Application of CR-39 microfilm for rapid discrimination between alpha particle sources
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Application of CR-39 microfilm for rapid discrimination between alpha particle sources. This work presents a new technique for discriminating between alpha particles of different energy levels. In a first study, two groups of alpha particles emitted from radium-226 and americium-241 sources were successfully separated using a CR-39 microfilm of appropriate thickness. This thickness was adjusted by chemical etching before and after irradiation so that lower-energy particles were stopped within the detector, while higher-energy particles were revealed on the back side of the detector.
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Application of CR-39 microfilm for rapid discrimination between alpha particle sources. This work presents a new technique for discriminating between alpha particles of different energy levels. In a first study, two groups of alpha particles emitted from radium-226 and americium-241 sources were successfully separated using a CR-39 microfilm of appropriate thickness. This thickness was adjusted by chemical etching before and after irradiation so that lower-energy particles were stopped within the detector, while higher-energy particles were revealed on the back side of the detector..
882 N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 8 8 1 e8 8 5
Matter (SRIM) program , the range of 5-MeV alpha particles particles in CR-39. The figure clearly shows that the greater
in CR-39 is 28.9 mm. In general, the range is highly correlated the energy of the alpha particles, the longer their range. These
with the energy of the incident alpha particles . Conse- range values were 33.3 mm for the 5.49-MeV particles and
quently, this generates a problem in the case of different alpha 27.0 mm for the 4.78-MeV particles.
particles having very close energy levels. In that case, the
ranges will be very similar, and the ability to discriminate 2.2. CR-39 microfilm preparation and chemical etching
between alpha particles will seem difficult to achieve. process
Previous works on alpha spectroscopy have developed a
matrix of energy equations as a function of the track diameter Thin sheets of CR-39 microfilm (Fukuvi Chemical Industry
[17e20]. However, these approaches have used complicated Company, Tokyo, Japan) with C12H18O7 molecular composi-
geometric analyses of the track parameters, as well as cali- tion, 100 mm uniform thickness, and 1.32 g/cm3 density were
bration curves of the track diameter versus alpha energy. cut by a laser into pieces with dimensions of 1 1 cm2. To
Another work on alpha particles from radon gas and radon determine the rate of the chemical etching process, five pris-
daughters used two detectors . The first was a CR-39 track tine CR-39 microfilms were etched under standard etching
detector to determine the incident fluence; the second was an conditions in a 6.25N aqueous solution of NaOH maintained at
LiF thermoluminescent detector to deduce the average energy 70 C by a water bath for 6 hours . During the etching pro-
of the alpha particles. However, that study was time cess, a magnetic stirrer was used to achieve uniform etching
consuming and required calibration of the two detectors. and to prevent accumulation of the etchant material on the
Therefore, it is important to search for a faster and less surfaces of the microfilms. After etching, the microfilms were
complicated method of alpha spectroscopy. thoroughly rinsed with distilled water and dried in open air.
In this work, we present a new method using a CR-39 The thickness of each microfilm before and after etching was
microfilm for the discrimination of the energy of alpha parti- measured using a sensitive micrometer; the average value of
cles emitted from two different sources. The method is based the bulk etching rate was found to be 1.06 mm/h, according to
on the experimental observation that the greater the energy of the following equation:
an alpha particle, the longer its range in the material. There-
fore, by adjusting the thickness of a CR-39 microfilm to match Dd
Bulk etch rate ¼ (1)
the range of higher-energy alpha particles, low-energy parti- 2t
cles will stop within the microfilm, whereas high-energy where Dd is the thickness reduction and t is the etching time.
particles will pass the microfilm and can be revealed on the Our results for the etch rate agree exactly with those reported
back side by chemical etching. It can readily be understood by Yamauchi et al . In their work, it took about 40 hours to
that, under these conditions, discrimination of the two energy reduce the thickness of an unirradiated microfilm from
levels is achieved accordingly. This work is a continuation of 100 mm to 15 mm.
our previous work on improving radiation measurements Next, a fresh set of six 100-mm-thick CR-39 microfilms was
using the CR-39 detector . etched for 30.6 hours using the abovementioned etching
conditions until the thickness of the residual active layer of
each microfilm was reduced to 35 mm. This particular thick-
2. Materials and methods ness is sufficient to prevent possible backscattering of alpha
particles from a thick substrate at the back side of the detec-
2.1. Alpha-particle sources tor. Indeed, alpha particles can penetrate the detector to the
substrate, bounce from the substrate surface, and then enter a
We used two different alpha-particle sources from the second time into the detector, which may contribute to the
commercially available reference standards. One source was tracks at the back side of the detector. To ensure that alpha
Ra, which emits alpha particles with a kinetic energy of 4.78 particles stop before reaching the substrate, the detector was
MeV; the other was 241Am, which emits alpha particles with a etched to a thickness slightly larger than 33.3 mm (i.e., thicker
kinetic energy of 5.49 MeV. Using the Bateman equation, we than the range of the highly energetic 5.49-MeV alpha parti-
calculated the present activity of the two sources at the time cles from 241Am in CR-39).
of this study and found that both had the same activity of 150 Afterward, one blank microfilm was randomly selected
nCi (5.55 kBq). In order to calculate the range of alpha particles and used as a control. The front and back sides of the control
in CR-39, we employed SRIM simulation software , avail- microfilm were scanned by a manual optical scanner to
able on the Internet. We chose the Transport of Ions in Matter determine the existence of possible background tracks. Sur-
(TRIM) section of the software to generate a list of stopping face defects or high-density pits were not found in the control
power and range values. The calculations were completed for microfilm, and the background tracks were easily distin-
99,999 helium ions per simulation, a default used by the guished. The mean value of background track density was
software. Fig. 1 is a plot of ionization, that is, the energy loss of measured and found to be 4 ± 3 tracks/cm2. This low count
the incident alpha particles to the target electrons as a func- value indicated that the microfilm in hand had not been
tion of the penetration depth in the CR-39 target. The dotted irradiated previously. At this point, the control microfilm un-
curve represents the 4.78-MeV alpha particles emitted from derwent no further processing and was stored for future
Ra, and the solid curve represents the 5.49-MeV alpha reference. It is worth noting that all the microfilms used in this
particles emitted from 241Am. End points of the curves work were kept away from the external environment in a
represent the maximum penetration depth of the alpha clean room under controlled laboratory conditions. This
N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 8 8 1 e8 8 5 883
Fig. 1 e Stopping power of 4.78-MeV and 5.49-MeV alpha particles as a function of the penetrating depths in CR-39 target
(calculated by SRIM-2013 software).
makes the collection of further background tracks from the particles used in this study. To do so, two microfilms were
environment or from some unaccounted-for source unlikely monoenergetically irradiated. One microfilm was exposed
to happen. Nonetheless, we placed these microfilms on thick only to high-energy long-range particles from the 241Am
aluminum substrates to block any possible exposure to the source, and the other microfilm was exposed only to low-
environment occurring at the back side. In such a situation, energy short-range particles from the 226Ra source. Fig. 2
there is no way for alpha particles to reach the back side shows a representative image of the etched tracks observed
except by coming through from the front side. on both sides of the microfilm, the front side of which was in
contact with the 241Am source. As can be seen, the front and
2.3. Irradiation, counting, and energy identification of back sides of the microfilm have the same number of tracks.
alpha particles This observation clearly indicates that all alpha particles
entering the front side penetrated the microfilm thickness and
Each microfilm was irradiated with alpha particles by placing appeared at the back side. Therefore, it can be concluded that
the point sources (226Ra and 241Am) in close contact with the the microfilm thickness is, indeed, about the same as the
front side of the microfilm for 5 seconds. After irradiation, the range of these 5.49-MeV alpha particles.
irradiated CR-39 microfilms were etched again in a 6.25N Conversely, no tracks were observed at the back side of the
NaOH solution at 70 C for a short time interval of 2 hours. microfilm whose front side was in contact with the 226Ra
After the etching, etched pits along the tracks of alpha parti- source. This indicates that all alpha particles incident on the
cles in the microfilm became visible under an optical micro- front side were stopped within the microfilm and remained
scope and could be counted using an automated counting embedded in it. Hence, the range of these 4.78-MeV alpha
system. The system setting can positively identify the pits and particles is shorter than the microfilm thickness. The above
ignore false positives. The characteristics of this system, and results confirm the possibility of using this very simple and
the procedure for track registration and analysis were practical procedure to discriminate completely and with cer-
described in detail in a previous publication . The numbers tainty between alpha particles of two different energy levels.
of etch pits on the front and back sides of each microfilm were For further quantitative analysis, Table 1 shows the num-
determined and verified by manual counting. Furthermore, ber of tracks counted at the front and back sides of the three
background radiation was taken into account by subtracting CR-39 microfilms after these films were irradiated by a com-
the number of etch pits counted in the control (unexposed) bination of alpha particles emitted from the 226Ra and 241Am
microfilm from the number of etch pits counted in the irra- sources together.
diated microfilms. Discrimination of the alpha particles with The data in Table 1 clearly reveal that the alpha particles
two different energy levels is simply based on track counting from both sources entering the microfilm produced nearly
on both sides of the microfilm, without need for calibration 4,800 visible tracks at the front side of the microfilm, whereas
curves of the track diameter versus alpha energy. those arriving at the back side of the microfilm produced only
approximately 2,400 visible tracks. Assuming that the number
of alpha particles is determined by counting the visible tracks
3. Results and discussion in the microfilm, it can be deduced that out of all the alpha
particles from the two sources incident on the front side of the
Initially, it was important to verify experimentally that the CR- microfilm, only approximately half arrived at the back side of
39 microfilms prepared with 35-mm thickness were appro- the microfilm. Most likely, these are the long-range 5.49-MeV
priate for discriminating between the two ranges of alpha alpha particles from the 241Am source. By subtracting the
884 N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 8 8 1 e8 8 5
Fig. 2 e Recorded tracks for alpha particles with 5.49-MeV energy and normal incidence in CR-39 microfilm. The field of view
taken at a specific position shows tracks on (A) the front side and (B) the back side.
natural and manmade radiation. However, further study is
Table 1 e Counts of the number of tracks of alpha
necessary to determine clear criteria and/or significant re-
particles in CR-39 microfilms.
strictions on the conditions for which the method is appli-
Counts on Counts on
cable. In particular, challenges may arise if the energy of the
front side back side
alpha particles is not known or if the alpha particles are
CR-39 microfilm #1 4,783 2,387 comparable in energy, such that their separation in energy is
CR-39 microfilm #2 4,770 2,391
too small. In addition, it may become increasingly difficult to
CR-39 microfilm #3 4,777 2,379
Average 4,777 ± 7 2,386 ± 6
quantify each population of particles if the separation be-
tween the high-energy cutoff of one particle is too close to that
of the other due to broadening of the Bragg peak. At present,
track counts on the front and back sides of the microfilm, it the method has been used for binary discrimination, not
can be found that nearly half of the incident alpha particles spectroscopy. With more development, there is certainly po-
remained embedded in the microfilm. Most probably, these tential to move in the direction of spectroscopy.
are the short-range 4.78-MeV alpha particles from the 226Ra
source. It is interesting to note that the numbers of alpha
particles embedded in the microfilm and those that pene-
trated the microfilm are exactly the same. Essentially, this
In this work, we have developed a simple and rapid method of
result confirms that the 226Ra and 241Am sources have equal
using CR-39 microfilms to discriminate between alpha parti-
activity, which is in agreement with our calculations of the
cles of two different energy levels. The method proved effec-
present activity of the sources, as mentioned in Section 2.1.
tive in identifying alpha particles emitted from different
The findings in this work provide new insight into using
sources with suitable different energy levels. This makes the
CR-39 microfilms to distinguish between alpha particles of
method an appropriate option for nuclear science research
different energy levels. The method is reliable, accurate, and
and environmental radiation measurement. The method is in
suitable for environmental radiation measurements. It is
the first phase of experimentation, and future work will
relatively fast because of short etch times. In addition, it is
extend this study by further optimization of the microfilm and
simple because there is no need for calibration curves of the
implementation of the method in advanced and complex
track diameter versus the energy of the incident alpha parti-
cles. Moreover, in the case of a mixed source emitting multiple
alpha particles (n), the required number of microfilms for the
identification of alpha particles must be n e 1. For instance, Conflicts of interest
two CR-39 microfilms with different thickness are sufficient to
distinguish between 222Rn and its progenies 214Po and 218Po The authors have no conflicts of interest to declare.
Finally, it should be noted that this reported method re- Acknowledgments
quires prior knowledge of the alpha particles to be recorded
and appropriate preparation of the CR-39 microfilm, in addi- The authors wish to acknowledge the support provided for
tion to suitable separation in energy of the alpha particles this work by King Fahd University of Petroleum & Minerals
under investigation. In practical applications, the method can through project number SB141003. The authors also thank
be used for environmental radiation monitoring. The method Professor Toshiyuki Iida and his lab members at Osaka Uni-
can also be used to discriminate between different types of versity, Japan, for their valuable collaboration.
N u c l e a r E n g i n e e r i n g a n d T e c h n o l o g y 4 9 ( 2 0 1 7 ) 8 8 1 e8 8 5 885
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