with a 400 nA ion current; together with the typical pattern of H,, NH, and CO,, a set of peaks in the mass range 26-30 is also shown. It is correlated with molecular dimer hydrocarbons. In the same fig. 1 (lower section) the mass spectra detected during the target heating (without ion irradiation) at high temperature are reported. In the temperature range from ambient temperature to 200°C the emission from the target is negligible; a fast increase in the mass signals is observed just above 200 o C and it reaches a maximum at 220 OC. The mass spectrum during the pyrolysis is quite different compared to the mass spectra recorded during ion irradiation. The main difference is the strong decrease of mass 2 (hydrogen emission) and mass 17 (ammonia emission). The time dependence (or ion fluence dependence) of the peaks with mass 2, 16 and 44 is shown in fig. 2 during ion irradiation with 200 keV helium, 400 nA current. Mass 16 has been used as a typical marker for the ammonia emission because its interference with other signals is negligible. The behaviour of each peak as a function of time exhibits a quite different trend: for mass 2, the signal reaches the maximum value very fast after the beam hits the target; for mass 44, the delay to get the maximum is higher and, in terms of fluence, to about 2 X lOI ions/cm2; for mass 16, the delay is quite large up to a fhence of 3.6 x lOI ions/cm2. All the signals show, for large fluence irradiation, an exponential decay with a long tail up to a fluence of about 3 X 1015 ions/cm2.

0

40

80

Time(set) Fig. 2. Time dependence of peaks of mass M = 2, M = 44 and M =16, respectively, during ion irradiation with a 200 keV helium beam with a 400 nA current.

The quadrupole signals reported in fig. 2 can be converted into molecules emitted per second. by using a calibration standard procedure. At the maximum value of the quadrupole signal we get the following values: 172 mass-2 molecules/ion; 95 mass-44 molecules/ion and 43 mass-16 molecules/ion, respectively. A very convenient way to describe the ion irradiation effect of alanine is to report the chemical yield (molecules emitted per 100 eV deposited energy) for each mass, as given in table 1 for helium and argon beams. Alanine exhibits the interesting property to trap for a long time (more than one year) radicals after irradiation [5]. The radical density can be recorded by using a standard electron spin resonance system (EPR). From the variation of EPR signals, as reported in ref. [5], per 100 eV deposited energy one is able to obtain a chemical yield for radical production which can be directly compared to the molecular emission. It is well established that the main component in the EPR signals comes from the radical CH,-CH’XOOH where the band of the amino group is broken. Since such radicals can be considered as precursors for ammonia molecule formation, an interesting plot is

Table 1 Chemical yield (molecules/100 ion bombardment Beam

Helium Argon

(keV1

Energy

LET [keV/pm]

200 250

240 2350

eV) of ejected

molecules

under

Chemical yield [X IO-’ molecules/100 M=2

M=16

M=44

8.6 8.0

2.2 1.6

4.8 2.9

eV]

363

A.M. For, et (11./ Amino mid decomposrtion

4. Conclusions

,d

,o'

13

10"

10"

10'

lo"

lo'

Dose(Gy)

Fig. 3. Chemical production per

yields for radical formation 100 eV deposited energy absorbed dose.

and for ammonia as a function of

in fig. 3 where both chemical yields for radical production (R/100 eV) and for ammonia production (NH,/100 eV) are reported as a function of absorbed dose (Gy). The EPR signal is for alanine irradiated with y-rays from a “‘CO source while the molecular emission is for ion irradiated samples. However, it is established that the fluence dependence of EPR signals for y-rays, electrons and ion irradiation shows a very similar trend given

[71. From fig. 3 we derive the information that at low doses the chemical yield for radical production shows a linear dose response without molecular emission. Chemical yields indicate that more than 100 radicals are combined to yield one emitted molecule. At about 1 MGy the EPR signals collapse to zero while the molecular emission shows a strong increase up to 100 MGy. The physical meaning of this trend is quite simple, showing that, for high energy deposition, density recombination of radicals becomes more and more important with the ejection of final products as molecules (NH, and CO,). The hydrogen emission seems to be always present; however, we are not able to detect the hydrogen signal below lo4 Gy.

The experimental results obtained during this investigation show that ion decomposition of alanine produces NH,, CO, and H, emission which is not due to a thermal effect. The energy deposited in alanine generates radicals which combine to give stable molecules. The chemical yield for such processes at high fluence regimes ( - 3 x lOi ions/cm2 for helium) are of the order of 10-l molecules/100 eV. The interest for alanine lies in its use as an ideal one-hit detector at doses up to near-saturation levels (- lo5 Gy). In this region. in fact, no radiation damage occurs and the only radical production and process of radical-radical reaction give the linear dose response in EPR signals [8].

Thanks are due to Prof. S. Onori of the Istituto Superiore di Sanita of Rome for the support of the experimental data of EPR measurements by y-irradiated alanine, and to Prof. G. Foti for useful discussions and suggestions during this work.

References

111A.L. Lehninger.

Biochimtca, ed. N. Zanichelli (1987). G. Foti. L. Torrisi. V. Pirronello and G. Strazrulla. Radiat. Eff. 65 (1982) 167. R.A. Haring. L. Calcagno. A. [31 R. Pedrys. D.J. Oostra. Haring, and A.E. De Vrien, Nucl. Instr. and Meth. B17 (1986) 15. B. Sundqvist and R.E. Johnson. [41 A. Hedin, P. Hakansson, Phys. Rev. B31 (1985) 1780. [51 J.W. Hansen. K.J. Olsen and M. Wille. Radiat. Prot. DOS. 19 (1987) 43. of Stopping Cross Sections for 161 J.F. Ziegler, Handbook Energetic Ions in All Elements. vol. 5 (Pergamon, New York. 1980). [71 J.W. Hansen and K.J. Olsen, Radiat. Res. 104 (1985) 15. P.L. Indovina, S. Onori and A. Rasati, PI A. Bartolotta, Radiat. Prot. Dosim. 9 (1984) 277.

121 G. Ciavola,

VI. POLYMERS

/ ORGANIC-S