White noise in spectral rendering

[Ine-123]

I want to create a simulation of apple tissue based on the bulk optical properties (absorption coefficient, scattering coefficient and anisotropy factor). I used this code for the scene, but get always white noise (fireflies?). I already tried changing some parameters (different integrators, higher sample count, brighter light source) but it doesn't seem to help. Anyone suggestions?

<?xml version='1.0' encoding='utf-8'?>

<scene version="0.6.0">
<integrator type="volpath"> 
    <integer name="maxDepth" value="100"/>
</integrator>


<shape type="obj">                                 
    <string name="filename" value="appleFilled.obj"/>
    <transform name="toWorld">
		<translate x="0" y="0" z="0"/>
        <scale value="1.0" /> 
    </transform>

    <medium name="interior" type="homogeneous"> 
		<spectrum name="sigmaS" filename="sigmaS.csv"/>
		<spectrum name="sigmaA" filename="sigmaA.csv"/> 
		<phase type="hg">
			<float name="g" value="0.8313"/>
		</phase>
    </medium>

    <bsdf type="roughdielectric">
        <string name="distribution" value="beckmann"/>
        <float name="alpha" value="0.304"/>
        <float name="intIOR" value="1.33"/>
        <float name="extIOR" value="1.0"/>
    </bsdf>

</shape>

<sensor type="perspective">   
   <float name="focusDistance" value="6"/>
   <float name="fov" value="40"/>
   <string name="fovAxis" value="x"/>
   <transform name="toWorld">
	<lookat target="0, 0, 0" origin="0, 0, 0" up="0, 1, 0"/>
   </transform>

       <sampler type="independent">
          <integer name="sampleCount" value="128"/> 
       </sampler>

      <film type="hdrfilm">
          <boolean name="banner" value="true"/>
          <integer name="height" value="768"/>
		<integer name="width" value="768"/>
      <string name="pixelFormat" value="spectrum"/>
      <rfilter type="gaussian"/>
      </film>
</sensor>


<emitter type="directional">
    <transform name="toWorld">
        <lookat origin="0,0,0.3" target="0.1, 0, 0" up= "0,1,0"/>   
    </transform>  
</emitter>