Alpha particle track production in polycarbonate foils amplified by electrochemical etching

Nucl. Tracks, Vol. 3, pp. 185-196 Pergamon Press Ltd. 1979. Printed in Great Britain.

A L P H A P A R T I C L E TRACK P R O D U C T I O N IN P O L Y C A R B O N A T E FOILS A M P L I F I E D BY E L E C T R O C H E M I C A L E T C H I N G * GARY B. STILLWAGON Radiation Therapy Department, Veterans Administration Medical Center, Memphis, TN 38104, U.S.A. and KARL Z. MORGAN

Georgia Institute of Technology, School of Nuclear Engineering, Atlanta, GA 30332, U.S.A.

(Received 10 July 1978; in revised form 5 March 1979) Abstract--We have examined two of the parameters concerned with alpha-particle track production in 250/zm thick polycarbonate foils as part of our work to find the average dose delivered to the endosteal surface of bone out to a distance of 10/~m by 2 3 9 p u alpha particles. These parameters are (i) the efficiency of track production by g-particles from a '~thin" source as a function of absorber thickness and (ii) the track diameter as a function of etching time. The efficiency was determined by varying the air pressure in our vacuum-sealed alpha-calibrator over a wide range and observing the energy degradation and number of alpha particles incident on a surface barrier detector from a 2.0 #Ci 2 3 9 p u s o u r c e . The polycarbonate foils were then irradiated by g-particles at the same air densities (T and P correction included) and using the same geometry as before; after which they were electrochemically etched. When the number of tracks vs absorber thickness was plotted, a curve showing the "Bragg-peak" resulted. The peak was approximately a factor of two above the flat portion of the curve. For the track diameter vs etching time determination, foils were irradiated by alpha-particle sources, and then electrochemically etched for times ranging from 15 min to 4 h. No tracks were observed below 30 rain of etching. A plot of track diameter vs etching time for the tracks observed is presented. Two size groups, or catagories, of tracks are observed on g-particle-irradiated foils after electrochemical etching. A reason for the two groups is proposed.

1. I N T R O D U C T I O N THE PARAMETERS necessary to be k n o w n to employ the electrochemical etching system utilizing polycarb o n a t e foils in fast n e u t r o n dosimetry applications has been e x a m i n e d in the past (Sohrabi, 1975; Sohrabi a n d M o r g a n , 1976; Su, 1977; Somogyi, 1977; Stillwagon et al., 1977). The research described here a n d elsewhere (Stillwagon, 1978) however, has focused u p o n the a d a p t i o n of the electrochemical etching eystem for alpha-particle internal dosimetry applications. Several parameters need to be evaluated before this dosimetry system can be utilized for internal dosimetric work. Two of these p a r a m e t e r s are described herein, namely (i) the efficiency of p r o d u c t i o n of etchable g-particle tracks using a " t h i n " g-particle source a n d (ii) the

track diameter as a function of etching time. Descriptions of additional work appear elsewhere (Stillwagon, 1978).

2. EXPERIMENTAL P R O C E D U R E In the process of electrochemical etching ( T o m m a s i n o a n d Armellini, 1973), alternating voltage is applied between two c o m p a r t m e n t s of a c h a m b e r such as the one s h o w n in Fig. 1. Seven 250/~m thick p o l y c a r b o n a t e foils separate the two c o m p a r t m e n t s of the chamber. The etching equipment is s h o w n in Fig. 2, where the audio-frequency generator, amplifier, etching c h a m b e r a n d voltmeter can be seen. T h r o u g h o u t this work, T r a n s i l w r a p p o l y c a r b o n a t e foils o b t a i n e d from the Transilwrap C o m p a n y , Doraville, Georgia, U.S.A., were used.

*This research was supported in part by the Division of Biomedical and Environmental Research, U.S. Department of Energy. 185

186

G. B. STILLWAGON and K. Z. MORGAN

Proper selection of foil is crucial, as our earlier work demonstrated (Sohrabi, 1975; Stillwagon et al., 1977, 1978). To examine the efficiency of s-particle track production, a device called the vacuum-sealed alpha-calibrator (Fig. 3) was designed during this research work. It provides a constant source-to-detector distance; one of the detectors was a surface barrier detector and the other the polycarbonate foil. Different absorber thicknesses can be simulated by varying the air pressure in the chamber. The source used consisted of 2.0/~Ci of 239pu. First, various air densities, using the standard temperature and pressure corrections, were used to irradiate an Ortec surface barrier detector, which provided input into a Model 8100 Canberra multichannel analyser. The total number of a-particles and peak energies were noted. The intrinsic efficiency of the surface barrier detector for the detection of s-particles that have entered the sensitive volume was assumed to be 100 %. The top, made to hold polycarbonate foils, was then employed and, to insure similar geometry and irradiation conditions, ceramic rings were obtained from the Ortec Company identical to the rings employed in the construction of their surface barrier detectors. These were placed on the foil at the position formely occupied by the surface barrier detector. After irradiation at the same air densities, the foils were etched under standard conditions (for 4h in 28 '~o KOH at 24~'C, utilizing an applied potential of 800 V at 2 kHz). To examine the track diameter as a function of etching time, the 250 #m thick polycarbonate foils were irradiated by one of three a-particle sources, two 0.1/~Ci 241Am sources, and one 0.2#Ci mixednuclide source containing 239pu, 241Am and 244Cm. These foils were then etched for varying times between 15 min and 4 h at room temperature (24 °C) in 45 ~o by weight KOH using an applied voltage of 800 V at 2kHz. 3. RESULTS The results obtained from the study to determine the efficiency of the production of etchable ~tparticle tracks as a function of polycarbonate foil thickness using alpha particles from a "thin" source shown in Fig. 4, where equivalent polycarbonate foil thickness was calculated from the air thicknesses of various densities. Each point on the curve repre-

sents the average of more than one foil, usually two or three foils, and the error bars correspond to one standard deviation. No correction for attentuation in the small gold layer on the face of the surface barrier detector was employed in plotting Fig. 4, since the surface barrier detector was not in place during the polycarbonate foil irradiations. The shape of the curve in Fig. 4 is quite clearly the familiar Bragg-type curve, peaking at 27.5/~m thickness of polycarbonate. The peak of the curve was of a height slightly more than twice that of the flat portion. Also, the peak fell off sharply down to zero at 34/~m thickness of polycarbonate, corresponding to the range of the 239pu or-particle in polycarbonate, and again toward the flat portion of the curve on the left-hand side of the peak. The total number of counts used in the efficiency determination was 1.6516 x 107 counts. This value was the average of two separate 24 h irradiations and was recorded on the Canberra multichannel analyser system described above. If the intrinsic efficiency of the surface barrier detector for these alpha particles was less than 100 %, the curve in Fig. 4 would still retain the same shape but would be shifted down slightly. Our background was low enough to be considered neglible. In our fast neutron work, when both sides of the foil are counted, our background was about 2 tracks cm -2 (Stillwagon et al., 1977, 1978; Stillwagon, 1978). Only one side of our 250pm thick foils was counted in the s-particle research, leading to a background of 1 track cm 2 The results of the measurements of track diameter as a function of etching time are plotted in Fig. 5, for times ranging from 15 min to 4 h. Each point represents the average of the extremes in the measurements of track diameter performed using a filar micrometer in place of our microscope eyepiece at a magnification of 430 x. The error bars in this case actually mark the error ranges, because they indicate the extreme values obtained for track diameter in each case. No tracks were observed for an etching time less than 30 min. The two curves reflect the presence of the two track sizes that were observed for the alpha-particle-irradiated foils and will be discussed below Isee also Stillwagon and Morgan, 1979). 4. DISCUSSION Figure 5 contains two curves for the following reason. After the polycarbonate foils were irradiated

PRODUCTION

AND ELECTROCHEMICAL

ETCHING OF e-TRACKS

FIG. 1. A Plexiglass etching chamber.

187

188

G . B . S T I L L W A G O N and K. Z. M O R G A N

FIG. 2. Electrochemical etching apparatus showing etching chamber, audio frequency generator, amplifier and voltmeter.

PRODUCTION AND ELECTROCHEMICAL ETCHING OF s-TRACKS

FIG. 3. Close-up of the vacuum-sealed alpha calibrator showing the vacuum hook-up, two-compartment design and the calibrated source stand.

189

190

G.B. STILLWAGON

a n d K. Z. M O R G A N

Fl(i. 6. Tracks created by the 5.44 MeV e-particles from Z41Am at a source-to-dosimeter distance of 2 mm in air. Irradiation time = 14 h (a) Without foil masking. (b) With foil masking. Etching parameters were :45 % KOH at 24 °C for 5 h; electrical parameters; 800V, 2 kHz.

P R O D U C T O N A N D E L E C T R O C H E M I C A L E T C H I N G O F or-TRACKS

1.0

)¢ 0.6

.~_ ._t2 LO

0.2:

0

I

I

I

8

16

24

32

p,m polycarbonate FIG. 4. Efficiency of track production as a function of absorber thickness. Polycarbonate absorber thickness was calculated from g cm -2 air. Incident ct-particle count rate was 1.6516 x 107 per 24 h.

191

with ~t-particles, etched by the standard technique and viewed, it appeared that there were two distinct groups or categories of tracks on the foils (see Fig. 6). There were a few large tracks and many small tracks; but as shown in Fig. 7, the number of large tracks decreased and the number of small tracks increased with increasing absorber thickness. To check if this phenomenon of the two categories of tracks was real, the diameters of the tracks on the foil were measured and the relative frequency of the various diameters plotted (Fig. 8). Here the relative frequency, expressed in ~0, of the track diameters appearing on the x-axis in steps of 0.5/~m, is plotted histogram fashion. One can see clearly two categories of tracks present on the foil, represented graphically by the two large peaks of similar magnitude, one at diameters between 3.5 and 4.0pm and / the second between 10.0 and 10.5 #m. In general, no tracks were observed smaller than 3.0 pm and none were observed greater than 115.5/~m throughout these measurements. Histograms similar to Fig. 8 can be drawn also for each of the foils used to generate Fig. 7. Two of these are presented in Figs.

0.9

g

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~o E

::L

4-

64

L_"

._= 48

o

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i

0.5

3t -5 0.3

32 t

,~

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o Small tracks

0.1

I

0

I

I

2

Etching time,

1

I

3

4

0

l 4

l 8

l 12

l 16

i 20

I 24

Equivalent /~m of polycarbonafeabsorber

h

FIo. 5. Variation of track diameter with etching time. Upper curve represents the large.diameter tracks, and the lower curve represents the smaller-diameter tracks.

FIG. 7. Change in the fraction of the total number of tracks on a foil in each track size grouping as a function of ct-particle energy (expressed as #rn of absorber). Upper curve is for large tracks and lower curve for small tracks.

192

G . B . S T I L L W A G O N a n d K. Z. M O R G A N

IO

8

4

tf

2

Track diameter,

FIG. 8.

mnr

I0

14

16

~m

Relative frequency of track diameters after co-particle exposure for a foil etched for 1 h at 800 V, 2 kHz, 24°C in 45 % KOH.

12

I0

8

== 6

4

2

0

F

I0

20

30

40

50

Track diameter,

60

-

70

80

]

90

I00

p.m

FIG. 9.

Relative frequency of the various track dt:tmdcl-, I~,l- a foil tt~mg a 24#m absorber thickness between the source and the foil. Ratio of small tracks to large tracks is 31:26. Etching conditions were : 800 V, 2kHz, 28 % KOH, 24 ~C and 4 h etching time.

9 and 10 for a b s o r b e r thicknesses 24 # m a n d 2/~m of polycarbonate, respectively. By viewing Fig. 9 for 24 # m a n d Fig. 10 for 2 /~m a b s o r b e r thickness, the d r a m a t i c shift from the small to the large diameters can be seen easily. The a b s o r b e r changes the ~particle energy: the thicker a b s o r b e r resulting in

smaller kinetic energy of the a-particles incident on the dosimeter. T h e d a t a o b t a i n e d from these foils were used to generate Fig. 7, using plots of cumulative probability of the occurrence of a particular track diameter N(d~)/N as a function of track diameter, as

P R O D U C T I O N AND ELECTROCHEMICAL ETCHING O F a-TRACKS

193

22 18 m

>~ 14 ~r I0

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m

6

o

l-h, 0

I0

20

30

40

"7 50

60

70

Track diameter,

80

90

I00

I10

120

#m

FIG. 10. Relative frequency of the various track diameters for a foil using a 2/~m absorber between the source and the foil. Ratio of small tracks to large tracks is 12:78. Etching conditions were :800V, 2 kHz, 28 % KOH, 24°C, and 4 h etching time.

described elsewhere (Stillwagon, 1978). Examination of the raw data reveals that the change in the fraction of the total number of tracks in each size grouping was not due to changes in the number of tracks in one group alone but instead the variation was caused by changes in both groups. Any explanation of this phenomenon must describe the mechanism whereby the number of small tracks decreases with increasing energy and the number of large tracks increases with increasing energy. Incidentally, this is opposite to the trend one would observe if the small tracks were caused by alpha particles possessing enough kinetic energy to have an LET on the "flat" portion of the Bragg curve and the large tracks by c~-particles with LET under the "Bragg peak", i.e. of low energy. One plausible explanation for the appearance of two categories of tracks on the polycarbonate foils after ~-particle irradiation is as follows. It is possible that the smaller tracks could have been created by the ~-particles themselves and the larger tracks by the recoil of carbon and oxygen nuclei present in the polymer molecules composing the polycarbonate. Of course, these latter interactions would not be of the elastic collision type found in fast neutron work, because of the + 2 electric charge of the co-particle. Instead, we could envision a Coulombic repulsion event during which the target nucleus does not remain stationary, like classical Rutherford scattering, but instead is "pushed" out of its lattice site by this repulsive force, inelastically.

This event would be possible especially for high energy a-particles and large angles of ~-particle deflection, since more energy could potentially be imparted to the recoiling nucleus than in the opposite situation (low energy alpha particles and small angles of deflection). This type of interaction was introduced by Rutherford many years ago (Rutherford, 1911; Rutherford et al., 1930). Except for the work using protons and helium nuclei, much of this early work was concerned with heavier nuclei such as argon, gold and copper, in which this effect would be considerably reduced in importance owing to the difference in mass between an ~-particle and, say, a gold nucleus. In polycarbonate and tissue, the main target nuclei present are carbon, oxygen and protons. Two pieces of experimental evidence suggest this explanation for the appearance of large tracks. First, the track diameters for the large group of tracks are approximately the same as we have observed for recoil particle tracks after fast neutron irradiation (Su and Morgan, 1978). Second, the number of large tracks increased and the smaller tracks decreased with decreasing absorber thickness (cf. Fig. 7). The increase in large tracks would then be caused by the greater number of ~-particles possessing high energy. (The initial energies of ~particles from the sources used in our experiment lay between 5 and 6 MeV). Thus, with thinner absorber more a-particles have sufficient energy to "push" the polymer nuclei out

194

G. B. S T I L L W A G O N and K. Z. M O R G A N

of their lattice sites; because, as energy increases, smaller and smaller angles of c~-particle deflection will give sufficient energy to the target nucleus to push it out of its lattice site, i.e. the larger the range of angles that can cause the event the higher the probability of the event. The decrease in smaller tracks would be caused by the LET of the ~particles getting away from the Bragg peak where the highest LET is found. Energies near this Bragg peak may be the only region where c~-particles possess large enough magnitude of (-dE/dx) to have a large probability of producing tracks of small diameter. With a larger absorber present, many more ~-particles could be expected to be found within the high LET portion of the Bragg curve and fewer possessing high energy and low -dE/dx. A decrease in the number of large diameter tracks, but an increase in the number of small diameter tracks as the Bragg peak is approached, could be anticipated with this explanation. The data of Rutherford also support this explanation of the increase in numbers of large tracks with decreasing absorber thickness. His curve for helium nuclei as

.~

targets clearly shows an increase in numbers of counts with increasing o-particle energy (cf. Fig. 11). The increase as v0 is approached is caused by Helium target nuclei recoiling into the ZnS screen along with scattered c~-particles; and as Rutherford et al. (1930) indicate in the caption accompanying the figure, this effect increases with increasing ~-particle energy. Here Vo/V is the ratio of the velocity of the o-particle used in the study, Vo to the various velocities considered, v. The alpha sources used were "radium active deposit" which was 218p0, 214pb and 214Bi in equilibrium and "thorium active deposit" which was 22STh. It is believed that this effect could have ramifications in the field of internal dosimetry. To illustrate, we could examine the energy expected to be possessed by these light recoiling nuclei. A vector diagram for the interaction is shown in Fig. 12, where O A is the momentum vector for the incident o-particle (mvl), O B for the scattered o-particle (mY2), 4) the angle of deflection, and B A the m o m e n t u m vector for the recoiling nucleus ( M V ) . By the law of cosines, (MV) 2 = (my1) 2 + (mY2) 2 - 2(mY2) (mY2)cos~),

20 B

8 =o =5

,,9. ~o E +

\

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Inverse square calculation) I 0.5

I LO

I 1.5

I 2.0

(VolV)2 FIG. 11. Graph showing the number of counts received on a ZnS screen as a function of a-particle velocity. Y-axis is the number of counts observed divided by that expected from an inverse squares calculation and the X-axis is the velocity squared of the c~-particle used divided by the square of the ~-particle velocity considered (from Rutherford et al., 1930).

A F1G. 12. Vector diagram describing the collision of an c~particle and a nucleus. The momentum vectors are: OA for the incoming ~-particle, OB for the scattered ~-particle and BA for the recoiling nucleus.

PRODUCTION

AND ELECTROCHEMICAL

E T C H I N G O F or-TRACKS

195

Table l. The energy, AE, of the recoiling nucleus; the scattered ~-particle energy, E 2 and the ratio of final to initial orparticle velocity, a; shown for various angles of deflection of the scattered ~t-particle. Et = 5.1 MeV (initial alpha energy). V~= 1.567 x 10° cm s- 1 and m~= 6.647 x 10- 24 g. Parameter K is the ratio of the mass of the recoiling nucleus to that of the o-particle. 30°

Carbon Oxygen

K

p

3 4

0.956 0.967

60 °

E2 AE (MeV) (MeV) 4.66 4.77

0.44 0.33

=

Ez

AE

p

E2

AE

p

E2

AE

0.843 0.881

3.63 3.96

1.48 1.44

0.707 0.775

2.55 3.06

2.55 2.04

0.523 0.621

1.50 1.97

3.71 3.13

m v 2 - m v 2.

If we define K = M / m a n d v 2 = p v 1 where p < l , from these two equations we quickly find K-

then

1 = p 2 ( K + 1 ) - 2 p cos~b,

or

cos~b +

1

150 °

p

a n d by conservation of energy M V2

90 °

x / K 2 - sin2~b.

If we n o w assume an initial or-particle energy of 5.1 MeV, approximately equal to that of a 239pu 0cparticle, we can calculate the energy expected to be given to the recoiling nucleus. These values appear in Table 1 for various angles, ~b, E 2 is the scattered or-particle energy a n d AE is t h a t - g i v e n t o - t h e recoiling nucleus. F o r gold at ~b= 90 °, AE = 0.21 M e V a n d for a l u m i n i u m AE = 1.33 M e V ; so we can see how m u c h more i m p o r t a n t this type of scattering becomes for light nuclei (i.e. C a n d O) in these studies t h a n was the case for heavy nuclei studied by Rutherford. In a h e a d - o n C o u l o m b i c repulsion event with a c a r b o n nucleus almost 4 MeV is i m p a r t e d to this nucleus. The implications in internal dosimetry are now becoming clearer. Instead of this 4 M e V being imparted to the tissue t h r o u g h ionization by an ~tparticle, as we would assume in calculations of dose equivalent, this 4 M e V has been converted to a more efficient energy-imparting means, i.e. a c a r b o n nucleus. This could cause more severe d a m a g e to cells, especially in the close vicinity of the a l p h a emitter, t h a n we would expect when considering only or-particle energy deposition. Also, we have a

direct m e c h a n i s m n o w present to cause c h r o m o some breakage. With this interaction, a t o m s simply could be removed or k n o c k e d out of a D N A chain, causing irreparable d a m a g e to this chain a n d possibly o t h e r chains owing to the higher L E T of the removed c a r b o n or oxygen nucleus. This is a direct m e c h a n i s m for p e r m a n e n t change in a cell nucleus that should be considered during discussion of carcinogenicity a n d other cellular effects. One interesting side note is the possibility t h a t the effect described a b o v e m a y never have been observed if the foils h a d not been composed of light nuclei as in tissue, i.e. h a d n o t been tissue equivalent. This effect could actually be occurring in tissue as well as in p o l y c a r b o n a t e since the main c o m p o n e n t s of tissue are nuclei whose masses are not greatly different from that of the or-particle. Therefore large energy transfer can result from the or-particle to the target nucleus in tissue, as we have observed in polycarbonate. P h o t o g r a p h i c emulsions, on the other hand, are not tissue equivalent; almost 17 ~o of the atomic composition of N T A emulsions consists of the heavy nuclei, silver a n d bromide. In polycarb o n a t e foils we are observing the actual track created by the incident particle, amplified m a n y times; b u t in the emulsions we are only observing silver ions reduced by the passage of the ionizing particle close by. Discovery of the "two track-size" effect on p o l y c a r b o n a t e foils was m a d e by measuring track diameters easily distinguishable with a c o m m o n optical microscope. The corresponding effect on the emulsion would appear as a n a r r o w track of reduced silver ions up to the point of collision, then a wide track of reduced silver ions c o r r e s p o n d i n g to the recoil nucleus for a 180 ° collision. O f course n o tracks would be seen from recoils of silver or bromide, since they are too heavy to receive e n o u g h energy from the or-particle

196

G. B. S T I L L W A G O N and K. Z. M O R G A N

to move far enoug h to produce a recognizable track on the photographic emulsion (Rutherford, 1911; Rutherford et al., 1930). Owing to the inherent difficulties in reading particle tracks (angular dependence causes some tracks to be unrecognizable if angle of incidence is close to the normal; high magnification required to read emulsions and background or artifacts; etc.) and non-tissue equivalence, the two track sizes would not be expected to be observable in large enough numbers to reproduce the histograms describing this effect as reported in this paper. The observance of effects such as these is one argument in favour of using a tissue equivalent material whenever possible to detect the interactions of radiation with tissue. If this suggestion is not followed, important interactions may be overlooked simply because the dosimeter selected does not interact with the radiation in the same manner as tissue. We cannot pretend to know beforehand when tissue equivalence is not necessary. If this interaction is occurring in tissue to the degree indicated by the data presented here, the recoil reaction may be more important than first seemed to be the case, even though LET is considered to b e the only quantity of prime importance for the purpose of or-particle dosimetry (Haque, 1966). REFERENCES

Haque A. K. M. M. (1966) Energy expended by alpha particles in lung tissue, Br. J. appl. Phys. 17, 905. Rutherford E. (1911) The scattering of a and fl particles by matter and the structure of the atom, Phil. Mag. 21, 669. Rutherford E., Chadwick J. and Ellis C. D. (1930)

Radiation from Radioactive Substances, Ch. 9. Cambridge University Press. Somogyi G. (1977) Processing of plastic track detectors, Nucl. Track Detection l, 3. Sohrabi M. (1975) Electrochemical etching amplification of low-LET recoil particle tracks in polymers for fast neutron dosimetry. P h . D . Dissertation, Georgia Institute of Technology, Rep. ORO-4814-5. Sohrabi M. and Morgan K. Z. (1976) Some studies on the development and applications of recoil particle track amplication by electrochemical etching for fast neutron dosimetry. Rep. ORO-4814-9. Stillwagon G. B. (1978) The microdosimetry of plutonium 239 in bone using electrochemical etching and polycarbonate foils. Ph.D. Dissertation, Georgie Institute of Technology. Stillwagon G. B. and Morgan K. Z. (1979) Letter to the Editor concerning the discovery of two track sizes on polycarbonate foils after alpha particle irradiation and electrochemical etching. Hlth Phys. 36, 741. Stillwagon G. B., Su S. J. and Morgan K. Z. (August 1977) Some studies on the development and application of particle track amplification by the electrochemical etching method for the purpose of dosimetry. Progress Report, Georgia Institute of Technology. Stillwagon G. B., Su S. J. and Morgan K. Z. (1978) Letter to the Editor concerning our determination of bulk etching rate of polycarbonate foils. Hlth Phys. 34, 735. Shian-Jang Su (1977) Neutron dosimetry using electrochemical etching. Trans. Am. nucl. Soc. Stud. Conf., Raleigh, N. C., 11-29. Su S. J. and Morgan K. Z. (1978) Energy dependence of fast neutron dosimetry using electrochemical etching. Trans. Am. nucl. Soc. Stud. Conf., Gainesville, Florida. Tommasino L. and Armellini C. (1973) A new technique for damage track detectors. Radiat. Effects 20, 253.