Counting Results for Acid used to soak KamLAND AmBe Sources
Zelimir Djurcic and Andreas Piepke
University of Alabama
Introduction and Results
In this note we summarize new leak test results for several AmBe sources intended
for deployment into KamLAND's liquid scintillator. At this point two AmBe source
have been delivered from LSU to the KamLAND site. Both commercial sources were
manufactured by Amersham. One of the sources contains 3.33 MBq of
241Am the other 1.85 MBq. In addition UA has shipped another
AmBe source to Japan (7/22/2002). These sources will be called A, B and C from
All sources shipped to Kamioka are classified as sealed
sources. However, testing any release of activity into a medium
surrounding the source has to be done to tight activity limits in order to preserve
KamLAND's low background. We have therefore decided to provide an independent
leak test. Initial acid soak tests done with both LSU AmBe sources were performed in
Japan in May 2002 by Bob Svoboda. The counting of the acid showed clear net activities
in both acid samples. This finding has been
reported earlier. The sources have been kept in acid for an unknown length of
time. It was found that sources A and B released
13.6±1.5 Bq and 1.5±0.3 Bq
into the 60 ml of acid used for the soaking, respectively. The soaking was then repeated
with water. The water samples were counted and no 241Am
activity was found. The activity limits derived for the water samples of sources A and B
(60 ml of water, contained in glass bottles, were counted in the same geometry as before)
0.71±0.52 Bq and 0.49±0.47 Bq,
It was decided to repeat the initial acid wash in order to understand whether the
activity initially removed from the sources was due to a surface contamination or
supported by leakage of activity into the solvent. On June 28 both sources A and B were soaked
in dilute nitric acid (molarity is not exactly known) for 24 hours. This operation
was performed in Japan by Bob Svoboda and Andreas Piepke. The acid samples of
both sources and a blank were then shipped to UA for counting. Unfortunately only
sample B survived the shipment. In this note we present the results obtained by counting
this sample. As in the previous measurement a statistically
significant (5.5·Sigma) 241Am activity was found in the wash acid.
The measured activity is with 620±160 mBq slightly smaller
than that measured before for source B (soaking time 24 hours).
Please note that the source had been
mechanically cleaned between the two soakings.
In parallel UA's AmBe source has been soaked for 9 days
in 0.1 molar HNO3 to provide a backup solution. Results for this source are
presented as well. For this sample no 241Am activity was
detected. We can place a 90% c.l. limit of 80 mBq of activity released into the acid
for this sample. If we assume
a constant transfer of activity into the acid the activity limit is 8.8 mBq/day.
Procedure and Data
Am is not well soluble in organic solvents. To provide a stringent leak test the sealed
AmBe source to be used in KamLAND have been immersed in 0.1 molar HNO3 which
does dissolve Am. The fact that the same source, B, did show a leakage in dilute
acid but no leak in water underscores the importance of the right medium
(see the previous report).
To obtain a quantitative estimate for the release of activity into the liquid
scintillator we would need to scale the measured activities reported here by the
ratio, r, of the solubility of Am in scintillator divided by the
solubility in dilute nitric acid. r is not known at this time. However,
r is for sure less than one. If r is indeed much smaller than
unity (as expected) then
the acid test provides a stringent test of source leakage. At this point
it is not known whether the activity is released at once or is proportional
to the exposure time, or a combination of both. It is thus not clear whether
it makes sense to divide the activity by the exposure time, as was done in all
The acid used in the source soaking tests has been counted at UA
using our low background Ge detector.
A 59.6 keV gamma is emitted with a branching
ratio of b=0.359 gammas/decay
in the 241Am alpha decay. The level scheme is shown in the following
Figure 1: Level scheme of 241Am alpha decay, as taken from the
Table of Isotopes, 1998 edition.
In our tests we were looking for a full
absorption line at 59.6 keV.
Due to the low energy of the emitted photons, absorption losses in the passive
detector materials can severely degrade the detection efficiency.
The detection efficiency has been experimentally calibrated for the glass bottles
used. We made use of an open Am activity, dissolved in acid. It was found that
the combined efficiency is:
The following data sets were used in the activity analysis:
UA Am Be source (C) (blue spectrum)
Figure 2 shows the measured energy spectra, normalized to counts per day. The soak sample
of source B shows a clear peak at 59.6 keV, the expected energy for 241Am. The
soak sample of source C shows no such peak. The fact that the neighboring peaks show more
or less the same intensity and widths demonstrates that the data has been taken under
the same conditions. The detector background is a bit lower than the soak spectra. This
event excess is caused by K/Th/U activities contained in the glass of the bottles.
The use of plastic bottles of the same size would allow to obtain tighter activity limits.
LSU 1.85 MBq Am Be source (B) (red spectrum)
Ge detector background (purple spectrum)
The following figures show the different data sets separately, together with a
a Gaussian peak fit.
Figure 3: Source B (weaker LSU source)
Figure 4: Source C (UA source)
Figure 5: Detector background
The x-axis is in channel number, the y-axis in counts/channel. Fit parameter
1 corresponds to the peak area (in counts), parameter 2 to the peak centroid
(in channels) and parameter
3 to the peak's FWHM (also in channels). The remaining two parameters describe the linear
background. For source C and the background no peaks were found. For these fits the
peak position and width was fixed to the value obtained in the analysis of source B.
As can be seen from figure 5 a small peak at 59.6 keV (or 173.7 channels) is present. This
(2.2·Sigma) net rate has to be subtracted from the source data. It should be noted
that it would be better to subtract blank data taken in the presence of the glass bottles.
In case of source C this might help to achieve an even tighter activity limit.
For source B the blank was lost during transport.
To arrive at the above activities the fitted peak areas were divided by the measuring time,
the branching ratio and the detection efficiency.
The AmBe sources provided by LSU show signs of Am leakage. In case of source B this
has been demonstrated twice now. Although leakage seems to be the
most likely explanation
a repeated contamination through the use of contaminated plastic bags (into which the
sources were unfortunately put back after the acid treatment) or other materials is
not excluded. This can only be shown by yet another round of measurements.
As UA's AmBe source (shipped to Japan) shows no sign of contamination
its deployment seems to be the safest and fastest way to provide an energy calibration for
This page is maintained by A. Piepke
Last update July 24, 2002